adaptacion skeletal rodents

Skeletal Indicators of Locomotor Adaptationsin Living and Extinct Rodents

Joshua X. Samuels* and Blaire Van Valkenburgh

Department of Ecology and Evolutionary Biology, University of California-Los Angeles,Los Angeles, California 90095

ABSTRACT Living rodents show great diversity intheir locomotor habits, including semiaquatic, arboreal,fossorial, ricochetal, and gliding species from multiplefamilies. To assess the association between limb mor-phology and locomotor habits, the appendicular skele-tons of 65 rodent genera from 16 families were meas-ured. Ecomorphological analyses of various locomotortypes revealed consistent differences in postcranial skel-etal morphology that relate to functionally importanttraits. Behaviorally similar taxa showed convergent mor-phological characters, despite distinct evolutionary his-tories. Semiaquatic rodents displayed relatively robustbones, enlarged muscular attachments, short femora,and elongate hind feet. Arboreal rodents had relativelyelongate humeri and digits, short olecranon processes ofthe ulnae, and equally proportioned fore and hind limbs.Fossorial rodents showed relatively robust bones,enlarged muscular attachments, short antebrachii anddigits, elongate manual claws, and reduced hind limbelements. Ricochetal rodents displayed relatively proxi-mal insertion of muscles, disproportionate limbs, elon-gate tibiae, and elongate hind feet. Gliding rodents hadrelatively elongate and gracile bones, short olecranon proc-esses of the ulnae, and equally proportioned fore and hindlimbs. The morphological differences observed here canreadily be used to discriminate extant rodents with differ-ent locomotor strategies. This suggests that the methodcould be applied to extinct rodents, regardless of ancestry,to accurately infer their locomotor ecologies. When appliedto an extinct group of rodents, we found two distinct eco-morphs represented in the beaver family (Castoridae),semiaquatic and semifossorial. There was also a progres-sive trend toward increased body size and increased aquaticspecialization in the giant beaver lineage (Castoroidinae).J. Morphol. 269:1387–1411, 2008. � 2008 Wiley-Liss, Inc.

KEY WORDS: ecomorphology; functional morphology;locomotion; rodents; beavers; convergent evolution

The order Rodentia includes about 2,000 livingspecies and has members that occupy nearly everyterrestrial habitat on Earth. Compared with mostother mammalian orders, rodents span a widerange of body sizes, from <10 g in some mice to>50 kg in the capybara, Hydrochaeris, and >500kg in Phoberomys and Josephoartigasia, extinctrelatives of the pacarana (Nowak, 1999; Sanchez-Villagra et al., 2003; Rinderknecht and Blanco,2008). Rodents have also evolved a diverse arrayof ecological specializations including semiaquatic,

arboreal, fossorial, jumping, and gliding forms.Similar adaptations are seen in distantly relatedfamilies suggesting widespread convergence andparallelism over the course of rodent evolution(Howell, 1930; Nevo, 1995; Lacey et al., 2000;Stein, 2000). Many previous studies have com-pared rodents with particular locomotor habits totheir more generalized terrestrial counterparts;however, few studies have examined locomotorcharacteristics across a wide range of specializa-tions (Bou et al., 1990; Casinos, 1994; Elissamburuand Vizcaıno, 2004). This study examines the mor-phology of a broad sample of living rodents to: (1)identify skeletal characteristics linked to locomotorbehaviors; and (2) identify which characteristicsdiffer between groups. These skeletal features arealso used to infer the locomotor behavior of extinctrodent species and improve our understanding ofrodent evolution.

Ecomorphological analyses have often been usedto examine locomotor habits of extant species andto infer behavior in their extinct counterparts(e.g., Van Valkenburgh, 1987; Stein, 1988; Lessaand Patton, 1989; Lessa and Stein, 1992; Biknevi-cius, 1993; Fernandez et al., 2000; Argot, 2001;Gingerich, 2003; Elissamburu and Vizcaıno, 2004;O’Keefe and Carrano, 2005). These studies use avariety of statistical techniques to relate morpho-logical variables to the ecological niches occupiedby species. Their goals are to characterize the mor-phological space occupied by modern taxa with dis-tinct ecologies and then infer the ecologies ofextinct groups from the morphological spaces theyoccupy. To date, few such morphometric compari-sons have examined locomotor adaptations acrossrodent groups. Previous studies of rodents usingecomorphological analysis include the analysis ofspecializations for digging in gophers (Lessa andPatton, 1989; Lessa and Thaeler, 1989; Lessa and

*Correspondence to: Joshua X. Samuels, Department of Ecologyand Evolutionary Biology, 621 Charles E. Young Drive South, Uni-versity of California-Los Angeles, Los Angeles, CA 90095-1606,USA. E-mail: [email protected]

Published online 5 September 2008 inWiley InterScience (www.interscience.wiley.com)DOI: 10.1002/jmor.10662

JOURNAL OF MORPHOLOGY 269:1387–1411 (2008)

� 2008 WILEY-LISS, INC.

Stein, 1992), semiaquatic locomotion in muskrats(Stein, 1988), and locomotor specializations in cav-iomorph rodents (Elissamburu and Vizcaıno,2004).

This study includes 65 rodent genera from 16living families that show terrestrial, semiaquatic,arboreal, semifossorial, fossorial, ricochetal, andgliding locomotor habits (Table A1). We predictrodents with similar locomotor habits will showsimilar morphologic features as a result of conver-gent or parallel evolution. Selection pressure forfunctionally advantageous structures should leadto similarities between taxa regardless of theirancestry. Postcranial material from extinct rodentscan be compared with that of extant rodents todetermine whether these species had similar loco-motor adaptations and lifestyles. Here we applythese methods to extinct members of the familiesCastoridae (beavers) and Sciuridae (squirrels).

MATERIALS AND METHODS

A total of 293 individuals of 65 extant rodent genera (67 spe-cies) from 16 families were used in this study (average n 5 4.4,n for each species can be found in Table A1). These taxa werechosen to represent a wide range of body morphs and locomotorspecializations. Examination at the generic rather than specieslevel was used, because species within the same genus areexpected to have similar form. Members of both sexes wereused, and only adult, wild-caught specimens were examined. Aliterature survey was used to classify each species into one ofseven locomotor groups: terrestrial, semiaquatic, scansorial/ar-boreal, semifossorial, fossorial, ricochetal, and gliding (Table 1)(Howell, 1930; Nevo, 1999; Nowak, 1999; Lacey et al., 2000).Each locomotor group includes members of multiple families.Additionally, 15 extant species from other mammalian orderswere used to assess the feasibility of applying our results to abroader mammalian sample (Table A2). Specimens of 11 extinctbeaver species from three beaver subfamilies, as well as theextinct giant marmot Paenemarmota barbouri were also exam-ined in order to infer their locomotor behaviors (Table A3). Skel-etal specimens from extant and extinct species were examinedat the following institutions: Museum of Natural History of LosAngeles County, Los Angeles; Donald R. Dickey Collection ofthe University of California, Los Angeles; Museum of Verte-

brate Zoology, University of California, Berkeley; University ofCalifornia Museum of Paleontology, Berkeley; Idaho Museum ofNatural History, Pocatello; University of Kansas Natural His-tory Museum, Lawrence; Field Museum of Natural History,Chicago; American Museum of Natural History, New York;United States National Museum of Natural History, Washing-ton, D.C.; and Hagerman Fossil Beds National Monument,Hagerman.

A set of 20 osteological characteristics were measured to thenearest 0.01 mm using digital calipers (Chicago 12‘‘EDC). Meas-urements included lengths and midshaft diameters of the limbbones, as well as lengths of various functionally important mus-cular insertions (Fig. 1, Table 2). These measurements wereused to compute a set of 16 functional indices (ratios) indicativeof locomotor adaptations (Table 3); some indices were novel tothis study, whereas others were adopted from previous studies(Tarasoff, 1972; Hildebrand, 1985; Stein, 1988; Elissamburuand Vizcaıno, 2004). Indices represent overall limb proportionsand mechanical advantages of the primary muscles used inlocomotion.

Ratios are commonly used in biological studies because theyreflect functional and easy to understand features of organisms.However, the use of ratios can pose problems when used in sta-tistical analyses as they tend to violate assumptions of normal-ity and homoscedasticity included in parametric tests (Sokaland Rohlf, 1995). However, many studies have found the use ofratios in multivariate statistical analyses to be robust (Corruc-cini, 1987, 1995; Van Valkenburgh and Koepfli, 1993; Elissam-buru and Vizcaıno, 2004), and additional considerations can al-leviate some issues related to their use. Arcsine transformationof ratios prior to analysis can help restore normality to the dataset (Atkinson et al., 2004; Christiansen, 2006).

Rodents examined in this study include a wide range of bodysizes; thus, it is necessary to assess the influence of body sizeon structures being examined. One method that can be used toexamine the interaction between size and shape utilizes thegeometric mean (GM) (Jungers et al., 1995; Madar et al., 2002).The GM is a size variable derived from the nth root of the prod-uct of n measurements, and the ratio of any particular mea-surement to the GM is a Mosimann shape variable (Mosimannand James, 1979). Log transformed raw measurements wereregressed against log GM scores (as a proxy for body size) to an-alyze interspecific allometry, with equations in the form of:

log y ¼ logaþ b log x

where x, body size (log GM); y, measurement; a, y-intercept;and b, slope. Negative allometry was indicated by slopes signifi-cantly <1.0, positive allometry by slopes significantly >1.0, andisometry by slopes not significantly different from 1.0 (Schmidt-

TABLE 1. Locomotor categories used in the analyses and their definitions

Locomotor category Definition

Terrestrial (T) Rarely swims or climbs, may dig to make a burrow (but not extensively), may show saltatory behavior(quadrupedal only), never glides (e.g., rats and mice).

Semiaquatic (Sa) Regularly swims for dispersal, escape, or foraging (e.g., beavers and muskrats).Arboreal (A) Capable of and regularly seen climbing for escape, shelter, or foraging (includes scansorial species; e.g.,

tree squirrels and erethizontid porcupines).Semifossorial (Sf) Regularly digs to build burrows for shelter, but does not forage underground (e.g., ground squirrels).Fossorial (F) Regularly digs to build extensive burrows as shelter or for foraging underground (e.g., gophers and mole

rats). Display a predominantly subterranean existence.Ricochetal (R) Capable of jumping behavior characterized by simultaneous use of the hind limbs, commonly bipedal

(e.g., kangaroo rats).Gliding (G) Capable of gliding through the use of a patagium, commonly forage in and rarely leave trees (e.g., flying

squirrels).

Complete lists of extant species included and their classifications are in Appendix (Table A1 and A2. Species were assigned to cate-gories deemed most appropriate, but given that there is some gradation between categories, individual species may be capableof placement within more than one category (e.g., the European water vole is commonly known for swimming and burrowingbehaviors).

1388 J.X. SAMUELS AND B. VAN VALKENBURGH

Journal of Morphology

Nielsen, 1993; Read and Tolley, 1997). The null hypothesis of b5 1.0 was tested with a t-test in the form of:

ts ¼ ðb� 1Þ=SEb

where b, slope; SEb, standard error of the slope; d.f. 5 n 2 2,and a 5 0.05.

Another potential problem associated with using ratios isthat differences in the numerator, denominator, or both maylead to similar values for a ratio in functionally distinct forms(Emerson, 1985). For example, shortening a proximal segment(without a change to the distal segment) and elongating a distalsegment of a limb (without a change in the proximal segment)could produce equal values for an index but with distinctly dif-ferent consequences for the function of that limb. Regression oflog transformed raw variables (used to compute the indices)versus log GM scores can be used to examine the influence ofthe numerator or denominator on a functional index.

The functional indices and GM-transformed shape variableswere tested for significant differences among locomotor groupsusing multivariate analysis of variance (MANOVA) withScheffe’s F and Tamhane’s T2 procedures used for post hoc com-parisons. Stepwise discriminant function analysis (DFA) wasused to determine linear combinations of variables that maxi-mize separation among rodent taxa belonging to the seven loco-motor groups. This technique reviews and selects variables forinclusion in the model that contribute most to discriminationamong groups. These tests were chosen because they are com-monly used for these types of ecomorphological analyses andhave been effectively used to study the locomotor specializationsof other rodents (for examples see: Stein, 1988; Lessa and Stein,

1992; Elissamburu and Vizcaıno, 2004). Resultant discriminantfunctions were also used to classify the extinct rodent speciesinto locomotor groups; only seven of the surveyed extinct spe-cies were complete enough to be included in this step of theanalysis (Table A3). Bivariate plots and linear regressions wereused to visualize indices used in the analysis and aid in inter-pretation of extinct taxa represented by incomplete specimens.Statistical analyses were performed using SPSS 13.0.

In addition to extinct rodents, 15 extant species from othermammalian orders and the capybara, Hydrochaeris hydrochae-ris, were included as unknowns in the classification phase ofthe discriminant analysis. Capybaras were not included in con-struction of the model as they are known for both cursorial andsemiaquatic habits, in clear violation of the assumption ofmutually exclusive groups in DFA. Also, capybaras use all four-webbed feet when swimming, a fundamentally different modefrom hind limb or tail paddling locomotion seen in most othersemiaquatic rodents (Nowak, 1999). Nonrodent species werechosen to represent a diverse array of locomotor types, but stillconsistent with locomotor modes of the rodents studied (i.e.,semiaquatic and arboreal taxa were included, but not forelimbdominated swimmers or brachiators).

RESULTSAllometry

The possible influence of allometry was assessedusing regressions of log transformed measure-ments against the log GM score (a measure ofbody size) for each individual. Many characters inthe total sample of rodents displayed slight devia-tion from isometry, but these deviations did nottend to be significant at the P < 0.05 level (Table4). Femoral epicondylar breadth (FEB) showed asignificant positive allometry, whereas metatarsalthree length showed a significant negative allome-

TABLE 2. Osteological measurements used in the analyses

Measurement Abbreviation

Greatest length of the humerus HLMidshaft mediolateral diameter of the

humerusHMLD

Length of the deltopectoral crest DPCLEpicondylar breadth of the distal humerus HEBGreatest length of the radius RLFunctional length of the ulna FULMidshaft mediolateral diameter of the ulna UMLDLength of the olecranon process of the ulna ULOLGreatest length of metacarpal 3 MC3LGreatest length of proximal phalanx of digit

3 of manusMph3p

Greatest length of terminal phalanx of digit3 of manus

Mph3t

Greatest length of the femur FLMidshaft anteroposterior diameter of the

femurFAPD

Height of the greater trochanter of thefemur

FGT

Epicondylar breadth of the distal femur FEBGreatest length of the tibia TLMidshaft mediolateral diameter of the tibia TMLDLength of tibial tuberosity TSLGreatest length of metatarsal 3 MT3LGreatest length of terminal phalanx of digit

3 of pesPph3t

Measurements used are illustrated in Figure 1.

