original slaying dragons: limited evidence for … reptiles, the largest living lizard (the komodo...

ORIGINALARTICLE

Slaying dragons: limited evidence forunusual body size evolution on islands

Shai Meiri1*, Pasquale Raia2 and Albert B. Phillimore3

INTRODUCTION

Giant tortoises, enormous flightless birds and huge bears,

alongside minute deer, tiny lizards, dwarf elephants and, lately,

pygmy humans all spring to mind when the body sizes of

island vertebrates is discussed. Body size evolution on islands is

perceived to be fast (Lister, 1989; Millien, 2006) and has

produced extreme phenotypes, with the smallest or the largest

1Department of Zoology, Faculty of Life

Sciences, Tel Aviv University, 69978 Tel Aviv,

Israel, 2Dipartimento di Scienze della Terra,

Universita Federico II, 80138 Naples, Italy,3Division of Biology, Imperial College at

Silwood Park, Ascot SL5 7PY, UK

*Correspondence: Shai Meiri, Department of

Zoology, Faculty of Life Sciences, Tel Aviv

University, 69978 Tel Aviv, Israel.

E-mail: [email protected]

ABSTRACT

Aim Island taxa often attain forms outside the range achieved by mainland

relatives. Body size evolution of vertebrates on islands has therefore received

much attention, with two seemingly conflicting patterns thought to prevail: (1)

islands harbour animals of extreme size, and (2) islands promote evolution

towards medium body size (‘the island rule’). We test both hypotheses using body

size distributions of mammal, lizard and bird species.

Location World-wide.

Methods We assembled body size and insularity datasets for the world’s lizards,

birds and mammals. We compared the frequencies with which the largest or

smallest member of a group is insular with the frequencies expected if insularity is

randomly assigned within groups. We tested whether size extremes on islands

considered across mammalian phylogeny depart from a null expectation under a

Brownian motion model. We tested the island rule by comparing insular and

mainland members of (1) a taxonomic level and (2) mammalian sister species, to

determine if large insular animals tend to evolve smaller body sizes while small

ones evolve larger sizes.

Results The smallest species in a taxon (order, family or genus) are insular no

more often than would be expected by chance in all groups. The largest species

within lizard families and bird genera (but no other taxonomic levels) are insular

more often than expected. The incidence of extreme sizes in insular mammals

never departs from the null, except among extant genera, where gigantism is

marginally less common than expected under a Brownian motion null. Mammals

follow the island rule at the genus level and when comparing sister species and

clades. This appears to be driven mainly by insular dwarfing in large-bodied

lineages. A similar pattern in birds is apparent for species within orders. However,

lizards follow the converse pattern.

Main conclusions The popular misconception that islands have more than

their fair share of size extremes may stem from a greater tendency to notice

gigantism and dwarfism when they occur on islands. There is compelling evidence

for insular dwarfing in large mammals, but not in other taxa, and little evidence

for the second component of the island rule – gigantism in small-bodied taxa.

Keywords

Birds, dwarfism, evolution, gigantism, island biogeography, island rule, lizards,

mammals.

Journal of Biogeography (J. Biogeogr.) (2011) 38, 89–100

ª 2010 Blackwell Publishing Ltd www.blackwellpublishing.com/jbi 89doi:10.1111/j.1365-2699.2010.02390.x

species of many clades being insular (Hooijer, 1967; Berry,

1998; Greer, 2001; Glaw et al., 2006; Whittaker & Fernandez-

Palacios, 2007; Hedges, 2008; Losos & Ricklefs, 2009). For

example, the St Helena earwig, Labidura herculeana, the

Indonesian stick insect, Pharnacia serratipes, and the New

Zealand giant wetas (Deinacrida spp.) are probably the largest

representatives of their clades (Chown & Gaston, 2010).

Similarly, the world’s largest bat (Smith et al., 2003) is the

Philippine-endemic golden crowned flying fox, Acerodon

jubatus, the largest Quaternary bird was the Madagascan

elephant bird, Aepyornis maximus, and the largest raptor was

the New Zealand endemic Haast’s eagle, Harpagornis moorei

(Worthy et al., 2002; Murray & Vickers-Rich, 2004).

