Journal of Herpetology, Vol. 22, No, 3, pp, 323-330, 1988Copyright 1988 Society for the Study of Amphibians and Reptiles

Feeding Habits of the Diamond Python, Morelia s. spilota:Ambush Predation by a Boid Snake

DA VID J. SLIP AND RICHARD SHINE

if....

Zoology AOS, University of Sydney, NSW 2006, Australia

ABSTRACT- Diamond pythons (Morelia s. spilota) are large (to 3 m) snakes of temperate-zone coastaleastern Australia. Foraging behavior was studied by observation of telemetered snakes, and diets deter-mined by dissection of museum specimens and collection of fecal samples from wild-caught snakes. Fiveadult snakes monitored in the field by radiotelemetry were found to be ambush foragers. In summer,telemetered snakes spent more than 80% of their time coiled, usually in a disinctive ambush posture nearmammal trails. Telemetered snakes found feeding had occupied the site for at least a day beforehand. Thediet of adult diamond pythons consisted almost entirely of mammals (91% of 44 records), mainly Rattusrattus and R. fuscipes (52%). Birds (9%) were taken infrequently. Juveniles ate mainly mammals (69%) andreptiles (23%). Larger snakes consumed larger prey. Hatchlings in captivity showed a preference for copper-tailed skinks, Ctenotus taeniolatus, but would also take newborn mice. Feeding occurred mainly in latespring, summer and early autumn. In terms of feeding biology, diamond pythons may be more similar toviperid snakes of other continents than to the elapids or colubrids with which they are sympatric.

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The food that an animal eats, and theway it obtains that food, are central aspectsof a species' ecology, Trophic interactionsmay influence the distribution and abun-dance of a predator (and perhaps its prey),and certainly will directly affect daily ac-tivity P'!~terns and habitat use, Foragingbehavior may determine rates of energyintake and survivorship, and hence mayhave wide-ranging effects on life-historyvariables, either as direct phenotypic ef-fects, or as a basis for the evolution of mod-ified morphology, physiology, behavior orreproductive biology (e.g, Vitt and Cong-don, 1978; Ballinger, 1983). Feeding habitsof snakes are of particular interest, as theyshow remarkable adaptations for locating,capturing, subduing and ingesting verylarge prey items (see Pough, 1983). Avail-able published data on snake foraging areconcerned primarily with active searchingpredators belonging to the families Colu-bridae and Elapidae (e.g. Godley, 1980;Greene, 1984). Sit-and-wait (ambush) pre-dation also is common in snakes, but hasbeen described in detail only for viperids(e.g. Reinert et aI., 1984; Duvall et aI., 1985).

Boid snakes are widely distributed intropical and subtropical areas throughoutthe world, but have attracted little scien-tific study under field conditions. Thesesnakes are of interest from several points

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of view, including their large size, con-stricting behavior, and ability to consumeextremely large prey items (e.g. Fitzsi-mons, 1930). In the present paper, we de-scribe food habits and foraging behaviorof diamond pythons (Morelia s. spilota) insoutheastern Australia. This subspecies oc-cupies a relatively small range in coastalNew South Wales, whereas the related M.s. variegata occurs over most of the rest ofthe Australian continent. Adult diamondpythons average approximately 170 cmsnout-vent length (Slip, 1986), and areblack dorsally with yellow flecks. Neo-nates are mottled brown, and the ontoge-netic shift in color is gradual (pers. obs.).We obtained data from observation of free-ranging radiotracked snakes, combinedwith dissection of museum specimens andanalysis of fecal and stomach content sam-ples from field-collected snakes. Becausejuvenile pythons are rarely encountered inthe field, we examined dietary preferencesof neonates in the laboratory. This work ispart of a larger study on the ecology ofdiamond pythons (Slip, 1986; Slip andShine, 1988a, b).