Fig. 1. Measurements of the limb skeleton used in thisstudy. HL, humerus length; HMLD, humerus mediolateral di-ameter, DPCL, deltopectoral crest length, HEB, humeral epi-condylar breadth, RL, radius length, FUL, functional ulnalength, UMLD, ulna mediolateral diameter, ULOL, olecranonprocess length, MC3L, metacarpal 3 length, Mph3p, manusdigit 3 proximal phalanx length, Mph3t, manus digit 3 terminalphalanx length, FL, femur length, FAPD, femur anteroposteriordiameter, FGT, greater trochanter height, FEB, femoral epicon-dylar breadth, TL, tibia length, TMLD, tibia mediolateral diam-eter, TSL, tibial tuberosity length, MT3L, metatarsal 3 length,Pph3t, pes digit 3 terminal phalanx length. Detailed descrip-tions of measurements are included in Table 2.

LOCOMOTOR ADAPTATIONS IN RODENTS 1389


TABLE 3. Morphological indices used in analyses, their definitions, and inferred functional significance

Index Definition

Shoulder moment index(SMI)

Deltopectoral crest length divided by functional length of the humerus (DPCL/HL). Indicatesmechanical advantage of the deltoid and pectoral muscles acting across the shoulder joint.

Brachial index (BI) Functional length of the radius divided by functional length of the humerus (RL/HL). Indicatesrelative proportions of proximal and distal elements of the forelimb.

Humeral robustnessindex (HRI)

Mediolateral diameter of humerus divided by functional length of the humerus (HMLD/HL).Indicates robustness of the humerus and its ability to resist bending and shearing stresses.

Humeral epicondylarindex (HEB)

Epicondylar breadth of humerus divided by functional length of the humerus (HEB/HL). Indicatesrelative area available for the origins of the forearm flexors, pronators, and supinators.

Olecranon length index(OLI)

Olecranon process length divided by functional length of the ulna (ULOL/FUL). Indicates relativemechanical advantage of the triceps brachii and dorsoepitrochlearis muscles used in elbowextension. This is identical to the index of fossorial ability used by Hildebrand (1985).

Ulnar robustness index(URI)

Mediolateral diameter of ulna divided by functional length of the ulna (UMLD/FUL). Indicatesrobustness of the ulna and its ability to resist bending and shearing stresses, and relative areaavailable for the origin and insertion of forearm and manus flexors, pronators, and supinators.

Manus proportions index(MANUS)

Manus digit 3 proximal phalanx length divided by metacarpal 3 length (Mph3p/MC3L). Indicatesrelative proportions of proximal and distal elements of the manus and size of the palmar surface.

Claw length index(CLAW)

Manus digit 3 terminal phalanx length divided by pes digit 3 terminal phalanx length (Mph3t/Pph3t). Indicates the size of manual claws relative to the pedal claws.

Crural index (CI) Functional length of the tibia divided by functional length of the femur (TL/FL). Indicates relativeproportions of proximal and distal elements of the hind limb.

Femoral robustnessindex (FRI)

Anteroposterior diameter of femur divided by functional length of the femur (FAPD/FL). Indicatesrobustness of the femur and its ability to resist bending and shearing stresses (AP diameter isused due to transverse expansion of the femora in semiaquatic rodents).

Gluteal index (GI) Length of distal extension of the greater trochanter of the femur divided by functional length ofthe femur (FGT/FL). Indicates relative mechanical advantage of the gluteal muscles used inretraction of the femur.

Femoral epicondylarindex (FEB)

Epicondylar breadth of femur divided by the functional length of the femur (FEB/FL). Indicatesrelative area available for the origins of the gastrocnemius and soleus muscles used in extensionof the knee and plantar-flexion of the pes.

Tibial robustness index(TRI)

Mediolateral diameter of tibia divided by functional length of the tibia (TMLD/TL). Indicatesrobustness of the tibia and its ability to resist bending and shearing stresses.

Tibial spine index (TSI) Length of distal extension of the tibial tuberosity (spine) divided by functional length of the tibia(TSL/TL). Indicates relative mechanical advantage of the hamstrings and biceps femoris musclesacting across the knee and hip joints.

Pes length index (PES) Metatarsal 3 length divided by functional length of the femur (MT3L/FL). Indicates relativeproportions of proximal and distal elements of the hind limb, and relative size of the hind foot.

Intermembral index (IM) Functional lengths of the humerus and radius divided by lengths of the femur and tibia [(HL 1RL)/(FL 1 TL)]. Indicates the length of the forelimb relative to the hind limb.

Measurements used to calculate indices are illustrated in Figure 1 and described in Table 2.

TABLE 4. Interspecific allometric relationships for log transformed measurements regressed against body size (log GM)

Measurement (y) a b r2 SEb b 5 1

Humerus length 0.546 1.045 0.940 0.066949 b 5 1Humerus ML diameter 20.542 1.120 0.931 0.077332 b 5 1Humeral epicondylar breadth 0.024 1.019 0.881 0.094720 b 5 1Deltopectoral crest length 0.180 1.073 0.912 0.084444 b 5 1Radius length 0.593 0.981 0.944 0.060563 b 5 1Functional ulna length 0.599 1.002 0.946 0.060779 b 5 1Ulna ML diameter 20.840 1.149 0.866 0.114637 b 5 1Olecranon process length 20.105 0.996 0.756 0.143061 b 5 1Metacarpal 3 length 0.057 0.962 0.891 0.085089 b 5 1Manus proximal phalanx length 20.141 0.942 0.819 0.112097 b 5 1Manus terminal phalanx length 20.247 1.102 0.726 0.171443 b 5 1Femur length 0.717 0.958 0.939 0.061814 b 5 1Femur AP diameter 20.426 1.060 0.966 0.050411 b 5 1Greater trochanter height 20.339 1.049 0.833 0.118864 b 5 1Femoral epicondylar breadth 20.089 1.103 0.961 0.056372 b > 1Tibia length 0.843 0.888 0.851 0.094253 b 5 1Tibia ML diameter 20.509 1.098 0.926 0.078398 b 5 1Tibial tuberosity length 0.313 1.021 0.948 0.060284 b 5 1Metatarsal 3 length 0.608 0.671 0.526 0.161235 b < 1Pes terminal phalanx length 20.178 1.018 0.916 0.078293 b 5 1

Regression equations were calculated in the form log y 5 log a 1 b log x; x, GM score, y, measurement, a, y intercept, b, slope, r2,correlation coefficient, SEb, standard error. Significant deviations from isometry (b 5 1) at the P � 0.05 level are indicated with b> 1 for positive allometry and b < 1 for negative allometry.



try. Absence of interspecific allometry is not sur-prising, as none of the species studied are particu-larly massive. Our findings are consistent with thepattern of geometric similarity commonly found insmaller mammals (Biewener, 1990). Over therange of body mass included in this study, changesin limb posture and muscle mechanical advantagescan compensate for increasing size without strongskeletal allometry.

Analysis of Variance

Mean values and standard deviations of func-tional indices were calculated for locomotor groups(Table 5, species averages for each index areincluded in Table A4). MANOVA found all groupsto be significantly different (P < 0.001). Differen-ces for individual indices were assessed by univari-ate ANOVAs with Scheffe’s F and Tamhane’s T2post hoc procedures.

Numerous significant differences in functionalindices distinguish locomotor groups (Table 5), andsome of these are summarized as follows. Semia-quatic rodents were characterized by relatively ro-bust limbs (high HRI and FRI), enlarged olecranonprocesses (high OLI), longer distal hind limb ele-ments (high CI and PES), and enlarged muscularattachments (high GI, FEB, and TSI). Arborealrodents display elongate humeri and hands (lowBI and high MANUS), equal claw lengths (CLAW5 1.0), and relatively equal limb lengths (IM closerto 1.0). Semifossorial rodents showed relativelyshort antebrachii (low BI), enlarged olecranonprocesses (high OLI), and enlarged manus termi-nal phalanges (CLAW >1.0). Fossorial rodents dis-play many significantly different features fromother groups, including: enlarged muscles orgreater mechanical advantages of muscles (highSMI, HEB, OLI, GI, FEB, TSI), robust limbs (highHRI, URI, and FRI), shortened antebrachii andhands (low BI and MANUS), dramaticallyenlarged manus terminal phalanges (CLAW >1.5on average), and equal limb lengths (IM close to1.0). Ricochetal rodents were characterized byshort humeri (high BI), elongate tibiae (high CI,low TRI and TSI), proximal muscle insertion (lowTSI), elongate hind foot (high PES), and dispropor-tionately long hind limbs (IM <0.5 on an average).Gliding rodents possess elongate, gracile limbs(low HRI, URI, FRI, and TRI, but also reflected inlow SMI, HEB, OLI, GI, FEB, and TSI), relativelyequal limb lengths (IM closer to 1.0), elongatehand (high MANUS), and equal claw lengths(CLAW 5 1.0).

DFA

Stepwise DFA was performed using the samedata set as previous analyses, with the addition ofextinct rodent taxa as unclassified cases. Thisanalysis was run separately for functional indices

TABLE

5.Mea

nvalues

andstandard

deviation

sof

indices

forea

chlocomotor

group

Vari

able

Ter

rest

rial;

n5

14,

53

Sem

iaqu

ati

c;n5

8,

38

Arb

orea

l;n5

9,

46

Sem

ifos

sori

al;

n5

9,

44

Fos

sori

al;

n5

16,

67

Ric

och

etal;

n5

7,

27

Gli

din

g;

n5

4,

18

SM

I0.4

16

(0.0

62)S

a,S

f,F,G

l0.4

74

(0.0

42)T

,F,G

l0.4

47

(0.0

39)F

,Gl

0.4

63

(0.0

54)T

,F,G

l0.5

39

(0.0

59)T

,Sa,A

,Sf,

R,G

l0.4

35

(0.0

50)F

,Gl

0.3

36

(0.0

33)T

,Sa,A

,Sf,

F,R

BI

0.9

96

(0.0

73)S

f,F

,R1.0

57

(0.0

82)A

,Sf,

F,R

0.9

30

(0.0

60)S

a,R

,Gl

0.9

21

(0.0

91)T

,Sa,R

,Gl

0.9

08

(0.0

86)T

,Sa,R

,Gl

1.2

45

(0.1

65)T

,Sa,A

,Sf,

F,G

l1.0

56

(0.0

63)A

,Sf,

F,R

HR

I0.0

86

(0.0

11)S

a,S

f,F

,Gl

0.1

05

(0.0

19)T

,A,S

f,G

l0.0

94

(0.0

13)S

a,F

,Gl

0.0

95

(0.0

08)T

,Sa,F

,Gl

0.1

12

(0.0

12)T

,A,S

f,R

,Gl

0.0

94

(0.0

06)F

,Gl

0.0

62

(0.0

04)T

,Sa,A

,Sf.

F,R

HE

B0.2

47

(0.0

26)S

a,S

f,F,R

,Gl

0.3

07

(0.0

68)T

,A,F

,Gl

0.2

71

(0.0

21)S

a,F

,R,G

l0.2

89

(0.0

46)

T,F

,Gl

0.3

68

(0.0

57)T

,Sa,A

,Sf,

R,G

l0.3

10

(0.0

19)T

,A,F

,Gl

0.1

70

(0.0

14)T

,Sa,A

,Sf,

F,R

OL

I0.1

62

(0.0

25)S

a,S

f,F,G

l0.2

20

(0.0

49)T

,A,F

,R,G

l0.1

48

(0.0

22)S

a,S

f,F

,Gl

0.2

19

(0.0

48)

T,S

a,F

,R,G

l0.3

28

(0.0

80)T

,Sa,A

,Sf,

R,G

l0.1

46

(0.0

16)S

a,S

f,F

0.0

92

(0.0

13)T

,Sa,A

,Sf,

F

UR

I0.0

43

(0.0

09)S

a,S

f,F,G

l0.0

57

(0.0

14)T

,A,F

,R,G

l0.0

46

(0.0

09)S

a,F

,Gl

0.0

55

(0.0

15)

Sa,F

,Gl

0.0

67

(0.0

15)T

,Sa,A

,Sf,

R,G

l0.0

45

(0.0

12)S

a,F

,Gl

0.0

21

(0.0

05)T

,Sa,A

,Sf,

F,R

MA

NU

S0.6

24

(0.0

72)A

,F,G

l0.5

73

(0.0

73)A

,Gl

0.7

33

(0.0

81)T

,Sa,S

f,F,R

,Gl

0.5

78

(0.0

79)

A,G

l0.5

24

(0.0

83)T

,A,R

,Gl

0.6

17

(0.0

86)A

,F,G

l0.8

86

(0.0

57)T

,Sa,A

,Sf,

F,R

CL

AW

0.7

89

(0.2

10)S

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and GM transformed data; however, results didnot differ appreciably and only the more easily cal-culated and interpreted indices are discussed here.

Fourteen of the 16 indices were included in thestepwise discriminant model (Tables 6–9). Asexpected from the great differences between someof the locomotor types examined, the DFA showedgood separation of groups and was significant(Wilks’ k 5 0.002, F(6,276) 5 36.461, P < 0.001).The analysis yielded three discriminant functions

with eigen values >1 which accounted for 90.7% oftotal variance.