Among reptiles, the largest living lizard (the Komodo

dragon, Varanus komodoensis) and tortoises (the giant tor-

toises of Aldabra and the Galapagos, Geochelone gigantea and

Geochelone elephantopus) are insular endemics (Arnold, 1979;

Meiri, 2008). Islands also harbour the smallest members of

several clades: the smallest bird is believed to be the Cuban bee

hummingbird, Mellisuga helenaei (although Dunning, 2008b,

suggests that Thaumastura cora from mainland Peru is

smaller), and species of Caribbean Leptotyphlops and Sphaero-

dactylus are the world’s smallest snakes and lizards, respectively

(Hedges, 2008; Meiri, 2008). This perceived abundance of

insular size extremes is usually thought to be a response to the

low intensity of competition and predation both within and

across taxa: the absence of carnivorous mammals is most often

quoted as allowing the evolution of large size in birds (mainly

through the evolution of flightlessness; e.g. Bunce et al., 2005;

Murray & Vickers-Rich, 2004), reptiles (e.g. Case, 1978; Meiri,

2008) and small mammals (e.g. Angerbjorn, 1986; Adler &

Levins, 1994). Alternatively, this perceived pattern may simply

reflect an ascertainment bias, i.e. we may be more likely to

notice animals of extreme body size when they happen to live

on islands (Whittaker & Fernandez-Palacios, 2007).

By contrast, the island rule suggests that, rather than showing

size extremes on islands, insular populations should be closer to

the clade-wide median body size than their mainland counter-

parts (Lomolino, 1985, 2005). According to the island rule,

populations of small species will evolve larger size, populations

of large species will dwarf, and populations of average-sized

species will show little size evolution on islands (Lomolino, 2005;

Welch, 2009). Viewed at the clade level, the island rule predicts

stabilizing selection: on islands an individual should not be

either too large or too small. Once the optimum is reached (via

directional selection on the founding population), stabilizing

selection should maintain phenotypes around it. Interestingly,

an opposite pattern of disruptive selection and increased vari-

ance is often thought to prevail within populations on islands

(Van Valen, 1965; Scott et al., 2003; cf. Meiri et al., 2005a).

The island rule is thought to manifest the combined effects

of lower predation pressures on islands (Heaney, 1978),

character release in the absence of competitors (Dayan &

Simberloff, 1998) and the paucity of resources on islands

driving dwarfism in large-bodied forms (Lomolino, 2005).

Empirical evidence from terrestrial vertebrate studies provides

mixed support for this rule (Clegg & Owens, 2002; Boback &

Guyer, 2003; Lomolino, 2005; Meiri, 2007), with the best

support coming from data for mammals (Lomolino, 1985;

Price & Phillimore, 2007; Welch, 2009; but see Meiri et al.,

2004, 2006, 2008). Size evolution is often hypothesized to be

most drastic on small islands, with island area showing

complex interactions with body size (Heaney, 1978), but

empirical patterns are equivocal (e.g. Meiri et al., 2005b; Wu

et al., 2006; Schillaci et al., 2009).

These two hypotheses, that islands should harbour extreme

sizes, and that they should harbour taxa that are closer to a clade-

wide mode, need not necessarily be contradictory (Fig. 1). If a

clade, such as mammals, has a single size attractor (e.g. Brown

et al., 1993) then members of a subclade may evolve to a size

extreme: insular members of a subclade of large-bodied animals

(e.g. elephants) will be the smallest within this subclade, but not

across the larger clade as a whole. Similarly, insular members of a

subclade of small-bodied animals (e.g. shrews) may be the largest

members of this subclade, but not the largest mammals overall.

If, however, each subclade has its own optimal size that its insular

members evolve towards (Lomolino, 2005) then islands should

harbour few size extremes (Fig. 1c).

The seemingly conflicting discussion of insular size extremes

on the one hand, and insular medium sizes on the other hand,

stems in part from the different phylogenetic and temporal scope

of studies dealing with them. Studies of size extremes are usually

conducted at the inter-specific level (Glaw et al., 2006; Hedges,

2008), and often deal with extinct taxa (Kurten, 1953; Sondaar,

1977; Steadman et al., 2002; Raia et al., 2003). The study of

evolution towards medium sizes usually involves intra-specific

studies of insular and mainland populations of extant species

(e.g. Lomolino, 1985; Boback & Guyer, 2003; Meiri, 2007).

Evolutionary processes above and below the species level

may differ through, for example, species sorting and adaptive

radiation in the former versus inter-island and island–

mainland gene flow in the latter (Jablonski, 2008). As far as

we are aware McClain et al. (2006) and Welch (2010) present

the only purely inter-specific studies of the island rule,

comparing mean sizes within genera of deep sea (= ‘insular’

in McClain et al.) and shallow sea (‘mainland’) species, rather

than using comparisons within single species. An argument for

extending tests of the island rule to the species level is that the

selection pressures thought to promote convergence on a

median body size for island populations should act similarly

on island species. The major difference between the two

scenarios is that there should be less gene flow between species

than between populations, meaning that evolution should

proceed more rapidly in the former. However, there is no

obvious reason why the type of selection and adaptive optima

should differ in these two contexts. A major advantage of

intra-specific studies of insular size evolution is that they

compare very closely related taxa (i.e. different populations

within a species), usually from areas that are in geographic

proximity. Intra-specific comparisons thus control for much of

the variation in size that is unrelated to insularity. Restricting

ourselves to intra-specific studies, however, we may be missing

S. Meiri et al.

90 Journal of Biogeography 38, 89–100ª 2010 Blackwell Publishing Ltd

the more dramatic cases of insular size evolution, where

changes are drastic enough to merit different specific status.