MATERIALS AND METHODS

Diet.-Feeding records came from twosources: preserved specimens from the col-lection at the Australian Museum, and

324 D. J. SLIP AND R. SHINE

snakes collected in the field. Museum spec-imens were dissected and the contents ofthe stomach and intestine were removed.Often the only gut contents were a fewmammalian hairs in the lower intestine.Feeding records from freshly-captured an-imals in the field were obtained by pal-pation. If a bolus was detected in the stom-ach, the snake was forced to regurgitate,and the prey item was identified, measuredand forced back into the stomach (Fitch,1960). If fecal pellets were detected in thehind gut they were removed by palpationand collected. If no gut contents were ob-tained, snakes were held in isolation forup to two weeks at 26 to 28°C to allowpassage of feces. If no fecal samples werefound after this period the snake was as-sumed to be in unfed condition. Mammalhairs were identified by microscopic anal-ysis (Brunner and Coman, 1974), and byuse of a reference collection of fur samples.Prey length usually could be estimatedfrom skeletal remains in fecal and gut sam-ples, or mean lengths of mammalian preywere taken from Strahan (1983). Dates ofcapture of snakes were obtained from mu-seum and field records and grouped ac-cording to season for analysis of the sea-sonality of feeding.

Hatchling Food Preferences.-Food pref-erences in hatchlings were investigated bypresenting captive hatchlings with poten-tial prey items. Eleven hatchlings werehoused in individual glass and wire tanks40 x 30 x 20 cm, heated with 60 watt bulbsand positioned near a window to providenatural light. The tanks were maintainedat 25-30°C and water was provided.

When the hatchlings were 21 days old,they were presented with the followingpotential prey items: the skinks Ctenotustaeniolatus, Lampropholis guichenoti, Leiolo-pisma entrecasteauxii and Nannoscincus mac-coyi; hatchling dragons Amphibolurus bar-batus and Physignathus lesueurii; theterrestrial tree frog Litoria lesueuri; the bushcockroach Calolampra sp., un furred nest-ling laboratory mice and recently furrednestling laboratory mice (Mus domesticus).These species were chosen as representa-tives of naturally-occurring potential preytypes within the range of Morelia s. spilota.

The order of presentation of prey was ran-domized. Each snake was presented withone prey item, which was left in the tankwith the snake for 24 hours. If the preyremained after this time it was removedand another prey item presented. If theprey was consumed, one week was allowedbefore the next trial. When each hatchlinghad been exposed to all prey items theywere fed (some force-fed) an unfurrednestling laboratory mouse. After one weekthe experiment was repeated.

Foraging Behavior.-Eighteen adult dia-mond pythons were monitored by radio-telemetry to provide information on ther-moregulation, movements and habitat use(Slip, 1986); the present paper provides dataon general behavior of five of these snakes(those for which most detailed data wereavailable), and on feeding by another three.The two study sites used are valleys nearSydney, N.5.W. (Belrose and Cowan). Bothsites are steep and rocky, and covered indry sc1erophyll forest. Miniature 152 mHztem perature-sensi tive radiotransmitters(model TT-ID-lOOO, J. Stuart Enterprises,Grass Valley, California) were surgicallyinserted in the peritoneal cavities of snakesunder halothane anesthesia. Single three-volt lithium cell batteries (National BR-2j3A) provided an operational life of 10-13months. The transmitters were sealed witha waterproof coating of paraffin (80%) andelvax. The assembled unit measured ap-proximately 90 x 20 x 20 mm with a meanmass of 40.2 :t 3.65 g, representing 0.9 to3.9% of snake body mass. Most locations oftelemetered snakes were made on foot, butcontinuous records of snake temperatures(pulse interval) and activity (signal inten-sity) were obtained by an automatic re-cording system consisting of a TR-2E re-ceiver, H-frame antenna, TDP-2 digitalprocessor (Telonics Telemetry Consult-ants, Mesa, Arizona) and a Rustrak duallevel recorder. For further details on meth-odology see Slip (1986).

To investigate foraging mode, three maleand two female snakes were monitoredcontinuously by telemetry, each over a 72-hour period during summer. In additionto this continuous record of the amplitudeand period of the telemetry signal, snakes

PYTHON FEEDING ECOLOGY

TABLE1. Prey items identified from gut and fecalcontents of adult and juvenile Morelia s. spi/ota (adult= SVL :>150 cm, criteria for sexual maturity are givenin Slip and Shine 1987a).