The first discriminant function (DF1) accountedfor 54.1% of the variance and was positively corre-lated with intermembral index (IM) and negativelycorrelated with brachial index (BI), crural index(CI), and relative hind foot length (PES). Arboreal,fossorial, and gliding taxa tended to have positivescores for DF1 (Figs. 2 and 3) reflecting their tend-ency to use the limbs equally or show forelimbdominated locomotion, characterized by forelimbsand hind limbs of similar length (IM 5 1.0), aswell as nearly equally proportioned proximal anddistal limb segments (BI and CI). Ricochetal, semi-aquatic, and some terrestrial taxa showed negativescores for DF1 reflecting their hind limb domi-nated locomotion, characterized by elongate hindlimbs (low IM) and distally elongate hind limb ele-ments (high CI and PES). DF1 is not correlatedwith GM values (r2 5 0.000, P 5 0.99), and thus isnot influenced by body size.

The second discriminant function (DF2) ac-counted for 27.5% of variance and was positivelycorrelated with ulnar and humeral robustness(URI, HRI, HEB), olecranon length (OLI), andnegatively correlated with manus proportions(MANUS). Arboreal and gliding rodents showedstrongly negative DF2 scores (see Fig. 2) and bothshowed features that facilitate climbing, includingelongate humeri and ulnae (reflected in low HRI,HEB, and URI scores), distal elongation of themanus (high MANUS), and reduction of the olecra-non process (low OLI). Fossorial taxa had positivescores for DF2 and displayed relatively broadhumeral epicondyles (high HEB), large olecranonprocesses (high OLI), reduced manual phalanges(low MANUS), and robust forelimb elements (highHRI and URI). Some ricochetal rodents also hadpositive scores for DF2; this was primarily due to

TABLE 6. Discriminant analysis structure matrix, eigenvalues,and proportions of variance explained by each function

Discriminant function

1 2 3

SMI* 0.165 0.221 0.156BI 20.338 20.006 20.014HRI 0.090 0.394 0.243HEB 0.093 0.494 0.101OLI 0.290 0.534 0.249URI 0.131 0.367 0.189MANUS 20.055 20.514 20.330CLAW 0.227 0.237 20.145CI 20.515 0.110 0.257GI 0.028 0.242 0.053FRI* 0.105 0.284 0.297FEB 0.110 0.269 0.494TRI 0.128 0.162 0.237TSI 0.242 0.076 0.474PES 20.383 0.175 0.061IM 0.631 20.171 0.261Eigenvalue cumulative (%) 9.576 4.861 1.619Variance 54.1 81.5 90.7

Indices with asterisks were not included in the stepwise dis-criminant function model. Each index is defined in Table 3(SMI, shoulder moment index; BI, brachial index; HRI, humeralrobustness index; HEB, humeral epicondylar index; OLI, olecra-non length index; URI, ulnar robustness index; MANUS, manusproportions index; CLAW, claw length index; CI, crural index;FRI, femoral robustness index; GI, gluteal index; FEB, femoralepicondylar index; TRI, tibial robustness index; TSI, tibial spineindex; PES, pes length index; IM, intermembral index).

TABLE 7. Discriminant analysis classification matrix of rodent taxa

Observedgroup

%Correct

Predicted Group

Terrestrial SemiaquaticScansorial/

arboreal Semifossorial Fossorial Ricochetal Gliding

Original Terrestrial 86.8 46 0 0 7 0 0 0Semiaquatic 86.8 5 33 0 0 0 0 0Arboreal 97.8 0 0 45 1 0 0 0Semifossorial 90.9 1 0 2 40 1 0 0Fossorial 94.0 0 0 0 4 63 0 0Ricochetal 100.0 0 0 0 0 0 27 0Gliding 100.0 0 0 0 0 0 0 18Total 92.8 52 33 47 52 64 27 18

Crossvalidated Terrestrial 86.8 46 0 0 7 0 0 0Semiaquatic 86.8 5 33 0 0 0 0 0Arboreal 95.7 0 0 44 2 0 0 0Semifossorial 86.4 2 0 2 38 2 0 0Fossorial 94.0 0 0 0 4 63 0 0Ricochetal 100.0 0 0 0 0 0 27 0Gliding 100.0 0 0 0 0 0 0 18Total 91.8 53 33 46 51 65 27 18



shortening of the humerus (high HRI and HEB)and collinearity in the data set (some indices thatare correlated with DF2 were colinear with PESand IM). DF2 was not correlated with GM (r2 50.002, P 5 0.47), and is not influenced by bodysize.

The third discriminant function (DF3) accountedfor 9.1% of variance, and it was primarily associ-ated (and positively correlated) with FEB and tib-ial spine length (TSI). DF3 distinguished semia-quatic rodents, which tended to have positivescores for DF3, from all other groups (see Fig. 3).Hind limb paddling locomotion of semiaquaticrodents demands adaptations to minimize dragand maximize thrust, such as shortened femora(high FEB) and enlarged hind limb musculature(high FEB and TSI). DF3 scores tended to behigher in larger semiaquatic species, and DF3 isweakly correlated with GM (r2 5 0.067, P < 0.05).

The ability of the discriminant model to separatetaxa into locomotor groups was assessed using the

classification phase (in which individual specimensare excluded from creating the functions, but areinstead classified using models derived from theremaining specimens) (Table 7). This classificationshowed 92.8% correct classification of individuals,as well as 91.8% correct classification when caseswere cross-validated. Ricochetal and gliding groupshad 100% correct classification, and only the ter-restrial and semiaquatic groups had <90% correctclassification. Most misclassifications (11 of 21)were of individual specimens rather than entirespecies. However, three taxa were consistentlymisclassified by the discriminant analysis. All indi-viduals of the grasshopper mouse, Onychomys leu-cogaster, were misclassified by the discriminantanalysis as arboreal rather than terrestrial. Theterrestrial pacarana, Dinomys branickii, showedmisclassification of both individuals used in theanalysis as semifossorial. Arvicola terrestris, theEuropean water vole, had all 3 individuals mis-classified as terrestrial rather than semiaquatic.

TABLE 8. Discriminant analysis classification of other mammalian orders

SpeciesActualgroup

Most likelygroup P(D/G) P(G/D)

2nd mostlikely group

Hydrochaeris hydrochaeris T/Sa Sf 0.000 0.998 TCynocephalus volans G G 0.000 1.000 TMacropus robustus R Sa 0.000 1.000 RDasypus novemcinctus F F 0.000 0.999 SfCholoepus didactylus A T 0.000 0.930 FMephitis mephitis Sf Sf 0.011 0.998 FAilurus fulgens A A 0.004 0.836 SfProcyon lotor T T 0.086 0.512 SfMustela frenata T A 0.047 0.508 SfNeovison vison T Sf 0.004 0.748 TLontra canadensis Sa Sa 0.000 0.997 ATaxidea taxus Sf F 0.000 0.990 AGulo gulo T Sf 0.001 0.880 FPhoca vitulina A Sa 0.000 1.000 RSorex vagrans T F 0.106 0.994 SfScapanus townsendi F F 0.000 1.000 Sf

Nonrodent taxa were excluded from the discriminant model formation. Species classifications are indi-cated by: T, terrestrial; Sa, semiaquatic; A, scansorial/arboreal; Sf, semifossorial; F, fossorial; R, rico-chetal; Gl, gliding. P(D/G) represents the conditional probability of the observed discriminant functionscore, given membership in the most likely group. P(G/D) represents the posterior probability that acase belongs to the predicted group, given the sample used to create the discriminant model.

TABLE 9. Discriminant analysis classification of extinct taxa

SpeciesMost likely

group P(D/G) P(G/D)2nd Most

likely group

Castor californicusy Sa 0.010 1.000 ADipoides stirtoniy Sa 0.005 0.849 TProcastoroides idahoensisy Sa 0.001 1.000 ACastoroides ohioensisy Sa 0.000 1.000 TPalaeocastor fossory Sf 0.000 0.702 FPalaeocastor nebrascensisy Sf 0.000 0.504 FPaenemarmota barbouriy Sf 0.079 0.901 A

Species classifications are indicated by: T, terrestrial; Sa, semiaquatic; A, scansorial/arboreal; Sf, semi-fossorial; F, fossorial; R, ricochetal; Gl, gliding. P(D/G) represents the conditional probability of theobserved discriminant function score, given membership in the most likely group. P(G/D) representsthe posterior probability that a case belongs to the predicted group, given the sample used to createthe discriminant model. Extinct species are indicated by y.



The 16 extant species (1 rodent, 15 nonrodents)included in the classification phase of the analysisas unknowns produced mixed results (Table 8).Eight were correctly classified, but some of thosecorrectly classified showed discriminant functionscores well outside the range of the sample used tocreate the discriminant model. All 10 individualsof the cursorial and semiaquatic capybara, Hydro-chaeris hydrochaeris, were incorrectly classified assemifossorial.

Six extinct species from the Family Castoridaeand the extinct giant marmot Paenemarmota bar-bouri were complete enough to be included asungrouped cases in the classification phase of the

analysis (Table 9). Of these species, four were clas-sified as semiaquatic (Dipoides, Procastoroides,Castoroides, and Castor californicus) and threewere classified as semifossorial (Palaeocastornebrascensis, P. fossor, and Paenemarmota bar-bouri).

Classifications of some unknown taxa (bothextant and extinct) resulted in high posterior prob-abilities and low conditional probabilities (Tables 8and 9). This pattern can result from the unknowntaxon being closest to the centroid for a group(high posterior probability), but on the peripheryor outside of the observed morphospace for thatgroup (low conditional probability).

Fig. 2. Plot of first and second discriminant function scores for rodent locomotor types. Indi-vidual specimens for each species were collapsed to a single point. Numbers associated witheach point identify individual species (Table A1).



Bivariate Plots

To further examine morphological differencesbetween locomotor groups, bivariate plots, and lin-ear regressions are included for the nine indiceswith significant discriminating power. Bivariateplots include the linear measurements (log trans-formed) for each of the functional indices, plottedwith the numerator (on the y-axis) against the de-nominator (on the x-axis). Bivariate plots facilitatevisual comparisons across taxa and are especiallyuseful for inferring habits of extinct taxa, oftenrepresented by incomplete specimens.

Arboreal, semifossorial, and fossorial rodentshad lower BI scores than the other groups, with

values tending to be <1 (Table 5, Fig. 4a). Regres-sion of humerus and radius length versus the GMscores revealed low BI scores in arboreal rodentswere related to elongation of the humerus,whereas low scores in semifossorial and fossorialrodents were related to shortening of the ante-brachium. Fossorial species had significantlybroader humeral epicondyles than other rodentgroups (Table 5, Fig. 4b). The semiaquatic Castorand its extinct relatives also showed particularlylarge humeral epicondyles. Semifossorial and fos-sorial rodents had significantly longer olecranonprocesses than other rodents; however, manylarger semiaquatic rodents showed similar OLI

Fig. 3. Plot of first and third discriminant function scores for rodent locomotor types. Individ-ual specimens for each species were collapsed to a single point. Numbers associated with eachpoint identify individual species (Table A1).



scores (Table 5, Fig. 4c). Phalanges were elongaterelative to metacarpals in gliding and arborealrodents, resulting in greater scores for MANUSrelative to other groups (Table 5, Fig. 4d). MANUS

scores were particularly small in fossorial rodentsdue to distal shortening (Table 5, Fig. 4d). Curso-rial taxa would presumably show low scores forthis index due to elongation of the metacarpals,

Fig. 4. Log/log plot of ratio components, with the y-axis representing the numerator and the x-axis representing the denomina-tor. Units are in log (mm). Individual points represent the average for each species. Numbers associated with each point identifyindividual species (Table A1). Slopes for all regressions are significant at the P � 0.05 level. a: Brachial index (BI). Regression line:y 5 0.116 1 0.915 x. Standard error of the estimate 5 0.053. Correlation coefficient r 5 0.978. Dashed reference line representsequal proximal and distal proportions: y 5 0 1 1 x. b: Humeral epicondylar index (HEB). Regression line: y 5 20.439 1 0.928 x.Standard error of the estimate 5 0.109. Correlation coefficient r 5 0.920. c: Olecranon length index (OLI). Regression line: y 520.557 1 0.897 x. Standard error of the estimate 5 0.173. Correlation coefficient r 5 0.804. d: Manus proportions index (MANUS).Regression line: y 5 20.181 1 0.962 x. Standard error of the estimate 5 0.087. Correlation coefficient r 5 0.944. e: Crural index(CI). Regression line: y 5 0.157 1 0.943 x. Standard error of the estimate 5 0.060. Correlation coefficient r 5 0.969. Dashed refer-ence line represents equal proximal and distal proportions: y 5 0 1 1 x. f: Femoral epicondylar index (FEB). Regression line: y 520.782 1 1.067 x. Standard error of the estimate 5 0.099. Correlation coefficient r 5 0.937. g: Tibial spine index (TSI). Regressionline: y 5 20.434 1 1.014 x. Standard error of the estimate 5 0.097. Correlation coefficient r 5 0.931. h: Pes length index (PES).Regression line: y 5 0.053 1 0.734 x. Standard error of the estimate 5 0.142. Correlation coefficient r 5 0.791.



and thus using only this index to infer fossorialityis not recommended.

Ricochetal and semiaquatic rodents both had rel-atively high values for the CI, indicating relativelylonger distal elements (tibiae) than proximal ele-ments (femora) in the hind limbs (Table 5, Fig. 4e).The similar CI values in these groups were due tofundamentally different morphological features.Ricochetal rodents are characterized by elongationof both the femur and tibia relative to body size,with more pronounced tibial elongation (see Fig.5). On the other hand, semiaquatic rodents showed

shortening of the femur relative to body size andthe expected tibia length of a rodent their size.One exception to this is the African water rat, Col-omys goslingi, (no. 11 in Fig. 5a,b) which displayedthe expected femur length of a rodent its size, buta significantly lengthened tibia. Other rodentgroups studied tended to have more equally pro-portioned proximal and distal elements.

Because of pronounced shortening of the femuras well as enlarged femoral epicondyles semia-quatic and fossorial rodents had significantlygreater values for FEB than all other rodent

Figure 4. (Continued).



groups (Table 5, Figs. 4f and 5a). The length of thetibial tuberosity (spine) relative to tibia length wassignificantly shorter in ricochetal and glidingrodents than in all other groups, as a function oftibial elongation (Table 5, Figs. 4g and 5b). Highervalues for TSI were seen in large bodied semia-quatic rodents (e.g., Castor), some arboreal rodents(porcupines and South American climbing rats),and many fossorial taxa. Ricochetal rodents, andto a lesser degree semiaquatic rodents, had signifi-cantly greater relative pes lengths than the otherlocomotor groups (Table 5, Fig. 4h). Elongation ofthe limbs in gliding rodents separates them fromother groups; consequences of this can be seen inFigure 4b–d,f,g.