Thus intra-specific studies will omit from consideration, for

example, Elephas falconeri, which evolved to just 1% of the

mass of its mainland ancestor (Roth, 1992), and even studies

conducted on species within genera will miss members of

insular endemic genera such as the c. 100 kg insular rodent

Amblyrhiza inundata (Biknevicius et al., 1993) and the largest

gecko, Hoplodactylus delcourti (Russell & Bauer, 1986).

Here we examine, in a purely inter-specific fashion, whether:

(1) species with extreme body sizes (i.e. the largest and smallest

species) in a subclade are more often insular endemics than

expected by chance given the number of insular endemics in

the subclade; and (2) if there is evidence for a pattern

consistent with the island rule above the species level using

mean sizes of insular and mainland species within clades.

We use nearly complete datasets of species body sizes of

mammals, birds and lizards. For mammals (only) a compre-

hensive species-level phylogeny and an excellent late Pleisto-

cene fossil record exist. This meant that we were also able to

use a sister-clade comparison, and include data for species

that went extinct since the end of the last glacial, which

include some of the most pronounced cases of insular size

evolution.

MATERIALS AND METHODS

Data

Body size data are species specific. Body size (snout–vent lengths

in mm) and insularity data for lizards (Appendix S1a in the

Supporting Information) are from an updated version of the

No island rule, insular size extremes

Island rule, insular size extremes No island rule, insular size extremes

Island rule, no insular size extremes

Reversed island rule, insular size extremes

No island rule, no insular size extremes

Body size of mainland taxa

Body size of mainland taxa

Bod

y si

ze o

f is

land

tax

aB

ody

size

of

isla

nd t

axa

(a) (b) (c)

(d) (e) (f)

Figure 1 Schemes showing possible ways in which patterns of body size on islands and mainland within taxa (size extremes or average

sizes) can combine to produce patterns of body size across taxa (slope <1, slope = 1, slope >1). x-axis, mainland body size; y-axis,

island body size; dashed line, a line with an intercept of 0 and a slope of 1; solid line, slope returned by standardized major axis regression

(can mask the dashed line). The body size frequency distributions depict insular (top) and mainland members of a clade (bottom). The

mainland mean size is indicated by the vertical, dashed line. (a) The null distribution. No insular size extremes, no island rule (slope of 1,

similar insular and mainland size ranges). (b) There are size extremes because island taxa have wider size distributions, but no island

rule because mean sizes are similar (slope of 1). (c) The island rule holds (small animals evolve larger size and large ones dwarf, slope <1),

but no size extremes because island taxa have narrower size distributions. (d) The island rule holds (slope <1), and insular clades are

of extreme size – members of large-bodied clades are smallest in their clade and members of small-bodied clades are the largest in their clade.

(e) Island members are proportionally always larger than mainland ones, hence there is one type of size extreme, but no island rule (no

dwarfing of large animals, slope = 1). (f) There are size extremes, but in the opposite direction than predicted under the island rule. Small-

sized insular animals are extremely small, large ones are extremely large (slope >1).

Island vertebrates and body size extremes

Journal of Biogeography 38, 89–100 91ª 2010 Blackwell Publishing Ltd

dataset of Meiri (2008). Lizard taxonomy follows Uetz et al.

(2009).

Body mass data (in g) for birds are from Dunning (2008).

Where mass data were reported for multiple subspecies the mean

of these measurements was taken (intra-specific data were first

log10-transformed). For each species overall mean body mass

was estimated across means for female, male and unsexed birds.

We obtained data on avian insularity from McCall (1997), and

supplemented and verified them using Avibase (http://

avibase.bsc-eoc.org) and regional guides (Appendix S1b). Tax-

onomy follows Clements (1998), and we included species that

had been extirpated since the extinction of the dodo (17th

century). As we were interested in the effect of insularity on

birds, we excluded birds that forage at sea, which we defined as all

members of the Alcidae, Diomedeidae, Fregatidae, Hydrobat-

idae, Laridae, Pelecanidae, Pelecanoididae, Phaethontidae,

Phalacocoracidae, Procellariidae, Rynchopidae, Spheniscidae,

Stercoraridae, Sternidae and Sulidae families.

Body mass and insularity data for mammals are from the

2007 version of Smith et al. (2003) (data kindly provided by

Felisa Smith), supplemented with literature data (see Appen-

dix S1c for species for which we obtained data from sources

other than the Smith et al. database). Mammal insularity was

verified using Wilson & Reeder (2005), regional guides and

palaeontological accounts of fossil species. We use only fossil

mammals that went extinct since the end of the last glacial.