Snakes

Prey

Mammals

Potorous tridactylusTrichosurus vulpeculaPseudocheirus peregrinusPetaurus brevicepsPerameles nasuta

Rattus fuscipesRattus rattusMus domesticus

Pseudomys novaehollandiaeOryctolagus cuniculus

Birds

Manorina melanocephalaMelopsittacus undulatusUnidentified birds

Reptiles

Amphibolurus barbatusCtenotus taeniolatusUnidentified skink

111

were located soon after sunrise and ob-served continuously for four hours. Theirpositions and behaviors were then checkedthree or four times at approximately two-hour intervals, and the snakes were ob-served continuously for about two hoursimmediately before sunset. Nocturnal ac-tivity was determined from changes in theamplitude of the radio signal. Movementof the transmitter relative to the receiverresults in a change in amplitude, and acrude calibration of movement was ob-tained by noting amplitude response on achart recorder while simultaneously ob-serving the telemetered snake. For exam-ple, it was possible to distinguish posturalreadjustments (such as moving from a tightcoil to a loose coil) from continuous move-ment, and periods of no movement. Threecategories of behavior were used for theanalysis: (1) moving, determined from di-rect observation or telemetry data; (2) bask-ing, determined from direct observation (itwas assumed no basking took place atnight), defined as when a snake was lyingloosely coiled or uncoiled in direct or fil-tered sunlight; and (3) coiled (not moving),

325

500

EE- 400

~ 300(!jZW...J 200>-WIX:c.. 100

.. --

. .S' --..."'" .-.. .,:

50 100 150 200 250

SNAKESVL (cm)

FIG.1. The relationship between snout-vent length(SVL) of diamond pythons and the lengths of prey:'taken.

determined from direct observation and

telemetry data.

3

RESULTS

Diet.-Of the 108 M. s. spilota examined,49 (45%)contained prey items in the stom-ach (8) or produced fecal samples (41). Sin-gle prey items were identified from mostsnakes, but six snakes contained two itemseach and one contained three. The foodtypes recorded were mammals (49 records,86%),birds (5 records, 9%) and reptiles (3records, 5%). The mammals most common-ly recorded were rats (two Rattus species,40% of all records) and possums (threespecies, 19%; Table 1). The three reptilianprey items came from juvenile snakes.

A comparison of snake body length toestimated prey length (Fig. 1) shows thatsmall snakes took only small prey such asreptiles, mice and sugar gliders (Petaurusbreviceps). Larger snakes also took smallprey, but in addition they consumed largeanimals such as brush tail possums (Trich-osurus vulpecula) and rabbits (Oryctolaguscuniculus). Some prey items were very large:for example, a brush tail possum consumedby a 3.2 kg snake was estimated to weighapproximately 2 kg.

Morelia s. spilota show distinct seasonaldifferences in frequency of feeding (Table2) with a higher proportion of recently-fedsnakes captured in summer (73.9%) than inother seasons (0-42%). No snakes capturedduring winter showed evidence of recentfeeding. These seasonal differences are sig-nificant (R x C chi-square from Zar, 1974;X2 = 28.610, v = 3, P < 0.001). No adult

Juveniles Adults

136

1 11

2 121 84 31

5

326 D. J. SLIP AND R. SHINE

TABLE2. Monthly variation in the numbers of Morelia s. spilota containing food items in the digestive tract(letters indicate months of the year, and asterisk indicates gravid females).

males were found with prey items duringspring (N = 13) and no prey items wererecorded from gravid females (N = 4, Table2).

HatchlingFoodPreferences.- Three of the11 hatchling pythons used showed no in-terest in any of the food items. The re-maining eight hatchlings all took Ctenotustaeniolatus when offered, and furred labo-ratory mice on at least one occasion. Sixalso took un furred mice, and two hatch-lings took Leiolopisma entrecasteauxii. Noneshowed interest in any of the other preyitems offered (Table 3).

Foraging Behavior.- Telemetered snakeswere often observed in a coiled or looselycoiled posture with the head positioned atthe top of'the coil and inclined at an angleof about 15° above horizontal. Generallythis position was adopted partially undercover, with the head oriented toward openground, and often on the edge of mammalrunways such as those used by rats, rabbitsand bandicoots. These runways could beidentified by the presence of scats orscratchings.