DISCUSSION

The average rodent is capable of an impressivearray of locomotor behaviors; they can climb, dig,and swim without extensive morphological special-izations. Nevertheless, many locomotor specialistshave evolved (often in independent lineages) andcan be found in nearly every terrestrial habitat.Distantly related rodents surveyed in this studythat have similar locomotor behaviors generallyshow convergent morphological characters. Differ-ences between the locomotor modes studied aregreat enough to readily discriminate groups andaccurately classify group membership.

Because the ultimate goal of this study is infer-ence of extinct species’ locomotor habits, the fol-

lowing discussion will begin by covering which fea-tures can be used to discriminate groups and howaccurately these features classify living species.Next, we discuss how each group of specializedrodents compares to their more generalized terres-trial counterparts and assess the functional impli-cations of group differences. Finally, we discusswhat application of these methods reveals aboutevolution in some extinct rodents.

Statistical Analyses

Both multivariate analyses and bivariate plotsrevealed correlations between skeletal form andlocomotor function in rodents. The DFA showedgood separation of locomotor groups studied here(Figs. 2 and 3). Discriminant function 1 separatedforelimb dominated arboreal, fossorial, and glidingtaxa from the hind limb dominated semiaquaticand ricochetal taxa. This function is primarilyrelated to limb proportions, including CI, IM, andrelative pes length. Discriminant function 2 sepa-rated climbers and gliders from more terrestrialand fossorial taxa. Manus proportions and robus-ticity of the limbs were linked to separation alongthis axis. Finally, discriminant function 3 sepa-rated semiaquatic rodents from all other groups.This separation was related to shortening of thefemur and enlarged attachments of hind limbmuscles.

Consistent differences between locomotor groupscan be used to accurately classify the locomotion of

Fig. 5. Log/log plots of femur and tibia lengths versus geometric mean scores. Individual points represent the average for eachspecies. Numbers associated with each point identify individual species (Table A1). a: Femur length. Regression line: y 5 0.717 1

0.958 x. Standard error of the estimate 5 0.062. Correlation coefficient r 5 0.969. b: Tibia length. Regression line: y 5 0.843 10.888 x. Standard error of the estimate 5 0.094. Correlation coefficient r 5 0.922.



individual species. The discriminant analysis accu-rately classified extant rodents, with 93% of indi-viduals correctly placed. The overall success of thediscriminant analysis in classifying living speciesshows this method can be applied to fossil rodentspecies with some confidence, but the systematicmisclassification of a few taxa suggests caution iswarranted.

Confounding factors such as dietary habits andconflicting specializations for different locomotormodes should be considered. Misclassification of allindividuals of the grasshopper mouse, Onychomysleucogaster, as arboreal is likely due to its special-ized carnivorous/insectivorous diet. Onychomysdisplays relatively large humeral epicondyles andelongate manual claws when compared withclosely related terrestrial species, which facilitatethe capture and grasping of prey. The terrestrialpacarana, Dinomys branickii, was misclassified assemifossorial, but this is not surprising as the spe-cies has been documented to both dig and climbreadily (Nowak, 1999). Arvicola terrestris, the Eu-ropean water vole, shows both aquatic and fosso-rial adaptations, but was classified as neither. Thismay be partly linked to the size of water voles, butan alternative explanation is that conflicting spe-cializations for aquatic and fossorial habits resultin the absence of pronounced skeletal specializa-tions in Arvicola.

The 10 individuals of the capybara, Hydrochae-ris hydrochaeris, included as unknowns in theDFA were classified as semifossorial based on theirlimb proportions and robusticity; as was also thecase in Elissamburu and Vizcaıno (2004). Althoughthey rarely dig, capybaras are very large rodentsthat both swim and run, using all four legs forboth activities. Capybaras swim via quadrupedalpaddling, the use of both the fore and hind limbsto displace water may explain their classificationas semifossorial. Limb proportions in capybarasdiffer from the more common hind limb paddlingsemiaquatic rodents included in the DFA in sev-eral ways. Capybaras have a greater BI value(� 1.3 compared to the average of 1.0 in hind limbpaddlers) and much smaller CI value (�1.0 com-pared to the average of 1.3 in hind limb paddlers),and relatively shorter metatarsals relative to fe-mur length (pes length index). Development oflarger muscles and muscle attachments wouldfacilitate propulsion of a large body mass on landand in the water. Relatively robust bones are alsoexpected, as a way to accommodate both largerbody mass and stresses placed on the bones duringrunning and swimming.

Members of other mammalian orders includedas unknowns in the DFA were correctly classifiedin half of the cases. The colugo, Cynocephalus vol-ans, was correctly classified as gliding and pos-sesses the elongate, gracile limbs characteristic ofrodent gliders. Cynocephalus does, however, dis-

play some differences from the gliding rodents; ithas relatively longer radii (higher BI), shorterproximal phalanges (lower manus proportionsindex), and more equally proportioned fore andhind limbs (IM � 1.0). Colugo hands have rela-tively shorter proximal phalanges and more elon-gate long middle phalanges than those of glidingrodents. The armadillo, Dasypus novemcinctus,and the mole, Scapanus townsendi, were both cor-rectly classified as fossorial, with features in Sca-panus similar to those of fossorial rodents, butenhanced to a greater degree. Mephitis mephitis,(the striped skunk), Ailurus fulgens (the redpanda), and Procyon lotor (the raccoon) were cor-rectly classified as semifossorial, arboreal, and ter-restrial, respectively. The river otter, Lontra cana-densis, and the harbor seal, Phoca vitulina, wereboth classified as semiaquatic; Phoca is fullyaquatic but extended the trend toward femoralshortening seen in semiaquatic taxa, resulting inparticularly high CI and femur epicondylar indexscores.

The macropodid marsupial, Macropus robustus,was classified as semiaquatic rather than ricoche-tal, but this species fell well outside the range ofdiscriminant function scores seen in rodents. Theprimary reason for this misclassification was thehigh femur epicondylar index score of Macropuscombined with its extremely high CI score (tibiamore than 2x femur length). The two-toed sloth,Choloepus didactylus, is very similar to the arbo-real rodents in most features studied; however, thesloth was misclassified as terrestrial due to itsrather low manus proportions index scores. Slothhands display particularly short proximal pha-langes and elongate middle phalanges, thus grasp-ing is still facilitated by elongate digits, butthrough modification of different elements(Mendel, 1985). Sloths are also atypical climbers inthat their body is suspended below the limbs(Adam, 1999). The mustelids Mustela frenata, Neo-vison vison, Taxidea taxus, and Gulo gulo were allmisclassified. However, positions of these taxa(plus the river otter Lontra) relative to oneanother in shape space are similar to those seen incomparable rodents. The terrestrial Mustela, Neo-vison, and Gulo had moderate scores, while thesemifossorial Taxidea had higher discriminantfunction 1 and discriminant function 2 scores, andthe semiaquatic Lontra had relatively lower dis-criminant function 1 and higher discriminant func-tion 3 scores. This suggests similar evolutionarytrajectories for locomotor specializations in thisfamily, but with a centroid displaced in shapespace relative to rodents. Similarly, the shrew,Sorex vagrans, was misclassified as fossorial dueto relatively high shoulder moment index, humeralepicondylar index, and olecranon length indexscores. Compared with the terrestrial shrew, thehighly fossorial mole, Scapanus, has far greater



values for each of these indices and fell well out-side the range of values observed for rodents.Ancestrally, eulipotyphlans and rodents likely hadvery different limb proportions, but these resultssuggest similar modifications of the limbs for acommon function.

Misclassifications observed here are likelyresults of the separate phylogenetic histories ofthese groups. Limb proportions of the ancestors ofdifferent mammalian orders were likely quite dif-ferent, and thus application of our methods toinfer the habits of a broader sample of nonrodentmammals is not recommended. Additionally, asmany features that distinguish locomotor groupsare not exclusive to a single group, inference oflocomotor habits from a single or a few measure-ments should only be done with great caution.However, the ability of this method to classifyrodents suggests that application to other extinctmammals based on members of their own familyor order may be similarly successful.

Functional Analyses of Locomotor ModesSemiaquatic. Semiaquatic mammals show an

array of specializations to life in an aquatic envi-ronment while maintaining the ability to disperseacross or acquire food on land. Adaptations to asemiaquatic existence include soft tissue featureslike a fusiform body shape, webbed feet, and val-vular ears and nostrils (Howell, 1930); however,such features are not typically preserved in fossils.The primary mode of swimming in rodents is drag-based and utilizes alternate paddling of the hindlimbs, which includes a power stroke and recoverystroke (Stein, 1988; Fish, 1994, 1996). This type oflocomotion is facilitated by increasing the length ofthe paddling limb, increasing the surface area ofthe paddle, increasing thrust produced by the mus-culature (which aids in the power stroke), as wellas minimizing drag produced by the paddling limbduring the recovery stroke.

Semiaquatic rodents in our study display rela-tively robust bones (high values of humeral andfemoral robustness index), relatively short femora(indicated by high values of CI, femur robustnessindex, and femur epicondylar index), enlargedmuscular attachments (olecranon length index,femoral epicondylar index, and tibial spine index),and elongate hind feet (pes length index) whencompared with other rodents (Table 5). The shorterfemur in semiaquatic species, which shows a nega-tive allometry with body size, reduces the mechan-ical advantage of muscles acting in the powerstroke, and simultaneously minimizes induceddrag by allowing the hind limb to be broughtcloser to the body in the recovery stroke (Stein,1988). Semiaquatic rodents tend to use water forrefuge rather than foraging (like many other sec-ondarily aquatic mammals), and thus the mainte-

nance of swimming speed through mechanicaladvantage of the limb retractors may be under lessselective pressure than improving swimming effi-ciency through reduction of drag.

Reduction of the in-lever for the primary limbretracting muscles (i.e., gluteals, biceps femoris,semimembranosus, and semitendinosus) may becompensated by increases in their size (femoralepicondylar index, tibial spine index; Fig. 4f,g) togenerate the necessary propulsive forces duringthe power stroke. Increased femoral epicondyle(femoral epicondylar index) size in semiaquatictaxa increases the area of insertion for the limbretracting semimembranosus and also increasesthe areas of origin for the gastrocnemius and sol-eus muscles, which act in plantar flexion of thepes and are important in the power stroke. The ex-tensor digitorum longus also originates on the lat-eral epicondyle of the femur; this muscle acts intoe extension and likely aids in maintaining theshape of the paddle. The relatively large tibial tu-berosity (tibial spine index) increases the area ofinsertion for the limb retracting semimembrano-sus, semitendinosus, and the pelvic head of thebiceps femoris.

In semiaquatic species, the long olecranon proc-ess (olecranon length index; Fig. 4c) indicates thepresence of relatively large triceps brachii muscles,which facilitate digging as well as use of the fore-limb in turning and keeping the head raised dur-ing swimming. The relatively long olecranon proc-ess may also be linked to the presence and rela-tively large size of the anconeus and flexor carpiulnaris muscles. The anconeus acts as a forelimbextensor and rotator and may aid in turning whileswimming or in the manipulation of aquatic vege-tation (Stein, 1988). The enlarged hind foot (peslength index; Fig. 4h) of semiaquatic rodentsincreases the length of the paddling limb and alsothe size of the paddle, thus increasing propulsiveforces produced during swimming.

The particularly large humeral epicondyles(humeral epicondylar index; Fig. 4b) of the beaver,Castor, may be related to their use of the forelimbsto handle wood and vegetation. Precise handling ofvegetation by beavers would be facilitated byenlarged carpal flexors, digital flexors, and forearmpronators and supinators. In beavers the medialepicondyle of the humerus is the origin of theflexor carpi ulnaris, flexor digitorum group, palma-ris longus, and pronator teres, whereas the lateralepicondyle is the origin of the supinator (Young,1937).

Our results are consistent with previous studieson rodents and other semiaquatic mammals andsuggest that, when compared with their terrestrialrelatives, semiaquatic taxa are relatively moremuscular and often exhibit skeletal differencesrelated to the size, shape, and proportion of thelimbs (Howell, 1930; Young, 1937; Tarasoff, 1972;



Stein, 1988; Korth, 1994). The extent to which theskeleton and musculature are modified relates tothe fraction of time spent in water and body size(Dunstone, 1979; Stein, 1988).

Larger size in swimming animals createsincreased profile drag (O’Keefe and Carrano,2005). While soft-tissue adaptations like fringehairs and webbed feet may be sufficient in smallrodents, larger species require greater specializa-tion. Thus, it is not surprising that Castor is thelargest hind limb paddling rodent and also dis-plays the most pronounced aquatic specializationsof any living rodent. The trend towards a shorterfemur in larger species minimizes induced drag,whereas larger hind limb muscles (femoral epicon-dylar index, tibial spine index) and a larger paddle(pes length index) increase propulsion during thepower stroke, all of which aid in overcoming thegreater drag associated with larger body size.

Consistent with their smaller size, other extantsemiaquatic rodent species possess some, but notall, of the skeletal features that characterize thegroup in general. For example, the tiny Europeanwater vole, Arvicola terrestris, (�160 g) has limbproportions (CI, pes length index) similar to terres-trial or fossorial rodents. The African water rat,Colomys goslingi, (�60 g) is similar to other swim-ming taxa in its CI, but this is due to tibial elonga-tion rather than femoral shortening (Fig. 5b).Nevertheless, the FEB (femoral epicondylar index)of Colomys is enlarged like other semiaquatictaxa. The unusual proportions of Colomys mayreflect the distinctive wading behavior it useswhen hunting (Kingdon, 1974; Kerbis Peterhansand Patterson, 1995).