Only fossil species known to be confined to islands during the

last glacial were considered to be insular.

All analyses were conducted on log10-transformed measures

of body size, to bring the intra-class distribution of body size

closer to a normal distribution and to remove a relationship

between the mean and variance of a group (Lynch & Walsh,

1998).

Phylogeny

Complete species-level phylogenetic data exist only for mam-

mals, and we thus only use phylogenetic analysis on this group.

We used the species-level mammal supertree of Bininda-

Emonds et al. (2007). The tree was modified as follows: we

resolved taxonomic discrepancies between the tree and our

data using the taxonomy of Wilson & Reeder (2005), and

excluded species for which we had no size data and fossil

species whose island endemism was uncertain. Finally, where

the phylogenetic affinities of extinct species are well under-

stood we added them to the tree. Our modified tree includes

3961 species. Although this tree excludes some new phyloge-

netic data, it is the only comprehensive species-level tree

currently available for any vertebrate class.

Statistical analysis

Size extremes

For each of the four datasets we counted the number of genera

for which either the largest or the smallest species was insular.

Only genera having at least one insular member and one

continental member were included. We repeated this proce-

dure at the family and order levels (family level only for

lizards).

We then assessed whether the observed values departed from

the null expectation if insular status were randomly assigned

by randomly selecting n individuals per taxon (i.e. genus,

family or order), where n is the number of insular species in

that taxon. We then calculated the number of taxa for which

the largest or smallest representative had been randomly

assigned insular status. This process was repeated 10,000 times

to obtain a distribution of the null expectation for the

frequency of insular gigantism or dwarfism. We calculated the

proportion of the null distribution that was: (1) greater than or

equal to the observed value, and (2) smaller than or equal to

the observed value. The smaller of these proportions was

multiplied by 2 to give a two-tailed P-value.

Our randomization test for size extremes on islands should

be conservative for two reasons. First, we classify all species

that have at least one mainland population as ‘mainland’,

even though in some of these species the largest or smallest

populations may be insular (e.g. Kodiak brown bears, Ursus

arctos middendorffi). Second, if insular taxa are clustered

within a subclade (as is the case in mammals; Raia et al.,

2010), the variance in body size among insular members of a

subclade will be reduced, reducing the potential for the

evolution of size extremes. However, it is possible that the

ancestral members of subclades that colonize islands are

themselves biased in size with respect to the mainland

representatives of the subclade (e.g. they may be very large;

Lomolino, 1985) and this could generate an increased

incidence of insular size extremes without requiring further

evolution on islands. In the absence of adequate fossil data we

are unable to test this hypothesis.

Using the mammalian phylogeny we were able to compare

the observed incidence of size extremes with those generated

under a phylogenetically explicit null model. We conducted

1000 simulations of body size evolution across the whole

phylogeny following a Brownian motion (random walk with

constant variance) model using the evolve.phylo function in

the R library ‘ape’ (Paradis et al., 2004). A recent study on

body size evolution across the mammalian tree identified a

weak but significant signature of early burst evolution (i.e.

decelerating rates of phenotypic evolution; Cooper & Purvis,

2010). However, a constant-rate Brownian motion model did

not perform much worse, and in the context of our study

should represent an unbiased means of generating a phyloge-

netically explicit null expectation. The fossil record shows that

mammals have increased in body size through time according

to Cope’s rule (e.g., Alroy, 1998), which conflicts with a

Brownian model. Nonetheless, with respect to the hypotheses

being tested here, adding directionality (universally across

island and mainland lineages) to a random walk should not

bias our tests. For each simulation we quantified the incidence

of gigantism and dwarfism and in this manner generated a null

distribution. We then used two-tailed tests to establish the

S. Meiri et al.


probability of obtaining the observed incidences of gigantism

and dwarfism under the null Brownian model.

Mean size

We assessed whether the ‘island rule’ applies above the species

level. We calculated the mean log10 body size of insular and

mainland species within each genus. We tested the null

hypothesis that there are no differences between patterns of

mainland and insular size evolution across genera (Welch,

2009) by examining the slope of the island versus mainland

taxon means using standardized major axis (= reduced major

axis) regression. A slope <1 is expected under the island rule,

which predicts gigantism in small-bodied taxa and dwarfism in

large-bodied ones and a reduction of variance amongst insular

taxa as compared with their mainland counterparts (Price &

Phillimore, 2007). Different taxa within a taxonomic level will

vary in age, meaning that the expected variance of phenotypes

among them is likely to vary. This heterogeneity of expected

variance violates an assumption of standardized major axis

(SMA) regression. We therefore used a distribution-free

variant of the SMA test, as proposed by Welch (2009). The

SMA correlation coefficient (r) between x + y (where x is body

size on the mainland and y is body size on an island) and x ) y

was the test statistic. The observed correlation coefficient was

then compared with that observed when the identity of insular

means was randomized with respect to x and y (Welch, 2009).