Telemetered snakes were observed feed-ing in the field on three occasions. On 16

February 1982, at 1150 h, a female (SVL 195cm) was observed swallowing an adultbrushtail possum (Trichosurus vulpecula).The snake was in a fork of a 10 m Eucalyptusoblonga, about six m above the ground.When located on the previous two days,this snake had been coiled in the same fork.On 15 October 1983, at 1250 h, a femalepython (SVL 184 cm) was observed swal-lowing an adult rabbit. This snake had beenpreviously located two m away on 13 Oc-tober coiled tightly and partially concealedby long grass at the edge of a 30 cm widetrail, which was used as a runway by smallmammals. The snake completed swallow-ing in 45 minutes. On 14 December 1983,at 1200 h, a male diamond python (SVL165 cm) was observed swallowing an adultringtail possum (Pseudocheirus peregrinus)with an estimated body length of 350 mm.The snake was lying on leaf litter at thebase of a six m Casuarina sp. When locatedon the previous day, this snake had beencoiled 10 m away at the base of a Xanthorheasp.

Table 4 shows the amounts of time eachof five snakes spent coiled, moving andbasking over 72 hours. Heterogeneity chi-

TABLE3. Feeding preferences of hatchling Morelia s. spilota.

5 0 N 0 J F M A M Winter

Adult males Fed 0 0 0 3 14 2 2 1 0 0(>150 cm) Not fed 4 8 1 2 3 1 3 2 4 6

Adult females Fed 0 1 0 0 8 5 5 0 0 0(>150 cm) Not fed 0 2 4* 1 0 3 1 0 2 4

Juveniles Fed 1 1 6 1 0 1 1 2 0 0«150 cm) Not fed 0 2 0 1 1 0 0 1 2 3

Approx. SVL Approx. massPrey species (mm) (g) Eaten Not eaten

Amphibolurus barbatus 44 4 22Physignathus lesueurii 46 5 22Ctenotus taeniolatus 80 10 16 6Lampropholis guichenoti 40 3 22Leiolopisma entrecasteauxii 50 5 4 18Nannoscincus maccoyi 50 5 22Litoria lesueuri 50 8 22Calolampra sp. 30 1 22Un furred neonate Mus domesticus 20 5 11 11Furred neonate Mus domesticus 40 10 14 8

PYTHON FEEDING ECOLOGY 327

Activity budget, in hours, for snakes monitored continuously over 72 hours.

Total hoursMean :t SOMean proportion of time

Coiled

Number of hours spent in each activity

Moving Basking

57.559.551.560.559

28857.6 :t 3.58

0.80

108.5

16109

53.510.7 :t 3.03

0.15

4.544.51.54

18.53.7 :t 1.25

0.05

square analysis shows that individuals weresimilar in the proportions of time spent ineach of the three activities (X2 = 3.93, v =8,0.75 < P < 0.90). Snakes monitored con-tinuously in summer spent most of theirtime (80%) in a coiled position.

DISCUSSION

Diet.-Published data on the diet of Mo-relia s. spilota are generally consistent withthe results of our study, in that most au-thors consider birds and mammals to bethe commonest food items (Table 5). Ourdata suggest clearly that mammals are theprincipal component of the diet of adultM. s. spiliJta(contra McPhee, 1979). The lowincidence of birds in the diet may reflectthe infrequency of arboreal activity (Slipand Shine, 1988b). No eggs, reptiles or frogswere recorded from the digestive tract ofadult M. s. spilota. Published data on thediets of other species of pythonine snakesare primarily anecdotal, but mammals seemto be an important component of the diets

of the larger species such as Python molurus,Python reticulatus and Python sebae, withlarge individuals capable of taking verylarge prey (Pope, 1961; Branch and Hacke,1980). Mammals are probably the most im-portant component of the diets of mostAustralian pythons except Aspidites mela-nocephalus, which feeds mainly on reptiles(Shine and Slip, unpubl. obs.). The speciesof mammals eaten by M. s. spilota are main-ly nocturnal (Watts and Aslin, 1981; Stra-han, 1983) and as adult diamond pythonsare predominantly ambush predators, mostprey encounters would occur at night.

The significant seasonal variations ob-served in feeding frequency could be dueeither to differences in prey availability(perhaps because of seasonal habitat shiftsby the snakes) or to other behavioral shiftsin the snakes or their prey. Thus, for ex-ample, the high frequency of feeding insummer may reflect fluctuations in preydensity, as Rattus fuscipesoccurs in highestdensities at this time (Watts and Aslin,

Reference

TABLE5. Published data on the diet of diamond pythons, Morelia s. sri/ala.