Arboreal

Tree squirrels, climbing rats, vesper rats, anderethizontid porcupines are all good climbers, buthave been described as less structurally modifiedfor climbing than other arboreal mammals (Cart-mill, 1985). Nevertheless, arboreal rodents studiedhere consistently differed from the other groups ofrodents studied, particularly in features associatedwith grasping. Grasping is important for climbing(Wood Jones, 1953; Hildebrand, 1978; Van Valken-burgh, 1987), and the digits of arboreal rodentsare relatively elongate (manus proportions index,Fig. 4d). Arboreal rodents have relatively shortolecranon processes of the ulnae (olecranon lengthindex, Fig. 4c), which allows full extension at theelbow. They display relatively elongate humeri (BI,Fig. 4a), equally proportioned limbs (IM), andequal claw lengths (claw length index) on themanus and pes, as both fore and hind limbs areused in climbing. Major limb elements of arborealrodents are also relatively gracile (humeral, ulnar,femoral, and tibial robustness index values not sig-

nificantly different from terrestrial forms; Table 5).Our results are consistent with previous studieswhich found adaptations that facilitate climbing inrodents include overall joint mobility as well aselongate and gracile limbs (both proximally anddistally) (Thorington, 1972; Thorington and Hea-ney, 1981). However, joint mobility is largelyrelated to the shape of bone articular surfaces andthe structure of limb girdles (Cartmill, 1985), fea-tures not measured in this study. Many features inthe skeletons of arboreal rodents are consistentwith those seen in arboreal marsupials and treeshrews (Argot, 2001, 2002; Sargis, 2002a,b).

Semifossorial

Nearly all rodents do some digging and mostrodent families include species with fossorial hab-its, typically scratch-diggers. Scratch-diggingbehavior is typified by production of large forcesby the forelimbs; thus, shortening of the forelimb(reducing out-lever) and enlarged muscular attach-ments (increasing both in-lever and the area ofinsertion) both result in improved mechanicaladvantage for the primary digging muscles (Hilde-brand, 1985). In addition, enlarged terminal pha-langes with fast-growing claws on the manus areused in breakdown of the soil (Hildebrand, 1985;Stein, 2000). Consistent with previous observa-tions, the semifossorial rodents in this study dis-played relatively elongate claws on the manus(claw length index >1.0), shortened antebrachii(BI; Fig. 4a), and enlarged appendicular muscles.

Enlarged muscle attachments in semifossorialrodents include the deltopectoral crest of the hu-merus, epicondyles of the humerus, and olecranonprocess of the ulna (shoulder moment index, hum-eral epicondylar index, and olecranon lengthindex; Table 5). The deltopectoral crest acts as theinsertion for several groups of muscles importantfor digging, such as the deltoids which primarilyprotract the arm, the pectoral muscles which adductand retract the arm, and, in some species, the lat-issimus dorsi which retracts the arm. Digging isalso enhanced by enlarged humeral epicondyles,particularly the medial epicondyle which serves asthe origin of the manual flexors (flexor carpi radia-lis, palmaris longus, flexors digitorum profundis,and sublimis) and the forearm flexing pronatorteres (Holliger, 1916). The olecranon process actsas the insertion for the elbow extending dorsoepi-trochlearis, anconeus, and triceps brachii muscles,and the origin of the flexor carpi ulnaris (Lehmann,1963). Elbow extension, along with retraction ofthe arm, is the primary action used to break upand remove soil when most mammals dig. The me-chanical advantage of the gluteal muscles (glutealindex) of semifossorial rodents is also significantlygreater than that of their terrestrial relatives,



likely reflecting the importance of these muscles inresisting the tendency of the body to be pushedbackwards when digging. A recent study of squir-rels by Lagaria and Youlatos (2006) yielded similarresults to our own, with burrowing species show-ing significantly larger shoulder moment, epicon-dylar, and olecranon indices than their nonburrow-ing relatives. Skeletal modifications in semifosso-rial squirrels corresponded with extensivemuscular enlargement, including the humeralretractors, elbow extensors, forearm pronators andflexors, and manual and digital flexors (Lagariaand Youlatos, 2006).

Body size may have a significant impact on thedegree of specialization in semifossorial rodents.The small ground squirrels Ammospermophilus(�120 g) and Tamias (�75 g) are less specializedthan their larger relatives; however, their habit ofburrowing primarily for shelter and refuge meanstheir specializations need not be as extreme aslarger or more extensive burrowers. A similar pat-tern of more pronounced fossorial specializationsin larger squirrels was observed in Lagaria andYoulatos’ (2006) study.

Fossorial

Fossorial and subterranean rodent species areknown from five extant families (21 genera) andoccur on every continent except Australia and Ant-arctica (Stein, 2000). Not surprisingly, fossorialrodents exhibit many of the same adaptations seenin semifossorial species but to a greater degree(Lehmann, 1963; Hildebrand, 1985; Nevo, 1999).Like semifossorial rodents, fossorial rodents in thisstudy display elongate claws on the manus (aver-age claw length index >1.5), relatively short ante-brachii (BI), enlarged muscle attachments, includ-ing: the deltopectoral crest of the humerus, epicon-dyles of the humerus, and olecranon process of theulna (shoulder moment index, humeral epicondy-lar index, and olecranon length index). On an av-erage, the length of the deltopectoral crest in fosso-rial species is more than half the length of the hu-merus, and observed values are particularly highin subterranean species belonging to the Bathyer-gidae (mole-rats), Geomyidae (gophers), and Spala-cidae (blind mole-rats and zokors). The enlargedolecranon process increases the moment arm ofelbow extending muscles that insert there, includ-ing triceps brachii, dorsoepitrochlearis, and anco-neus. Moreover, the flexor carpi ulnaris, a power-ful flexor of the manus and the largest muscle inthe forelimb of gophers (Lehmann, 1963) origi-nates from the olecranon as well (Holliger, 1916)along with a second head of the flexor digitorumprofundis in some fossorial rodents (Gambaryanand Gasc, 1993). Relative olecranon lengths for theblind mole-rats Spalax and Nannospalax are

extremely high, with the process making up morethan 1/3 of the total length of the ulna. Finally,like semifossorial rodents, fossorial species alsodisplay high gluteal index, femoral epicondylarindex, and tibial spine index scores, related toenlarged gluteus medius, gastrocnemius, and ham-strings, respectively, all of which act to resist thetendency to move rearward when digging. Sincebodies of fossorial rodents tend to be pushed back-wards as soil resists digging, enlargement of thesemuscles aids in maintaining a stable position.

Relative to semifossorial rodents, the femur andtibia of fossorial species are reduced (Fig. 5a,b),and the forelimbs and hind limbs are more similarin length (IM). Fossorial rodents also show rela-tively more robust bones (humeral robustnessindex, ulnar robustness index, femoral robustnessindex; Table 5) that accommodate the compressiveand torsional stresses placed on the limbs whendigging (Holliger, 1916; Biknevicius, 1993; Stein,1993). Additionally, other than the claws and pisi-form (the insertion of the manual flexors) the man-ual bones of subterranean rodents are extremelyreduced (Stein, 2000). Distal shortening of themanus (manus proportions index; Fig. 4d) reducesthe out-lever and improves the mechanical advant-age of muscles employed during digging.

It is noteworthy that in contrast to other fosso-rial mammals, fossorial rodents display three dis-tinct digging modes: scratch, chisel-tooth, andhead-lift digging, which are evident in their skele-tal morphology. The two latter modes of diggingare primarily seen in highly fossorial, subterra-nean rodents with shortened, robust limbs andfeet, and modifications of the skull and jaws(Nevo, 1995; Stein, 2000). The primarily chisel-tooth digging bathyergids Cryptomys, Heliopho-bius, and Heterocephalus all show comparativelylow humeral epicondylar index and claw lengthindex scores compared to other fossorial rodents(Fig. 4c,f), reflecting the fact that teeth ratherthan claws are used to break up soil. However, thepresence of other fossorial adaptations in the post-cranial skeleton of these rodents (high shouldermoment index and olecranon length index) indi-cates the conserved role of the forelimbs and over-all limb proportions in removing soil when digging,regardless of digging mode. Chisel-tooth diggingbathyergids and spalacids also show enlarged hindlimb muscle attachments (femoral epicondylarindex, tibial spine index) to aid in resisting thetendency of the body to be pushed backwardswhile digging. Head-lift diggers show many fea-tures seen in chisel-tooth diggers, but as men-tioned previously the relative size of the olecranonprocesses in the blind mole rats Spalax and Nan-nospalax exceeds that of any other rodents. Thesehead-lift diggers also possess shoulder momentindex and ulnar robustness index values greaterthan any other rodents in the analysis.



Ricochetal

Many rodents show saltatory behavior, includingboth quadrupedal hoppers and complete bipeds.The most strongly adapted saltatory rodentsinclude kangaroo rats, jerboas, and springharesthat display ricochetal behavior, whereby the hindlimbs are used simultaneously to produce a jump.Previous research revealed saltatory and ricochetalanimals tend to show longer, more gracile hindlimbs, larger hind limb muscles, and reorientationof these muscles to improve velocity of the limbsduring the propulsive stroke (Hatt, 1932; Howell,1933; Gambaryan, 1974; Alexander et al., 1981;Emerson, 1985). Ricochetal rodents included inthis analysis are characterized by having hindlimbs much longer than their forelimbs (IM) dueto both elongation of the tibia (CI; Fig. 5b) andhind foot (pes length index; Fig. 4h). When com-bined, these result in hind limbs being more thantwice the length of the forelimbs on average (IM <0.5). Low tibial spine index scores in ricochetalrodents are related to a more proximal insertion ofthe semimembranosus and semitendinosus muscles,which decreases the in-lever of hind limb retractingmuscles and hence increases the speed of rotationabout the knee. Elongation of the tibia and pespaired with proximal insertion of muscles andreduction in digit number (seen in Dipodidae) all actto increase the speed of hind limb rotation and jumplength. The enlarged greater trochanter of the fe-mur (seen in high gluteal index scores, despite hav-ing relatively elongate femora) increases the me-chanical advantage for the gluteus medius, whichpowers hind limb retraction vital for jumping behav-ior. Saltatory rodents, like the quadrupedal jumpingmice Zapus and Napaeozapus, are similar to rico-chetal species in the structure and proportions ofthe limbs, but their adaptations are not as extreme.

Gliding

Gliding is seen in four extant families of mam-mals, including two families of rodents (Anomalur-idae and Sciuridae). These animals possess skinfolds (patagia) extending from the body to thelimbs that act as airfoils to provide the lift neces-sary for gliding. Previous work indicates that glid-ing mammals have an elongate appendicular skel-eton, especially proximally (humeri and femora)when compared with nongliders (Norberg, 1985;Casinos, 1994; Runestad and Ruff, 1995). Glidingsquirrels possess elongate antebrachii and showan increase in the ratio of forelimb to hind limblength when compared to similar sized tree squir-rels (Thorington and Heaney, 1981). These fea-tures increase the aspect ratio, reduce induceddrag, and reduce load on the patagium.

Gliding rodents in this study display relativelyelongate and gracile bones (low humeral, ulnar,

femoral, and tibial robustness index scores; Table5), features that increase the size of and decreaseloading on the patagium. Elongation of these boneswithout enlargement of many muscle attachmentsresults in low scores for many of the musclemoment arms (shoulder moment index, humeralepicondylar index, olecranon length index, glutealindex, femoral epicondylar index, tibial spineindex), with the consequence of reduced power andimproved ranges of motion for these muscles.These findings are similar to a recent study of fly-ing squirrels by Thorington et al. (2005); theyfound relatively smaller deltopectoral crests, hum-eral epicondyles, and olecranon processes in glid-ing squirrels. More proximal insertion of the pec-toralis and deltoid muscles in gliders would reducepower and increase mobility of the shoulder joint,whereas a smaller olecranon would allow fullextension of the elbow. When compared with mostnongliding rodents, the gliders studied also showrelatively elongate distal limb segments (high BI,CI; Fig. 4a,e).

As climbing is fundamentally linked to glidingbehavior in rodents, manual digits in glidingrodents are relatively elongate (manus proportionsindex; Fig. 4d) to improve grasping. Glidingrodents also display relatively equally propor-tioned limbs and equal claw lengths on the manusand pes (claw length index), similar to the arborealgroup. Both flying squirrels (Family Sciuridae)and scaly tailed flying squirrels (Family Anomalur-idae) show nearly identical postcranial propor-tions, despite their independent evolution. Skeletalfeatures of gliding rodents do not display signifi-cant allometric changes, suggesting larger glidersshould have significantly greater loading on theirpatagium (since area and mass show a 2/3 scalingfactor).

Application to Extinct Taxa

Past researchers have inferred dietary and loco-motor habits of some extinct rodent species,largely on the basis of qualitative assessment ofmorphologic characteristics, phylogenetic affinities,and depositional environments. For example,members of the Family Castoridae vary in limbmorphology and inferences concerning extinct cas-torid locomotion have been postulated for some ofthe nearly 50 species represented in the fossil re-cord, but detailed quantitative comparisons of loco-motor characteristics within and between subfami-lies are lacking. Hypothesized locomotor habitsfrom previous research on the Castoridae dividethe group into two specialized classes: fossorial/subterranean adapted beavers (Palaeocastorinaeand Migmacastorinae) and semiaquatic adaptedbeavers (Castorinae and Castoroidinae) (Martin,1987; Korth, 1994; Hugueney and Escuillie, 1995,1996; Korth, 2002; Korth and Rybczynski, 2003).



Although some authors propose members of thegiant beaver lineage (Castoroidinae) show adapta-tions to semiaquatic locomotion, others suggestthat they possess a relatively unspecialized skele-ton (Zakrzewski, 1969; Kurten and Anderson,1980; Korth, 2002).

Castoroidines increased in size over time tobecome some of the largest known rodents(Schreuder, 1929; Shotwell, 1970; Korth, 1994).There is evidence of tree-cutting behavior in onegenus (Dipoides); the cut wood is similar to that ofthe modern beaver, Castor, but has significantlynarrower cut marks suggesting different cuttingcapabilities between species (Tedford and Haring-ton, 2003; Rybczynski, 2007). Only one beaver ge-nus, Castor, survives at present, but at times inthe Tertiary Castor co-occurred with members ofthe giant beaver lineage including: Dipoides, Pro-castoroides, and Castoroides in North America andTrogontherium in Eurasia. If these giant beaversshow both tree-cutting and semiaquatic behaviors,then competition and consequent niche partition-ing may have occurred. Living beavers are consid-ered keystone species for forest and waterway eco-systems throughout their range (Naiman et al.,1986; Wright and Jones, 2002; Muller-Schwarze andSun, 2003). Presumably the influence of beavers onecosystems and community structure would havebeen similar in the past, thus understanding theirevolution and inferring their behaviors would aid ininterpretation of their paleoecological roles.