Ten thousand randomizations were conducted and the two-

tailed P-value was the smaller of twice the proportion of

randomized correlation coefficients that were either greater

than or equal to or less than or equal to the observed

correlation. If the observed SMA correlation between x + y

and x ) y was significantly greater than expected at random

this would support the island rule.

At the family level we adjusted the protocol to control for

clade species richness, so that rather than each species

contributing equally to the family mean, each species contrib-

uted equally to the genus mean and each genus then

contributed equally to the family mean. The same procedure

was repeated at the order level with the addition of taking the

mean across families in an order. Tests with species treated

independently (i.e. simple means of all species in a family or

order) gave qualitatively the same results (not shown).

Using the modified mammal tree we located nodes that

represented a partition between a solely insular and a solely

mainland clade and estimated the mean body size for each

clade. Clades including polytomies were excluded. The sister

clades we identified are shown as Appendix S2. As the clades

involved in each island versus mainland comparison vary in

age, then under a Brownian motion model of evolution the

expected variance of the mean values of these clades is expected

to vary, violating an assumption of SMA regression (Welch,

2009). Consequently, we estimated the statistical significance

of a departure from a 1:1 relationship between the size of

island and mainland taxa by applying 10,000 randomizations

of the identity of insular and mainland clades using the

distribution-free randomization approach of Welch (2009)

described above. All statistical analyses were conducted in R (R

Development Core Team, 2008) and all tests were two-tailed.

RESULTS

Lizards

Of 5380 lizards in our dataset, 1636 are insular endemics

(Appendix S1a). The largest lizard in 9 of 19 families is an

insular endemic (see size frequency histograms in Appen-

dix S3a), while the median expected number is five (P = 0.07).

At the genus level the number of largest insular species is no

different from the null expectation. The number of insular

species that are the smallest species in their genus or family is

likewise not statistically different from that expected by chance

(Table 1).

The SMA slope between the taxon mean masses on islands

versus mainland does not depart significantly from 1 at the

genus level (Fig. 2a). At the family level the slope estimate of

1.24 [95% confidence interval (CI) = 1.02–1.50] (Fig. 2b) is

significantly steeper than 1 using both SMA (r = 0.49,

P < 0.05) and the distribution-free randomization test

(P < 0.05, similar to the pattern depicted in Fig. 1f).

Table 1 Actual and expected numbers of taxa in which insular

members are the largest or the smallest, in different taxa at dif-

ferent taxonomic levels.

Clade Level

Number

of clades Largest� Smallest�

Lizards Genus 85 30 (36) 36 (35)

Lizards Family 19 9* (5) 7 (5)

Birds Genus 223 90** (71) 73 (71)

Birds Family 86 24 (21) 21 (21)

Birds Order 19 3 (3) 6 (4)

Mammals Genus 91 29 (36) 38 (35)

Mammals Family 39 9 (11) 10 (12)

Mammals Order 12 4 (3) 2 (3)

Mammals� Genus 101 35 (40) 41 (39)

Mammals� Family 47 14 (14) 12 (14)

Mammals� Order 14 5 (3) 3 (3)

Mammals [BM] Genus 91 28* (35) 38 (35)

Mammals [BM] Family 39 9 (11) 10 (11)

Mammals [BM] Order 12 4 (3) 2 (3)

Mammals� [BM] Genus 99 33 (37) 41 (37)

Mammals� [BM] Family 46 15 (14) 12 (14)

Mammals� [BM] Order 14 6 (3) 3 (3)

*, **Denote statistical significance at the a = 0.1 and 0.01 levels,

respectively.

�The median expected values derived from our randomizations are in

parentheses.

�Including extinct species. [BM] denotes analyses conducted using

Brownian motion simulations rather than randomizations.

Note that because we only have phylogenetic data for c. 92% of

mammalian species observed numbers of size extremes can differ

between non-phylogenetic and phylogenetic analyses.



Birds

Of 8069 bird species in our dataset, 1347 are insular endemics

(we have maximum mass data for all, and for 7552, including

1268 insular endemic species, we have data on mean size;

Appendix S1b). In 90 out of 223 avian genera the largest

member of the clade is an insular endemic. This is significantly

more than expected under our randomizations (median

expected = 71; P < 0.01, Table 1). However, at the family

and order levels (see size frequency histograms in Appen-

dix S3b) the largest member of a clade is no more often an

insular endemic than expected at random. The frequency with

which the smallest member of a clade is an insular endemic

does not depart from the null expectation for any taxonomic

level. Moreover, when we repeated the analysis at the genus

level using the maximum rather than mean body size (data are

the maximum reported for a species in Dunning, 2008),

neither the frequency of gigantism nor dwarfism departed

from the null expectation (67 observed insular maxima across

219 genera, median expected = 71, P = 0.53).