Diet

Birds and small mammals. The young feed on insects, frogs or bird eggs.One record of a 2.59 m snake eating a ringtail possum, one record of a 2.74 m

snake eating a brushtail possum.Rabbits, wallabies, possums, ducks, other birds, bird eggs, lizards, frogs and

snakes.

Warm-blooded animals such as birds, rats, mice, rabbits, and small possums.Principally birds, also consume mammals such as rats, mice, marsupials, pos-

sums and bandicoots.Birds and mammals.Rabbits and mammals of similar size, rats and mice.One record of Pseudocheirus peregrinus.Birds and mammals, three records of lizards.A variety of terrestrial vertebrates.One record of Hydromys chrysogastor.Adults feed on mammals and birds. Young feed exclusively on reptiles.

Krefft, 1869Lucas and le Souef, 1909

Waite, 1929

McKay, 1948McPhee, 1979

Worrell,1963Kinghorn, 1964Rose, 1974Gow, 1976Cogger, 1983Lunney and Barker, 1985Weigel and Worrell, 1985

TABLE4.

Snake Sex SVL(cm)

#1 F 195#5 F 184

#12 M 149#14 M 180#18 M 175

328 D. J. SLIP AND R. SHINE

1981). Diamond pythons are sometimes ac-tive on warm nights and often remaincoiled under light scrub in an ambush pos-ture, only retiring under heavy cover oncolder nights (Slip and Shine, 1988b). Theyalso select habitats with higher prey den-sities in summer and autumn than at othertimes (as determined from mammal trap-ping: Slip and Shine, 1988b). Thus, withthe exception of brooding females, snakeswould have the highest likelihood of preyencounter in summer and part of autumn.

The cessation of feeding by adult malesduring the spring (mating season) is in-teresting in that the snakes are in habitatscontaining reasonably high prey densitiesat this time. We hypothesize a specific dis-inclination to feed, perhaps because (i) alarge food item in the stomach would re-duce mobility (Garland and Arnold, 1983;Ford and Shuttlesworth, 1986), and hencepossibly reduce the likelihood of locatingfemales; and (ii) as they are ambush pred-ators, the time spent waiting for prey wouldreduce the time available to search formates. A similar cessation of feeding bymales during mating has been found inviperids .,(Prestt, 1971) and natricines(Aleksiuk and Gregory, 1974) but not ela-pids (Shine, 1975). Continued feeding inmale elapids may be possible because ela-pids search actively for prey and must alsosearch widely for mates: the two activitiesmay be combined (Shine, 1975). Also, sincemost elapids take relatively smaller preythan do viperids or boids feeding wouldbe less likely to reduce their mobility.

The absence of prey items from gravidfemales is consistent with the reduction infood intake widely recorded in gravidsnakes (Fitch and Shirer, 1971; Prestt, 1971;Klauber, 1972; Shine, 1979, 1980a; Reinertet al., 1984). Foraging may cease becauseof reduced foraging efficiency, increasedvulnerability to predation, or the difficul-ties of maintaining the thermal require-ments for optimum embryonic develop-ment while foraging (Shine, 1980a). Gravidfemale M. s. spilota thermoregulate care-fully and retire under cover on most nights,sometimes using the oviposition mound afew weeks before depositing the eggs (Slip,1986). Increased time spent under covermay reduce encounters with prey. Since

pythons subdue prey by constriction, theact of capturing prey may involve somerisk of damage to the eggs, as well as mak-ing the snake more obvious to potentialpredators (Shine, 1987).

Among the elapids sympatric with M. s.spilota, some cease feeding in winter, whilea small number of individuals of otherspecies contain prey (Shine, 1980b, C,1981).Shine (1980c) suggested that in colder areas,only snakes of relatively small body sizewould have the ability to maintain bodytemperatures high enough to permit win-ter feeding. Low temperatures reduce boththe digestive rate (Skoczylas, 1970; Green-wald and Kanter, 1979; Naulleau, 1983) andthe efficiency of prey capture (Greenwald,1974) in some reptiles. In winter, M. s. spi-lata will bask on sunny days and elevateits body temperature close to those reachedat other times of year, although mean dailybody temperatures are lower in winter thanin other seasons (Slip, 1986). However,winter nights are generally spent deep un-der cover (Slip, 1986) which may reducethe likelihood of encountering nocturnalmammalian prey. Thus, although M. s. spi-lata may not actively avoid feeding in win-ter, fluctuations in prey density andchanges in snake habitat use, while max-imizing prey encounter rate in summer,may considerably reduce it in winter.