The combination of bivariate plots and DFAused here allows inference and interpretation ofthe extinct rodent ecologies. Bivariate plots allowthe examination of morphological similaritiesbetween living rodents and potentially incompletefossil specimens. When more complete specimensare available, discriminant analysis allows compar-isons of overall similarity in locomotor morphologyseen in extant and extinct rodents. The three earlyMiocene beavers from the Palaeocastorinaeincluded in the analysis show adaptations sugges-tive of fossoriality. Pseudopalaeocastor barbouri,Palaeocastor nebrascensis, and Palaeocastor fossorall show large deltopectoral crests, humeral epi-condyles, and olecranon processes. Limb propor-tions in the palaeocastorine beavers are similar tothose of semifossorial and fossorial rodents; theyhave BI, CI, and pes length index scores very dif-ferent from living beavers (Fig. 4a,e,h). The twospecies of Palaeocastor have scores for their fosso-rial ability and claw length indices similar tohighly fossorial gophers and mole-rats; however,the IM scores are more similar to semifossorialrodents. Palaeocastor nebrascensis and P. fossorwere complete enough for inclusion in the discrim-inant analysis, which classified them as semifosso-rial. A semifossorial way of life is consistent withartifactual evidence of digging and fossilized bur-row systems associated with these beavers, whichare similar to extant prairie dog towns (Martinand Bennett, 1977).

Fig. 6. Plots of selected functional indices versus the log geometric mean (a proxy for body size). Symbols represent locomotortypes. Individual specimens for each species were collapsed to a single point. Numbers associated with each point identify individ-ual species (Table A1). a: Femur Epicondylar Index. Total fit line (Solid): y 5 14.643 1 7.821 x. Correlation coefficient r 5 0.164.Semiaquatic fit line (Dashed): y 5 10.398 1 15.704 x. Correlation coefficient r 5 0.814, P < 0.001. b: Tibial Spine Index. Total fitline (Solid): y 5 28.745 1 12.358 x. Correlation coefficient r 5 0.137. Semiaquatic fit line (Dashed): y 5 18.289 1 25.028 x. Correla-tion coefficient r 5 0.670, P < 0.001.



The discriminant analysis places the PlioceneCastor californicus within the semiaquatic group;this is reassuring as the early North AmericanCastor is essentially indistinguishable in postcra-nial morphology from the living species. Thebivariate plots confirm the overall similarity ofthese two species, with Castor californicus slightlylarger than Castor canadensis but both specieshaving the same limb proportions (Zakrzewski,1969).

All of the castoroidine beavers studied show fea-tures characteristic of semiaquatic habits. Theearly members of this lineage in North America,Monosaulax pansus and Eucastor tortus, show dis-tinctive semiaquatic adaptations and later mem-bers of the lineage show markedly shortened fem-ora (Figs. 4e and 5a), broad femoral epicondyles(Fig. 4f), and elongate hind feet (Fig. 4h). Dipoides,Procastoroides, and Castoroides were completeenough to be included in the discriminant analysis,which classified them as semiaquatic. These spe-cies have been considered a progressive lineageshowing increasing body size from the Miocene toPleistocene. The massive size (likely 50–100 kg) ofthe two North American giant beavers Procastor-oides and Castoroides, as well as the independentlylarge European giant beaver Trogontherium wouldresult in very high profile drag when swimming.Corresponding with the increase in size of the line-age through time there is an increase in aquaticspecialization. This increased specialization can beseen in the castoroidine discriminant functionscores, where Procastoroides and Castoroides havehigher discriminant function 1 and discriminantfunction 3 scores than Castor (the most aquatic liv-ing rodent). For the total rodent sample both femo-ral epicondylar index and tibial spine index showvery low correlations with body size, but in semia-quatic rodents the scores for these indices show adramatic increase with increasing body size (Fig.6a,b). There is significant positive allometry inFEB (Table 4), but the higher femoral epicondylarindex scores in semiaquatic species are primarilydue to a strong negative allometry in femur length(a trend that is not seen in other rodent groups).

Castor co-occurs repeatedly with other beaverspecies from the subfamily Castoroidinae in thefossil record of North America and Eurasia, raisingthe question of how these beavers avoided competi-tion. Although the hind limbs are similar, thereare important differences between the swimmingapparatus of these beavers. The broad, flattenedtail characteristic of Castor allows the undulatorymode of swimming that beavers use when sub-merged. Undulation of the tail and simultaneouspaddling of the hind feet when submerged resultin a significantly lower cost of transport for livingbeavers than for other semiaquatic mammals(Allers and Culik, 1997; Fish, 2000). Members ofthe Castoroidinae lack this specialized tail, and

thus the increased specialization of their limbswith size is likely more necessary for efficientswimming than in Castor.

Like the extant Castor, members of the Castoroi-dinae display particularly large humeral epicon-dyles (Fig. 4b). The medial and lateral epicondylesof the humerus are the origin of the carpal flexors,digital flexors, and forearm pronators and supina-tors in living beavers (Young, 1937). The enlargedepicondyles of the castoroidines suggest that, likeCastor, they may have used their forelimbs to han-dle wood and vegetation, consistent with the infer-ence of tree-cutting and lodge or dam buildingbehavior in these beavers (Tedford and Harring-ton, 2003; Rybczynski, 2007).

The presence of semiaquatic adaptations in theCastoroidinae paired with the tree-cutting behav-ior seen in Dipoides (Tedford and Harrington,2003; Rybczynski, 2007) suggest that these beaverslikely competed with Castor for food. The molarsof both lineages are lophodont and show high hyp-sodonty indices, with fully hypselodont teeth seenin all of the castoroidine beavers examined here(Stirton, 1935; Stirton, 1947). Differences in cut-ting performance and incisor structure suggest thetypes of trees favored by Castor and castoroidinebeavers may have been quite different (Rybc-zynski, 2007), potentially allowing for niche parti-tioning. Competition between the two lineagesmay have led to the progressive increase in bodysize seen in the Castoroidinae after the arrival ofCastor in North America around the Hemphillian.

The giant marmot, Paenemarmota barbouri,which is known only from the Pliocene, displaysmany features consistent with semifossorial habitsand it was classified as semifossorial in the dis-criminant analysis. Paenemarmota shows rela-tively large deltopectoral crests, humeral epicon-dyles, olecranon processes, and large manual ter-minal phalanges. Paenemarmota may have thusfilled a similar ecological role to living marmotsand ground squirrels; however its large size (simi-lar to that of a living beaver, see Fig. 6) may be ofconsequence to its ecology. Living marmots andground squirrels are commonly preyed upon bybadgers, wolves, coyotes, and birds of prey(Nowak, 1999). The large size of Paenemarmotawould require significantly larger burrow diame-ters than are seen for any living rodent, a factthat would make them vulnerable to predation bylarger carnivores. Repenning (1962) suggested thatthe co-occurrence of Paenemarmota and the hyae-nid Chasmaporthetes in Pliocene faunas may indi-cate the giant marmots were their preferred prey.If Paenemarmota were social, as are extant mar-mots and ground squirrels, it would potentiallyrepresent large and relatively easy to subdue preyfor many of the large carnivores in the Pliocene,including saber-toothed cats, borophagine dogs,and the previously mentioned hyaenids.



The combination of morphometric techniquesused here reveal how appendicular skeletal mor-phology reflects locomotor habits in rodents. Appli-cation of these methods allows objective inferenceand interpretation of locomotor habits of extinctrodents. Species with similar locomotor habits andmorphology from multiple rodent families provideseveral examples for the study of convergent evo-lution. Our future work will use these methods tofurther examine the evolution of locomotor adapta-tions in rodents and better reconstruct the roles ofrodents in past ecosystems.

ACKNOWLEDGMENTS

The following curators and collection managerskindly allowed access to specimens in their care: J.Dines and X. Wang (Museum of Natural History ofLos Angeles County), K. Molina (Donald R. DickeyCollection of the University of California, LosAngeles), E. Lacey (Museum of Vertebrate Zoology,University of California, Berkeley), P. Holroyd(University of California Museum of Paleontology,Berkeley), W. Akersten and M. Thompson (IdahoMuseum of Natural History), L. Martin (Univer-sity of Kansas), B. Simpson and M. Schulenburg(Field Museum of Natural History), D. Dively(American Museum of Natural History), R. Emry(National Museum of Natural History), and P.Gensler (Hagerman Fossil Beds National Monu-ment). Discussion with and comments by J.Meachen-Samuels, P. Adam, K. Koepfli, X. Wang,and D. Jacobs greatly improved this paper. Theauthors appreciate the editorial assistance of F.Harrison and thank him and an anonymousreviewer for their helpful comments on an earlierversion of this manuscript.

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APPENDIX

TABLE A1. Extant rodent species used in analysis of locomotor characteristics

ID. no. Species Common name Family n Locomotion type

1 Allactaga hotsoni Hotson’s jerboa Dipodidae 4 Ricochetal2 Ammospermophilus

leucurusWhite-tailed antelope

squirrelSciuridae 6 Semifossorial

3 Anomalurus pelii Pel’s scaly-tailedsquirrel

Anomaluridae 1 Gliding

4 Aplodontia rufa Mountain beaver Aplodontidae 8 Fossorial5 Arvicola terrestris

italicusEuropean water vole Cricetidae 3 Semiaquatic

6 Cannomys badius Lesser bamboo rat Spalacidae 1 Fossorial7 Castor canadensis American beaver Castoridae 10 Semiaquatic8 Chelemys macronyx Andean long-clawed

mouseCricetidae 5 Semifossorial

9 Clethrionomys(Myodes) californicus

Western red-backedvole

Cricetidae 5 Terrestrial

10 Coendou prehensalis Brazilian porcupine Erethizontidae 4 Arboreal11 Colomys goslingi African water rat Muridae 5 Semiaquatic12 Cryptomys hottentotes African mole-rat Bathyergidae 3 Fossorial13 Cynomys gunnisoni Gunnison’s prairie dog Sciuridae 6 Semifossorial14 Dinomys branickii Pacarana Dinomyidae 2 Terrestrial15 Dipodomys deserti Desert kangaroo rat Heteromyidae 4 Ricochetal16 Dipodomys merriami Merriam’s kangaroo rat Heteromyidae 5 Ricochetal17 Dipus (Jaculus)

aegypticusNorthern three-toed

jerboaDipodidae 2 Ricochetal

18 Erethizon dorsatum American porcupine Erethizontidae 6 Arboreal19 Geomys bursarius Plains pocket gopher Geomyidae 6 Fossorial20 Georychus capensis Cape mole-rat Bathyergidae 1 Fossorial21 Geoxus valdivianus Long-clawed mole

mouseCricetidae 7 Fossorial

22 Glaucomys sabrinus Northern flyingsquirrel

Sciuridae 7 Gliding

23 Heliophobiusargenteocinereus

Silvery mole-rat Bathyergidae 2 Fossorial

24 Heterocephalus glaber Naked mole-rat Bathyergidae 4 Fossorial25 Hydromys chrysogaster Golden-bellied water

ratMuridae 2 Semiaquatic

26 Hylopetes nigripes Palawan flyingsquirrel

Sciuridae 6 Gliding

27 Hyomys goliath Eastern white-earedgiant rat

Muridae 2 Terrestrial

28 Hystrix cristata Crested porcupine Hystricidae 5 Semifossorial29 Jaculus orientalis Greater Egyptian

jerboaDipodidae 4 Ricochetal

30 Marmota flaviventris Yellow-bellied marmot Sciuridae 5 Semifossorial31 Microtus californicus Meadow Vole Cricetidae 5 Terrestrial32 Myocastor coypus Nutria Echimyidae 6 Semiaquatic33 Nannospalax (Spalax)

leucodonLesser mole rat Spalacidae 4 Fossorial

34 Napaeozapus insignis Woodland-jumpingmouse

Dipodidae 2 Terrestrial

35 Nectomys squamipes South American waterrat

Cricetidae 3 Semiaquatic

36 Neofiber alleni Round-tailed muskrat Cricetidae 4 Semiaquatic37 Neotoma cinerea Bushy-tailed woodrat Cricetidae 7 Terrestrial38 Nyctomys sumichrasti Vesper rat Cricetidae 2 Arboreal39 Ondatra zibethicus Muskrat Cricetidae 5 Semiaquatic40 Onychomys leucogaster Northern grasshopper

mouseCricetidae 5 Terrestrial

41 Orthogeomys grandis Giant pocket gopher Geomyidae 5 Fossorial42 Oxymycterus

dasytrichusBurrowing mouse Cricetidae 3 Semifossorial

43 Pappogeomys(Cratogeomys)tylorhinus

Naked-nosed pocketgopher

Geomyidae 4 Fossorial

44 Paraxerus cepapi Smith’s bush squirrel Sciuridae 5 Arboreal45 Pedetes capensis Springhare Pedetidae 5 Ricochetal46 Perognathus parvus Great Basin pocket

mouseHeteromyidae 5 Terrestrial



TABLE A1. (Continued).