There was no evidence for the island rule at the genus and

family level in birds, with slopes equal to 1.00 and 0.98,

respectively (Fig. 3a,b). The slope at the order level, however,

was significantly shallower than 1 (SMA slope = 0.76, 95%

CI = 0.66–0.88, r = )0.70, P < 0.01; Prandomization < 0.01;

Fig. 3c), even when ratites are excluded (SMA slope = 0.78,

95% CI = 0.64–0.95, r = )0.56, P < 0.05; Prandomization <

0.05).

Mammals

Of 3961 extant mammal species, 670 are insular endemics (778

of 4213 when extinct species are included). The frequency of

insular endemics that are either the largest or smallest

members of their clades does not depart from the null

expectation derived by randomization at any taxonomic level,

either including or excluding extinct taxa (Table 1, Appen-

dix S3c).

In agreement with the results from randomizations, the

frequency of gigantism and dwarfism tended not to exceed the

null expectation generated under a single-rate Brownian model

on the mammalian phylogeny. This was true for all taxonomic

levels, both including and excluding extinct species, except for

Continental mean SVL (log10 scale)

Insu

lar m

ean

SV

L (lo

g10

sca

le) (a) (b)

Figure 2 Standardized major axis regression

of mean body size [log10-transformed

snout–vent length (SVL) in mm] on islands

versus continents for species in lizard genera

(a) and families (b). The dashed line has a

slope of 1 and an intercept of 0. The solid line

represents the standardized major axis

regression slope estimate.

Continental mean mass (log10 scale)

Insu

lar m

ean

mas

s (lo

g 10

scal

e) (a) (b) (c)

Figure 3 Standardized major axis regression of mean body size (log10-transformed mass, in g) on islands versus continents for species in

bird genera (a), families (b) and orders (c). The dashed line has a slope of 1 and an intercept of 0. The solid line represents the standardized

major axis regression slope estimate.

S. Meiri et al.


extant mammals at the genus level, where the frequency of

gigantism was marginally lower than expected (P = 0.1).

Mammals show a general tendency to follow the island rule

at the genus level. The observed slope of mean insular clade

mass versus mean continental clade mass is significantly less

than 1 for genera both including and excluding extinct species

(extant only: SMA slope = 0.96, 95% CI = 0.93–0.99, r =

)0.24, P < 0.05; Prandomization < 0.05, Fig. 4a; extant + extinct:

slope = 0.92, 95% CI = 0.89–0.96, r = )0.41, P < 0.01;

Prandomization < 0.05, Fig. 4d; see also Fig. 1c). The slope for

families is not significantly different from 1 (slope = 1.01, 95%

CI = 0.95–1.08, and 0.95, 95% CI = 0.89–1.01, with and

without extinct species, respectively, Fig. 4b,e). The slope for

orders likewise is not significantly different from 1

(slope = 1.01, 95% CI = 0.74–1.37, and 0.87, 95% CI =

0.76–1.05, with and without extinct species, respectively;

Fig. 4c,f).

When analyses were conducted using the mammalian

phylogeny, regressing mean body sizes within insular clades

on the mean size within their mainland sister clades returned a

slope significantly shallower than 1 using SMA (n = 91,

slope = 0.931, 95% CI = 0.877–0.987, r = )0.25, P < 0.01),

but not using randomizations (P = 0.155; Fig. 5a). Restricting

the analysis to insular versus mainland sister species, we get a

slope consistent with the island rule using both tests (n = 57,

slope = 0.870, 95% CI = 0.81–0.93, r = )0.49, P < 0.01;

Prandomization < 0.05, Fig. 5b).

DISCUSSION

The largest species in bird and lizard taxa tend to be insular

more frequently than expected were insular status assigned to

species at random. Interestingly, however, this holds within

bird genera (but not when species maximum rather than mean

masses are used), whereas lizards only show gigantism among

families. Furthermore, the lizard giants are often the result of

insular radiations (e.g. Hoplodactylus, Gallotia, Cyclura) on

islands lacking mammalian carnivores (here New Zealand, the

Canaries and the Antilles, respectively). Such absence of

mammalian predation has been hypothesized to promote

lizard gigantism by allowing for more foraging, as less time is

spent hiding from predators, and by enabling lizards to evolve

the role of top predators (Case, 1982; Meiri, 2008). In birds,

the largest members of genera that are insular seem to be quite


Insu

lar m

ean

mas

s (lo

g 10

scal

e)

(a) (b) (c)

(d) (e) (f)

Figure 4 Standardized major axis regression of mean body size (log10-transformed mass, in g) on islands versus continents for species in

mammalian genera (a, d), families (b, e) and orders (c, f). Plots (a)–(c) are for extant taxa only and (d)–(f) include extinct taxa (see

Materials and Methods). The dashed line has a slope of 1 and an intercept of 0. The solid line represents the standardized major axis




evenly distributed between continental shelf, volcanic and

continental plate islands (e.g. the Philippines, New Guinea,

Tasmania, Hawaii and New Zealand). This suggests that release

from predation on islands has often promoted gigantism in

lizards and perhaps gigantism associated with flightlessness in

birds (e.g. the New Zealand moas, Sylviornis; Russell, 1877;

McNab, 1994), but perhaps a different mechanism drove

gigantism among birds that retained their power of flight (e.g.

high population density; Blondel, 2000).