HatchlingFoodPreferences.-Reptiles ap-pear to be an important part of the diet ofjuvenile and hatch ling M. s. spilota. Weigeland Worrell (1985) suggested that youngdiamond pythons feed exclusively on rep-tiles, but our data suggest that hatchlingsand juveniles will eat mammals and birdsas well as reptiles. Reptiles may becomeless important as snakes grow to a size atwhich they can subdue and swallow largermammalian prey. The ontogenetic shift indiet away from reptiles may also reflectdifferences in foraging behavior betweenadults and juveniles. A similar dietary shifthas been identified in death adders (Acan-thophis antarcticus: Shine, 1980b), Europeanvipers (Vipera berus: Prestt, 1971) and insome rattlesnakes (Klauber, 1972), all ofwhich are predominantly ambush preda-tors as adults.

ForagingBehavior.-Our data on behaviorof radio-tracked pythons (Table 4) and ob-

PYTHON FEEDING ECOLOGY

servations of feeding in the wild indicatethat diamond pythons ambush their preyrather than actively search for it. Althoughwe have no information from the field ondiets or behavior of juvenile pythons, ourlaboratory studies suggest that coppertailskinks, Ctenotus taeniolatus, may be an im-portant dietary component. These skinksare abundant and extremely fast-moving,and almost certainly could be captured onlyby ambush predation. Hence, diamond py-thons of all ages are probably sit-and-waitpredators. One major ontogenetic shift maybe in the time of day at which feedingoccurs: the skinks eaten by juvenile py-thons are diurnal, whereas most of themammals eaten by adult pythons are cre-puscular or nocturnal. The predominanceof diurnal rather than nocturnal move-ments by adult snakes (Slip and Shine,1988b) may maximize their time availablefor nocturnal ambush predation.

Foraging mode is phylogenetically con-servative in snakes. In general, viperids areambush predators (e.g. Prestt, 1971; Klau-

. ber, 1972), whereas most colubrids, elapidsand acrochordids may search actively fortheir prey (e.g. Shine, 1977; Godley, 1980;Greene, 1'984; Shine and Lambeck, 1985).Thus, in terms of foraging behavior, dia-mond pythons are more similar to viperidsthan to the elapids or colubrids with whichthey are sympatric. Nonetheless, many au-thors have recognized that the apparentdichotomy between "sit-and-wait" and"active searching" is in reality a contin-uum, and closely-related species may differin their position on this continuum. Suchvariation is evident within each of the ma-jor families of snakes. Although most ela-pids are active foragers, species of at leasttwo genera (Acanthophis and Denisonia) relyon ambush predation (Shine, 1980, 1983).Most hydrophiids also search actively forprey, but Pelamis uses an unusual form ofsit-and-wait foraging (Kropach, 1975;Burns, 1984). Viperid snakes vary in theimportance of mobility in their foragingbehavior (e.g. Reinert et al., 1984). Suchvariation can occur even within species,perhaps ontogenetically or seasonally. Forexample, the colubrid Nerodia rhombifera isusually an active searcher, but has beenreported to use ambush predation in some

329

situations (Gillingham and Rush, 1974).Acrochordus arafurae does the same (pers.obs.). Hence, our data on Morelia spilotashow only that this species feeds primarilyby ambush; any attempts to generalize onforaging strategies of other pythons wouldbe premature at this stage. Detailed studiesof other pythons-especially tropicalspecies-would be very interesting in thisrespect.

Acknowledgments. - This paper compris-es part of an M.5c. thesis by the seniorauthor. We thank G. Ross and C. Shine forhelp with mammalian hair identification,R. Williams and G. Swan for capture ofpythons, R. Lambeck, P. Harlow, C. Jamesand T. Pople for assistance, and G. Griggfor advice and the loan of equipment. L.Schwarzkopf and J. Bennett provided com-ments on the manuscript. H. Cogger andA. Greer kindly allowed us to dissect Aus-tralian Museum specimens. Transmitterswere provided by the Peter Rankin Fund,and the N.S.W. National Parks and Wild-life Service provided permits for the work.

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Accepted: 5 August 1987.