ID. no. Species Common name Family n Locomotion type

47 Peromyscusmaniculatus

Deer mouse Cricetidae 5 Terrestrial

48 Petaurista petaurista Red giant flying quirrel Sciuridae 4 Gliding49 Phloeomys pallidus Northern Luzon giant

cloud ratMuridae 3 Terrestrial

50 Pygeretmus pumilio Dwarf fat-tailed jerboa Dipodidae 3 Ricochetal51 Rattus norvegicus Norway rat Muridae 5 Terrestrial52 Rattus rattus Black rat Muridae 5 Terrestrial53 Ratufa affinis Pale giant squirrel Sciuridae 5 Arboreal54 Rhizomys pruinosus Hoary bamboo rat Spalacidae 6 Fossorial55 Sciurus niger Fox squirrel Sciuridae 6 Arboreal56 Sigmodon hispidus Hispid rice rat Cricetidae 5 Terrestrial57 Spalax giganteus Giant mole rat Spalacidae 1 Fossorial58 Spermophilus beecheyi California ground


59 Sphiggurus mexicanus Mexican hairy dwarfporcupine

Erethizontidae 4 Arboreal

60 Tachyoryctes splendens East African mole rat Spalacidae 4 Fossorial61 Tamias palmeri Palmer’s chipmunk Sciuridae 6 Semifossorial62 Tamiasciurus

hudsonicusRed squirrel Sciuridae 5 Arboreal

63 Thomomys bottae Botta’s pocket gopher Geomyidae 6 Fossorial64 Tylomys nudicaudus Peter’s climbing rat Cricetidae 4 Arboreal65 Xerus inauris South African ground


66 Zapus princeps Western jumpingmouse

Dipodidae 2 Terrestrial

67 Zygogeomys trichopus Michoacan pocketgopher

Geomyidae 5 Fossorial

TABLE A2. Extant species used in analyses of unknown locomotor types, and in application to other mammalian orders

ID no. Species Common name Family n Locomotion type

68 Ailurus fulgens Red panda Ailuridae 1 Arboreal69 Choleopus didactylus Southern two-toed sloth Megalonychidae 1 Arboreal70 Cynocephalus volans Philippine colugo Cynocephalidae 1 Gliding71 Dasypus novemcinctus Nine-banded armadillo Dasypodidae 1 Fossorial72 Gulo gulo Wolverine Mustelidae 1 Terrestrial (Scansorial)73 Hydrochaeris

hydrochaerisCapybara Caviidae 10 Cursorial/Semiaquatic

74 Lontra canadensis Northern river otter Mustelidae 1 Semiaquatic75 Macropus robustus Hill wallaroo Macropodidae 1 Ricochetal76 Mephitis mephitis Striped skunk Mephitidae 1 Semifossorial77 Mustela frenata Long-tailed weasel Mustelidae 1 Terrestrial78 Neovison vison American mink Mustelidae 1 Terrestiral/Semiaquatic79 Phoca vitulina Harbor seal Phocidae 1 Aquatic80 Procyon lotor Raccoon Procyonidae 1 Terrestrial/Scansorial81 Scapanus townsendi Townsend’s mole Talpidae 1 Fossorial82 Sorex vagrans Vagrant shrew Soricidae 1 Terrestrial83 Taxidea taxus American badger Mustelidae 1 Semifossorial

TABLE A3. Extinct rodent species used in the analyses

ID no. Species Family/subfamily Age of specimens used

84 Castor californicusy Castoridae/Castorinae Pliocene85 Castoroides ohioensisy Castoridae/Castoroidinae Pleistocene86 Dipoides stirtoniy Castoridae/Castoroidinae Miocene87 Paenemarmota barbouriy Sciuridae/Marmotinae Pliocene88 Palaeocastor fossory Castoridae/Palaeocastorinae Miocene89 Palaeocastor nebrascensisy Castoridae/Palaeocastorinae Miocene90 Procastoroides idahoensisy Castoridae/Castoroidinae Pliocene-Pleistocene91 Pseudopalaeocastor barbouriy Castoridae/Palaeocastorinae Miocene92 Monosaulax pansusy Castoridae/Castoroidinae Miocene93 Eucastor tortusy Castoridae/Castoroidinae Miocene94 Trogontherium cuvieriy Castoridae/Castoroidinae Pleistocene (Schreuder, 1929)

Extinct species are indicated by y.



TABLE

A4.Speciesaverages

forfunctionalindices

usedin

theanalyses

IDn

o.S

pec

ies

SM

IB

IH

RI

HE

BO

LI

UR

IM

AN

US

CL

AW

CI

GI

FR

IF

EB

TR

IT

SI

PE

SIM

1Allactagahotsoni

0.4

66

1.2

39

0.0

93

0.2

92

0.1

56

0.0

36

0.6

88

0.7

74

1.5

61

0.1

09

0.0

72

0.1

55

0.0

49

0.2

46

1.1

08

0.3

67

2Ammospermop

hilus

leucu

rus

0.4

16

0.9

49

0.0

88

0.2

52

0.1

63

0.0

43

0.6

09

1.2

02

1.1

31

0.1

05

0.0

79

0.1

88

0.0

61

0.3

43

0.4

76

0.7

23

3Anom

aluru

spelii

0.3

26

0.9

22

0.0

60

0.1

48

0.1

00

0.0

28

0.9

25

0.9

71

1.0

42

0.0

48

0.0

56

0.1

24

0.0

43

0.3

80

0.2

28

0.8

13

4Aplodon

tiaru

fa0.4

99

0.9

05

0.1

12

0.3

43

0.2

23

0.0

55

0.5

91

1.6

77

1.0

62

0.0

79

0.0

91

0.2

27

0.0

79

0.3

97

0.3

110.8

29

5Arvicolaterrestris

italicu

s0.5

04

0.9

40

0.0

89

0.2

46

0.2

13

0.0

57

0.6

35

0.8

27

1.1

21

0.1

04

0.0

78

0.2

02

0.0

74

0.4

34

0.3

90

0.7

90

6Cannom

ysbadius

0.5

51

0.8

38

0.1

12

0.3

21

0.3

31

0.0

60

0.5

51

0.8

45

1.0

40

0.1

42

0.1

01

0.2

77

0.0

74

0.4

57

0.3

67

0.9

08

7Castor

canaden

sis

0.5

00

1.0

34

0.1

29

0.4

21

0.2

73

0.0

68

0.4

86

0.8

57

1.3

37

0.1

28

0.1

15

0.3

46

0.0

99

0.5

72

0.4

95

0.7

27

8Chelem

ysmacron

yx0.4

90

1.0

32

0.1

07

0.3

82

0.2

67

0.0

47

0.5

83

1.3

14

1.2

07

0.1

31

0.0

94

0.2

15

0.0

56

0.4

23

0.4

33

0.7

119

Clethrion

omys

(Myodes)

californ

icus

0.3

98

0.9

10

0.0

82

0.2

16

0.1

13

0.0

31

0.6

55

0.6

83

1.1

89

0.0

55

0.0

73

0.1

91

0.0

52

0.3

26

0.4

66

0.7

79

10

Coendou

prehen

salis

0.4

56

0.8

81

0.1

02

0.2

67

0.1

18

0.0

58

0.7

04

0.9

66

0.9

52

0.1

40

0.1

00

0.2

54

0.0

87

0.5

22

0.2

65

0.8

85

11Colom

ysgoslingi

0.4

45

1.2

28

0.0

89

0.2

67

0.1

27

0.0

37

0.6

55

0.6

79

1.4

89

0.1

18

0.0

82

0.2

16

0.0

51

0.2

95

0.7

87

0.6

46

12

Cryptomys

hottentotes

0.5

27

0.8

64

0.1

22

0.3

14

0.3

51

0.0

93

0.5

38

1.0

99

1.1

02

0.1

41

0.1

02

0.2

79

0.0

71

0.4

40

0.4

12

0.8

84

13

Cyn

omys

gunnison

i0.4

70

0.9

14

0.0

98

0.2

75

0.2

17

0.0

52

0.5

27

1.3

17

1.0

22

0.1

18

0.0

82

0.2

00

0.0

74

0.4

04

0.3

49

0.8

10

14

Dinom

ysbra

nickii

0.5

69

0.9

28

0.1

20

0.2

79

0.2

04

0.0

64

0.5

67

1.0

42

0.9

99

0.1

14

0.1

01

0.2

58

0.0

77

0.4

56

0.3

16

0.8

80

15

Dipod

omys

deserti

0.3

56

1.3

28

0.0

94

0.3

14

0.1

23

0.0

41

0.5

26

0.7

85

1.5

21

0.1

39

0.0

81

0.2

04

0.0

57

0.2

69

0.6

44

0.4

94

16

Dipod

omys

merriami

0.4

00

1.4

20

0.0

93

0.3

27

0.1

30

0.0

37

0.5

62

0.7

78

1.4

95

0.1

16

0.0

76

0.1

69

0.0

51

0.2

66

0.7

39

0.5

07

17

Dipus(Jacu

lus)

aegyp

ticu

s0.4

58

1.3

26

0.0

96

0.3

04

0.1

45

0.0

42

0.6

98

0.6

56

1.6

39

0.1

03

0.0

73

0.1

67

0.0

49

0.2

40

1.2

02

0.3

76

18

Erethizon

dorsa

tum

0.4

66

0.9

95

0.1

12

0.2

76

0.1

26

0.0

56

0.5

80

1.0

15

0.9

82

0.1

42

0.0

95

0.2

60

0.0

87

0.5

17

0.2

20

0.9

13

19

Geomys

bursarius

0.4

94

0.9

15

0.1

13

0.4

20

0.3

36

0.0

60

0.4

26

2.4

51

1.0

41

0.1

23

0.1

12

0.2

30

0.0

92

0.5

10

0.3

49

0.8

49

20

Georych

usca

pen

sis

0.5

88

0.9

13

0.1

05

0.2

83

0.3

30

0.0

93

0.5

12

0.9

40

0.9

92

0.1

31

0.0

94

0.2

31

0.0

89

0.5

09

0.3

08

0.9

30

21

Geoxu

svaldivianus

0.4

65

1.0

39

0.1

16

0.4

46

0.3

53

0.0

55

0.5

90

1.4

89

1.2

26

0.1

39

0.0

95

0.2

29

0.0

53

0.4

21

0.4

79

0.7

46

22

Glaucomys

sabrinus

0.3

18

1.1

29

0.0

64

0.1

72

0.0

91

0.0

16

0.9

05

0.9

46

1.1

73

0.0

72

0.0

58

0.1

42

0.0

45

0.3

110.3

79

0.8

43

23

Heliophob

ius

argen

teocinereu

s0.6

04

0.8

28

0.1

24

0.3

06

0.4

10

0.0

81

0.5

56

0.9

33

1.0

53

0.1

29

0.0

89

0.2

70

0.0

82

0.4

49

0.3

63

0.9

01

24

Heterocep

halusglaber

0.5

47

0.7

49

0.0

90

0.2

96

0.3

21

0.0

86

0.6

18

1.1

21

1.0

33

0.1

36

0.0

82

0.2

61

0.0

57

0.3

87

0.4

71

0.8

69

25

Hyd

romys

chrysogaster

0.5

10

0.9

62

0.0

91

0.2

94

0.2

23

0.0

50

0.5

72

0.6

88

1.2

49

0.1

29

0.0

98

0.2

30

0.0

76

0.4

67

0.6

37

0.6

46

26

Hylop

etes

nigripes

0.3

61

1.0

52

0.0

66

0.1

82

0.1

03

0.0

25

0.8

46

0.9

43

1.1

27

0.0

75

0.0

64

0.1

45

0.0

48

0.3

16

0.3

29

0.7

90

27

Hyo

mys

goliath

0.4

98

0.9

49

0.0

98

0.3

03

0.1

74

0.0

46

0.6

92

1.1

47

1.1

13

0.1

37

0.0

93

0.2

50

0.0

80

0.4

65

0.3

93

0.8

03

28

Hystrix

cristata

0.5

77

0.7

68

0.1

03

0.3

17

0.3

08

0.0

91

0.4

56

1.2

67

1.0

00

0.1

59

0.1

09

0.2

84

0.0

97

0.3

86

0.2

96

0.8

70

29

Jacu

lusorientalis

0.4

59

1.2

42

0.1

01

0.3

33

0.1

59

0.0

46

0.6

26

0.8

07

10.6

01

0.1

09

0.0

76

0.1

77

0.0

54

0.2

34

1.2

01

0.3

58

30

Marm

otaflaviven

tris

0.4

75

0.8

16

0.0

87

0.2

80

0.2

37

0.0

58

0.5

85

1.2

01

0.9

75

0.1

08

0.0

79

0.2

06

0.0

79

0.4

29

0.3

34

0.8

25

31

Microtusca

liforn

icus

0.4

58

0.9

80

0.0

78

0.2

26

0.1

66

0.0

40

0.6

50

0.8

57

1.2

36

0.0

78

0.0

87

0.1

79

0.0

59

0.4

16

0.4

95

0.7

86

32

Myo

castor

coyp

us

0.4

25

1.0

80

0.1

13

0.2

30

0.2

15

0.0

56

0.5

27

1.0

79

1.3

04

0.1

03

0.1

20

0.2

90

0.0

70

0.4

49

0.5

62

0.7

52

33

Nannospalax(Spalax)

leucodon

0.6

33

0.8

29

0.1

18

0.3

43

0.5

36

0.0

94

0.6

17

1.0

20

1.0

10

0.1

66

0.1

04

0.2

74

0.0

75

0.5

14

0.2

85

0.8

70

34

Napaeoza

pusinsignis

0.4

21

1.0

28

0.0

76

0.2

24

0.1

42

0.0

32

0.5

13

0.7

64

1.3

63

0.0

98

0.0

72

0.1

55

0.0

56

0.4

01

0.8

12

0.6

26

35

Nectomys

squamipes

0.4

55

1.0

110.0

78

0.2

60

0.1

72

0.0

36

0.6

22

0.6

58

1.2

51

0.1

37

0.0

92

0.2

09

0.0

66

0.4

29

0.6

07

0.6

55

36

Neofiber

allen

i0.5

26

1.0

52

0.0

90

0.3

25

0.2

17

0.0

52

0.6

13

0.8

50

1.2

95

0.1

27

0.0

98

0.2

53

0.0

69

0.4

00

0.5

48

0.7

57

37

Neotomacinerea

0.3

80

0.9

68

0.0

88

0.2

33

0.1

40

0.0

44

0.6

52

0.5

81

1.1

29

0.1

12

0.0

86

0.1

36

0.0

79

0.3

92

0.3

67

0.7

69

38

Nyctomys

sumichra

sti

0.5

13

0.9

74

0.1

01

0.3

23

0.1

63

0.0

43

0.8

49

0.8

73

1.0

66

0.0

74

0.0

91

0.2

38

0.0

84

0.4

10

0.3

83

0.7

47

39

Ondatrazibethicus

0.5

03

1.0

62

0.1

04

0.3

08

0.2

46

0.0

68

0.6

28

0.8

03

1.5

07

0.1

10

0.1

18

0.2

63

0.0

78

0.3

84

0.6

24

0.7

35

40

Onychom

ysleucogaster

0.4

37

1.0

12

0.0

90

0.2

94

0.1

75

0.0

45

0.5

68

1.1

75

1.1

75

0.0

75

0.0

74

0.1

95

0.0

57

0.3

38

0.4

84

0.7

59

41

Orthog

eomys

gra

ndis

0.5

32

0.9

46

0.1

08

0.3

73

0.3

10

0.0

63

0.4

71

2.2

67

1.0

29

0.1

34

0.1

15

0.2

44

0.0

93

0.4

50

0.3

54

0.8

80

42

Oxy

mycteru

sdasytrichus

0.4

40

1.0

48

0.0

96

0.3

27

0.2

46

0.0

57

0.4

92

0.9

43

1.2

19

0.1

43

0.0

87

0.2

22

0.0

60

0.3

68

0.5

26

0.6

91

43

Pappog

eomys

(Cra

togeomys)

tylorh

inus

0.5

67

0.9

89

0.1

12

0.3

86

0.3

19

0.0

63

0.4

25

2.1

07

1.0

79

0.1

34

0.1

31

0.2

51

0.0

86

0.4

61

0.3

41

0.9

04

44

Para

xeru

scepapi

0.4

22

0.9

30

0.0

90

0.2

63

0.1

59

0.0

47

0.7

60

0.9

34

1.1

63

0.1

01

0.0

81

0.1

97

0.0

64

0.3

49

0.4

38

0.7

25

45

Ped

etes

capen

sis

0.4

90

0.9

55

0.0

91

0.2

94

0.1

57

0.0

67

0.6

10

0.9

25

1.3

91

0.1

21

0.0

91

0.2

12

0.0

68

0.2

54

0.5

20

0.4

19

46

Perog

nathusparvus

0.3

73

1.0

44

0.0

74

0.2

49

0.1

55

0.0

42

0.6

50

0.9

08

1.2

55

0.1

02

0.0

77

0.1

96

0.0

56

0.3

62

0.6

62

0.7

04



TABLE

A4.(C

ontinued

).