Size extremes in insular mammals show no departure from

null expectations. However, mammals conform to the island

rule at the genus level, especially when fossil species are

included. This pattern also emerges when we compare sister

species, a better test of size evolution than comparing clade

members ignoring their phylogenetic affinities.

In all three taxa, deviations from a slope of 1 seem to stem

from small (mammals and birds) or large (lizards) members of

generally large-bodied forms (e.g. small elephants, artiodactyls,

ratites and ducks, large iguanas). Extinct mammals show the

most drastic cases of dwarfism. However, many of the recently

extinct insular lizards and birds are extremely large (Pregill,

1986; Blondel, 2000) and extinction in these taxa was probably

much more prevalent on islands than on mainlands, whereas

extinctions of large mammals were common in mainland

settings (Barnosky et al., 2004). Thus in early Holocene times

there were giant insular birds in orders now exhibiting an

overall tendency for small sizes on islands. Indeed very large,

recently extinct, insular birds include members of the Falcon-

iformes (e.g. Amplibuteo, Harpagornis moorei, Circus eylesi;

Worthy et al., 2002; Suarez & Olson, 2007) and Strigiformes

(e.g. Tyto riveroi, Ornimegalonyx oteroi; Alcover & McMinn,

1994), ratites (Dinornis, Aepyornis; Worthy et al., 2002),

Anseriformes (e.g. Cygnus falconeri and the ‘very large Hawaii

goose’; Milberg & Tyrberg, 1993; Paxinos et al., 1999),

Ciconiiformes (Threskiornis solitarius, perhaps a Pelecaniform;

Mourer-Chauvire et al., 1995), Gruiformes (Diaphorapteryx

hawkinsii, perhaps Aptornis; Holdaway, 1989; Raia, 2009),

Galliformes (e.g. Megavitiornis, perhaps Sylviornis; Steadman,

2006) and Columbiformes (Raphus cucullatus, Pezophaps

solitaria, Natunaornis gigoura; Steadman, 2006; Worthy,

2000). Thus, the apparent tendency for smaller mean size in

insular members of large-bodied avian orders may be a result

of human-mediated extinction (Steadman, 2006; Pavia, 2008)

rather than a feature of natural insular evolution.

The finding that bird genera harbour more insular giants

than expected by chance is surprising, given that the mean size

of species within bird genera does not seem to differ between

islands and mainlands (Fig. 2a). A thorough study of the avian

subfossil record may even reveal that when extinct taxa are

included a similar pattern will be revealed within families and

orders.

A common explanation for insular gigantism in birds and

reptiles (e.g. elephant birds, moas, Komodo dragons and the

giant skinks of Cape Verde and New Caledonia) is that they

have evolved large size on islands with no mammalian

competitors or predators (Russell, 1877; Case, 1978; McNab,

2002; Meiri, 2008). While we view this as a highly likely

explanation, we are not sure it can explain our finding that

island birds tend to be the largest members of their genera

more often than is expected by chance. Our impression from

the data is that these largest members of avian genera are

mostly found on large islands, rich in bird, reptile and often

mammal species. Being classified as congenerics of mainland

forms, insular giants seldom occupy niches vacated by

mainland mammals, and usually differ relatively little from

the size of their mainland relatives. Currently we are unable to

sufficiently explain why this pattern prevails, or why it holds

only for birds, and only within genera, and note that species

maximum sizes [probably an inferior size measure because it is

more sensitive to sample size (Meiri, 2007) but representing

about 7% more species (Dunning, 2008)] do not show the

same trend.

The evidence we find for the island rule in mammals

emerges primarily via insular dwarfism in large taxa. Curi-

ously, the tendency of large mammals to dwarf on islands (see

also Raia et al., 2010), which is corroborated by our phylo-

genetic tests, and when fossils are included, is also linked to the

absence of predators and competitors, and seems more

prevalent in herbivores than in carnivores (Raia & Meiri,

2006). McNab (2002) has claimed that gigantism in insular


Insu

lar m

ean

mas

s (lo

g10

sca

le) (a) (b)