IDn

o.S

pec

ies

SM

IB

IH

RI

HE

BO

LI

UR

IM

AN

US

CL

AW

CI

GI

FR

IF

EB

TR

IT

SI

PE

SIM

47

Perom

yscu

smaniculatus

0.4

08

1.1

86

0.0

81

0.2

79

0.1

41

0.0

35

0.7

12

0.9

56

1.3

26

0.0

37

0.0

74

0.1

60

0.0

50

0.3

23

0.5

13

0.6

77

48

Petauristapetaurista

0.3

39

0.9

92

0.0

57

0.1

57

0.0

81

0.0

23

0.8

88

1.0

58

1.0

70

0.0

67

0.0

56

0.1

23

0.0

41

0.2

86

0.2

40

0.7

87

49

Phloeomys

pallidus

0.4

64

0.8

99

0.1

06

0.2

95

0.1

68

0.0

56

0.7

73

0.8

26

1.0

22

0.1

36

0.0

95

0.2

51

0.0

97

0.4

61

0.3

09

0.8

30

50

Pyg

eretmuspumilio

0.4

14

1.2

65

0.0

97

0.3

04

0.1

54

0.0

39

0.6

45

0.7

94

1.5

52

0.0

95

0.0

77

0.1

67

0.0

52

0.2

57

0.9

94

0.3

72

51

Rattusnorvegicus

0.4

01

0.9

73

0.0

85

0.2

50

0.1

90

0.0

41

0.5

54

0.7

02

1.1

70

0.1

04

0.0

95

0.1

91

0.0

78

0.4

29

0.4

62

0.7

71

52

Rattusra

ttus

0.3

79

0.9

48

0.0

92

0.2

35

0.1

75

0.0

49

0.6

30

0.7

56

1.1

78

0.0

93

0.0

95

0.1

95

0.0

81

0.4

42

0.4

67

0.7

36

53

Ratufa

affinis

0.3

99

0.8

24

0.0

79

0.2

47

0.1

60

0.0

39

0.8

39

0.9

47

1.0

23

0.0

88

0.0

77

0.1

89

0.0

60

0.3

90

0.3

97

0.7

80

54

Rhizom

yspru

inosus

0.5

56

0.7

99

0.1

29

0.3

24

0.3

46

0.0

71

0.5

69

0.8

65

0.9

33

0.1

21

0.1

01

0.2

50

0.0

89

0.5

13

0.3

10

0.8

42

55

Sciuru

sniger

0.4

20

0.9

41

0.0

85

0.2

69

0.1

60

0.0

38

0.7

72

0.9

58

1.1

37

0.0

99

0.0

78

0.1

85

0.0

64

0.3

42

0.4

22

0.7

31

56

Sigmod

onhispidus

0.4

90

0.9

17

0.0

89

0.2

62

0.1

48

0.0

43

0.7

08

0.7

22

1.1

27

0.1

13

0.0

93

0.2

06

0.0

72

0.3

83

0.3

57

0.7

68

57

Spalaxgiganteus

0.6

40

0.7

36

0.1

26

0.2

84

0.5

87

0.1

08

0.5

69

1.0

30

1.0

21

0.1

38

0.1

02

0.2

46

0.0

82

0.5

07

0.2

17

0.8

59

58

Spermop

hilusbeech

eyi

0.4

31

0.9

01

0.0

96

0.2

66

0.1

81

0.0

49

0.6

39

1.0

18

1.0

35

0.1

02

0.0

85

0.2

00

0.0

81

0.3

72

0.3

93

0.7

70

59

Sphigguru

smexicanus

0.4

66

0.9

67

0.1

05

0.2

67

0.1

09

0.0

54

0.7

60

0.8

95

1.0

01

0.1

23

0.1

00

0.2

40

0.0

83

0.4

52

0.2

35

0.8

81

60

Tach

yoryctes

splenden

s0.5

58

0.9

41

0.1

07

0.3

07

0.2

74

0.0

59

0.4

99

0.7

92

1.0

46

0.1

25

0.1

02

0.2

47

0.0

88

0.4

47

0.3

19

0.8

51

61

Tamiaspalm

eri

0.4

17

0.9

79

0.0

89

0.2

69

0.1

82

0.0

51

0.6

57

0.9

12

1.1

43

0.0

89

0.0

81

0.1

86

0.0

60

0.3

33

0.5

00

0.7

39

62

Tamiasciuru

shudsonicus

0.4

12

0.9

45

0.0

82

0.2

60

0.1

67

0.0

37

0.7

36

0.9

51

1.1

50

0.0

87

0.0

72

0.1

83

0.0

52

0.3

47

0.4

74

0.7

37

63

Thom

omys

bottae

0.6

00

0.9

43

0.1

03

0.4

44

0.2

61

0.0

66

0.4

78

1.8

23

1.0

13

0.0

94

0.1

110.2

39

0.0

87

0.4

40

0.3

42

0.8

50

64

Tylom

ysnudicaudus

0.5

00

0.9

62

0.0

77

0.2

37

0.1

89

0.0

48

0.6

30

0.5

31

1.0

67

0.0

78

0.0

76

0.1

69

0.0

59

0.4

35

0.5

37

0.6

63

65

Xerusinauris

0.4

88

0.9

40

0.0

91

0.2

76

0.1

92

0.0

51

0.5

68

1.0

84

1.1

61

0.1

06

0.0

81

0.1

92

0.0

76

0.3

25

0.4

32

0.6

44

66

Zapusprinceps

0.4

28

1.0

97

0.0

85

0.2

55

0.1

46

0.0

33

0.6

26

0.6

84

1.3

51

0.1

08

0.0

76

0.1

57

0.0

54

0.3

91

0.7

58

0.5

90

67

Zyg

ogeomys

trichop

us

0.4

99

0.9

82

0.1

01

0.4

01

0.3

22

0.0

58

0.3

93

2.5

05

1.0

63

0.1

47

0.1

14

0.2

62

0.0

88

0.4

38

0.3

98

0.9

15

68

Ailuru

sfulgen

s0.5

110.7

74

0.0

76

0.2

76

0.1

59

0.0

50

0.6

72

1.0

73

0.9

83

0.1

25

0.0

71

0.2

07

0.0

57

0.3

69

0.3

59

0.9

01

69

Choloepusdidactylus

0.3

76

1.1

97

0.0

71

0.2

39

0.0

49

0.0

32

0.2

64

0.9

38

1.0

08

0.0

66

0.0

75

0.1

72

0.0

54

0.2

97

0.2

74

1.1

23

70

Cyn

ocep

halusvolans

0.3

03

1.4

33

0.0

46

0.1

36

0.0

46

0.0

14

0.5

52

1.1

63

1.0

62

0.0

46

0.0

48

0.0

96

0.0

38

0.3

13

0.1

74

1.0

1171

Dasypusnov

emcinctus

0.4

96

0.7

13

0.1

08

0.3

55

0.5

82

0.0

58

0.3

02

1.1

66

0.8

38

0.1

48

0.1

28

0.2

90

0.0

77

0.5

66

0.3

71

0.6

75

72

Gulo

gulo

0.5

21

0.8

07

0.0

80

0.2

89

0.1

68

0.0

47

0.4

52

1.0

75

0.9

81

0.1

20

0.0

78

0.2

28

0.0

71

0.3

35

0.3

96

0.9

09

73

Hyd

roch

aeris

hyd

roch

aeris

0.5

42

1.3

09

0.0

80

0.2

53

0.3

10

0.0

80

0.3

80

0.8

03

0.9

92

0.1

12

0.1

01

0.2

42

0.0

86

0.3

53

0.3

54

0.7

62

74

Lon

traca

naden

sis

0.5

72

0.7

110.0

90

0.3

78

0.2

91

0.0

63

0.6

47

0.9

39

1.2

78

0.1

30

0.0

95

0.3

44

0.0

86

0.5

35

0.6

05

0.8

01

75

Macrop

usrobustus

0.5

98

1.3

65

0.0

89

0.2

87

0.1

18

0.0

32

0.7

55

0.5

87

2.4

02

0.1

85

0.1

39

0.3

08

0.1

31

0.2

34

0.9

95

0.6

83

76

Mep

hitis

mep

hitis

0.5

02

0.8

23

0.0

92

0.3

12

0.1

93

0.0

58

0.4

22

1.4

47

1.0

17

0.1

51

0.0

92

0.2

26

0.0

58

0.3

02

0.3

13

0.7

88

77

Mustelafren

ata

0.4

09

0.6

70

0.0

69

0.2

34

0.1

76

0.0

46

0.6

91

1.1

24

1.0

88

0.1

08

0.0

77

0.2

03

0.0

62

0.3

13

0.4

01

0.7

90

78

Neovison

vison

0.6

01

0.7

05

0.0

75

0.2

74

0.2

12

0.0

53

0.5

06

0.8

26

1.1

02

0.1

22

0.0

92

0.2

33

0.0

79

0.3

77

0.4

97

0.8

04

79

Phocavitulina

0.5

33

0.9

13

0.1

77

0.4

51

0.3

31

0.0

76

0.8

19

0.9

04

2.1

90

0.3

71

0.1

47

0.6

58

0.2

07

0.4

54

0.6

35

0.7

20

80

Procyon

lotor

0.5

39

1.0

06

0.0

72

0.2

16

0.1

16

0.0

42

0.4

83

0.8

42

1.0

45

0.1

13

0.0

77

0.1

93

0.0

67

0.3

14

0.3

39

0.8

70

81

Sca

panustownsendi

0.7

39

0.7

93

0.5

82

0.8

62

1.0

10

0.2

40

0.5

84

1.9

98

1.1

95

0.1

42

0.1

16

0.2

83

0.0

92

0.3

65

0.2

65

0.6

54

82

Sorex

vagra

ns

0.4

75

0.9

42

0.1

36

0.4

52

0.3

19

0.0

74

0.6

84

0.7

08

1.2

94

0.0

86

0.0

93

0.2

62

0.0

84

0.3

78

0.4

09

0.7

53

83

Taxidea

taxu

s0.5

63

0.8

03

0.0

95

0.3

37

0.2

88

0.0

69

0.6

29

2.0

40

0.8

18

0.2

33

0.0

77

0.2

48

0.0

63

0.5

62

0.3

24

1.0

01

84

Castor

californ

icusy

0.5

14

1.0

84

0.1

40

0.4

01

0.2

45

0.0

64

0.5

14

0.9

52

1.3

04

0.1

33

0.1

41

0.3

72

0.1

01

0.4

69

0.4

89

0.7

45

85

Castoroides

ohioen

sisy

0.4

97

1.3

24

0.1

65

0.3

53

0.2

19

0.0

69

0.4

09

0.6

77

1.5

96

0.1

55

0.1

46

0.3

89

0.0

81

0.5

42

0.5

36

0.8

23

86

Dipoides

stirtoniy

0.4

84

1.0

39

0.1

42

0.3

83

0.2

48

0.0

66

0.5

00

0.7

50

1.3

04

0.1

24

0.1

05

0.2

53

0.1

08

0.4

55

0.4

85

0.7

53

87

Paen

emarm

otabarbou

riy

0.5

54

0.8

83

0.1

14

0.3

58

0.2

62

0.0

79

0.5

60

1.1

31

0.9

74

0.1

80

0.0

96

0.2

54

0.1

19

0.4

83

0.3

36

0.8

71

88

Palaeoca

stor

fossor

y0.6

01

0.9

06

0.1

61

0.3

58

0.3

09

0.0

85

0.5

89

1.9

52

0.9

69

0.1

44

0.1

20

0.2

48

0.1

01

0.4

19

0.2

98

0.7

13

89

Palaeoca

stor

nebra

scen

sisy

0.5

82

0.7

34

0.1

35

0.3

48

0.3

02

0.0

71

0.5

70

1.9

00

0.9

97

0.1

25

0.1

06

0.2

80

0.0

83

0.3

38

0.3

39

0.7

37

90

Proca

storoides

idahoensisy

0.4

74

1.2

70

0.1

66

0.3

99

0.2

48

0.0

70

0.5

63

0.6

29

1.3

30

0.1

42

0.1

40

0.3

62

0.1

02

0.5

77

0.5

85

0.7

71

Exti

nct

spec

ies

are

ind

icate

dby

y .



adaptacion skeletal rodents

Documents