Figure 5 Standardized major axis regression

of mean body size (log10-transformed mass,

in g) on islands versus continents within (a)

mammalian sister clades and (b) a subset of

the data in (a): only sister species [only clades

in (a) where both mainland and insular

sample sizes are 1]. The dashed line has a

slope of 1 and an intercept of 0. The solid line

represents the standardized major axis


S. Meiri et al.


birds is more likely in herbivorous taxa. Additionally, in lizards

insularity is often associated with large size and herbivory

(Troyer, 1983; Meiri, 2008). Gigantism may be favoured where

resources are abundant (McClain et al., 2006), and the size of

large carnivorous vertebrates may depend on the size of

available prey; thus islands lacking large herbivorous mammals

are likely also to lack large carnivores. Because mammals can

grow much larger than either birds or lizards, one might say

that even the largest avian and reptilian predators, Haast’s

eagle and the Komodo dragon, are not large predators

compared with large mammalian carnivores. Thus low preda-

tion and competition pressures on islands may tend to

produce both relatively small mammals and relatively large

lizards.

The nature of the islands that we study, in terms of their

area and isolation, climate, geology (e.g. whether they are part

of the continental shelf, part of a tectonic plate or volcanic)

and biogeographic settings (e.g. realm, ocean) may all affect

the mode of size evolution (Meiri et al., 2005b; Schillaci et al.,

2009). Moreover, these attributes may interact with the

ecological attributes of the different taxa themselves, such as

their functional group or guild, their diet and microhabitat

preferences, as well as their behaviour (Case, 1978; McNab,

1994; Raia & Meiri, 2006) in shaping the way that size evolves.

Such attributes of islands and taxa offer promising avenues for

research into size evolution on islands.

CONCLUSIONS

The evidence that insular conditions favour the evolution of

extreme sizes within clades is restricted to gigantism in lizard

families and, perhaps, bird genera, but is not found in these

groups at other taxonomic levels, and neither does it apply to

mammals. Furthermore, large insular lizards seem often to

result from radiations on oceanic islands with no mammalian

carnivores whereas giant insular bird species are scattered over

highly variable set of islands (S.M., unpublished). We thus

think it is unlikely that these two patterns have a common

explanation.

The island rule applies in a statistical sense to mammalian

species within genera, and between sister species. Biologically,

however, while dwarfism in large insular mammals seems

prevalent, we find no evidence for the second component of

the island rule – a general tendency for gigantism in small-

bodied mammals. Within small-sized lizard families insular

species are smaller than mainland ones, and within large-

bodied families insular species are larger than mainland ones,

reversing the island rule. These findings are consistent with

intra-specific studies (Lomolino, 1985; Meiri, 2007), suggesting

that similar selection pressures may operate to produce

patterns seen both within and between species. More compre-

hensive fossil data are needed to resolve the pattern of size

evolution in island birds. The different courses of size

evolution on islands taken by different taxa imply an

important role for contingency, as animals differing in their

ecology respond differently to the selective forces imposed by

agents such as resource abundance, predation and competi-

tion, which in turn differ across different islands.

ACKNOWLEDGEMENTS

We thank Liz Butcher and Barbara Sanger from the Michael

Way Library for their enormous help in obtaining literature

sources for data used in this work. Felisa Smith kindly

provided us with the latest version of the ‘Integrating

Macroecological Pattern and Processes across Scales’ (IMMPS)

working group mammalian mass database. We thank Ian

Owens for valuable discussion and Mark Lomolino, Craig

McClain, John Welch and two anonymous referees for very

helpful comments on earlier versions of this manuscript.

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SUPPORTING INFORMATION

Additional supporting information may be found in the online

version of this article:

Appendix S1 Body size and insularity status for (a) lizards,

(b) birds and (c) mammals.

Appendix S2 Insular and mainland mammal sister clades.

Appendix S3 Size frequency histograms of species within (a)

lizard families, (b) bird orders, (c) mammal orders.

As a service to our authors and readers, this journal provides

supporting information supplied by the authors. Such mate-

rials are peer-reviewed and may be reorganized for online

delivery, but are not copy-edited or typeset. Technical support

issues arising from supporting information (other than

missing files) should be addressed to the authors.



BIOSKETCHES

Shai Meiri is a senior lecturer at the Department of Zoology, Tel Aviv University. He is interested in trait evolution, the tempo and

mode of evolution, the evolutionary implications of biogeography and vertebrate evolution.

Pasquale Raia is a post-doctoral research fellow at the Department of Earth Science, University of Naples Federico II, and a

member of the Center for Evolutionary Ecology based at Rome III University. He is interested in large mammal evolution, both at

the organismal and community levels, in response to climate change and the effect of ecological interactions.

Albert Phillimore is an Imperial College Junior Research Fellow and is interested in the influences of ecology on evolution and

speciation.

Author contributions: A.B.P., P.R. and S.M. conceived the ideas and collected the data; A.B.P. analysed the data; S.M. led the writing.

Editor: K.C. Burns

S. Meiri et al.


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