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Wildlife Damage Management, Internet Center for

USDA National Wildlife Research Center -

Staff Publications

University of Nebraska Lincoln Year

Ontogenetic Shifts in Canion

Attractiveness to Brown Tree Snakes(Boiga irregularis)

John A. Shivik Larry Clark

Colorado State University, Fort Collins, CO,National Wildlife Research Center,

This paper is p osted at DigitalCommons@University of Nebraska - Lincoln.

http://digitalcommons.unl.edu/icwdm usdanwrc/651

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4 SHORTER COMMUNICATIONSOLSOK, M. 1997. Comparative development of thelarval chondrochranium and visceral skeleton ofHymer~ocilirus nd Xerzupus (Amphibia: Anura: Pip-idae).J .Morphol. 232:304.RADICE,. P. . W. NEFF,.H . SHIM,. J . BRUSTIS,

A K D G. M. MALA CINS KI.989. Developmental his-tories in amphibian myogenesis. Int. J Dev. Biol.33:325-343.Accepted: 19 January 1999.

jiiirrnaf Hrrpr.tology, Vol. 33 No. 2, pp. 331-336 1999C o p y r ~ ~ t999 Socie? for the Study of Arnphib~ans nd Rephles

Ontogenetic Shifts in Canionttractiveness to Brown Tree SnakesBoiga irregularis)

J O H N A. SHIVI K~N D LARRY CLARK?,Dqartrnetlt ofBiology, Colorarlo State Uniircrsity, Ft. Collins, Colorado80523 USA. E-mail: jsh~~ik@iamau.colostate.ed~~XationalWildlife Research Center, 1716 Henth Pnrkuwy, Ft. Collins,Colorado 80524 USA.The brown tree snake (Boiga irregularis) is a noctur-nal, primarily arboreal, rear-fanged colubrid native toparts of Australasia (Savidge, 1987; Greene, 1989).Throughout their range, brown tree snakes eat a \,a-riety of prey including lizards, rats, and birds(Greene, 1989; Shine, 1991; Rodda, 1992; Rodda et al.,iiz press). Brown tree snakes on Guam have a wide dietconsisting mainly of lizards and lizard eggs, but avariety o f other items were found in snake stomachs,including odd items such as cooked spareribs (Sav-idge, 1988). Salridge (1988)noted an ontogenetic shiftin Guam brown tree snake diets; small snakes con-sumed lizards and lizard eggs and larger snakes con-sumed birds, bird eggs, and mammals.Brown tree snakes were introduced to Guam in thelate 1940s or early 1950s as a passive stowrawray ncargo (Salridge, 1987; Rodda et al., 1992). Since the

brown tree snake s introduction on Guam, its popu-lation has irrupted: population densities may occa-sionally reach 50-100 snakeslha (Rodda et al., 1992).The snake has virtually extirpated the island s avifau-na (Savidge, 1987), and concern that the snake willinvade elsewhere has spawned intensive trappingprograms (US. Dep. Agric., 1996).Managers use 1il.e mouse lures in brown tree snaketraps. The desire to avoid using mice has given riseto a quest for inanimate attractants for brown treesnakes (Fritts et al., 1989; Shivik and Clark, in press).Substances such as blood and saliva have shownpromise in laboratory studies (Chiszar et al., 1992,1993, 1997, in press), but have proven ineffective inthe field (Rodda et al., 1997). Therefore, t is importantto validate laboratory methods with field tests. Fur-thermore, prelrious lures based on odors associatedPresent Address: National Wildlife Research Cen-ter, 1716 Heath Parkway, Ft. Collins Colorado 80524,USA. E-mail: ishivikQlamar colostate edu

with live mice were relatively ineffective n field trialsbecause live prey odors require a simultaneous visualcue to attract brown tree snakes into traps (Shivik,1998). Carrion lures produce capture rates similar tolive mice lures; however, carrion does not need to becoupled with a visual cue in order to attract browntree snakes (Shivik and Clark, 1997).It is important to investigate thoroughly the use ofcarrion-based odor as an inanimate attractant prior toincorporating this technique into a management strat-egy. Here, we hypothesized that lure type, specificallya live or dead lure, could attract di fferentsize classesand sexes of brown tree snakes. The objective of thisstudy was to test brown tree snakes on Guam for anontogenetic shift in the attractiveness of carrion.Snakes were collected during two studies on Guam.For both studies, we used wire mesh minnow trapsfitted wi th one-way doors, and placed traps 20 mapart (Linnell et al., in press). Trap lines were estab-lished in forest edge along roads and trails. In traps,we enclosed lures within hardware cloth boxes 17 7 20 cm boxes of 6 m m mesh) to prevent snakesfrom eating lures. To minimize extraneous biologicalodors, we cleaned traps with a high-pressure waterspray soaked them in a 1:60 bleach: water solutionfor >two hours, and sun-dried them before place-ment. We ran each trao-line for two niehts and eachline contained 10 trapsltreatment type (ordered ran-domly) . Traps were checked every morning, andsnakes were brought to a laboratory for measuringand sexing (probing hemipenes).

In the first study, we set 90 traps containing livemice, quartered dead mice, or empty control traps (10traps per lure type in three traplines). Traps were setduring April, 1997 adjacent to Tarague Beach, Guam(Shivik and Clark, 1997). For dead-mice traps, com-mercially purchased frozen mice were defrosted earlyin the day and allowed to rot in traps for two nights.Because previous work showed that the importanceof a visual cue was dependent upon rvhether lureswere live or dead mice, we replicated an earlier study(Shivik and Clark, 1997) and collected sex and lengthdata on captured snakes. We hypothesized that dif-ferent size classes of snakes may be attracted di ffer-entially to live or dead prey (as examined in Study I ,or to visually apparent or visually obscured prey.Traps contained live mice, dead mice, live mice ob-scured, or dead mice obscured. Lures rvere obscuredby wrapping their holders in black felt. Traps in thesecond study were set adjacent to Tarague Beach andHaputo Beach, Guam. We set 160 traps (10 traps perlure type in four trap lines) in March and 200 traps(f iv e rap lines) during August 1997.LVe examined differences in snake snout-ventlength (SVL)using analysis of variance (ANOVA). nStudy 1, we performed a one-way ANOVA to deter-mine i f snake size varied by lure type. Also, we useda log-likelihood chi-square to determine i f capturesdiffered by lure type and sex. In Study 2, we per-formed a two-way ANOVA examining the effec tsof alive or dead lure and a visual and odor or an odoronly lure. We used a Mantel-Haenszel chi square (Kir-by, 1993; Ott, 1993) to determine i f male and femalesnakes showed differentialattraction to trap lures.In the first study, we captured 22 snakes using livemice, 14 snakes using dead mice, and two snakes in

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SHORTER CO MMUNICATIONS 335

FIG. 1. Mean snout-vent length (SVL) and SE ofsnakes captured using live mice (LM; N = 22) andquartered dead mice (DM; N = 14) in traps set adja-cent to Tarague Beach, Guam during April 1997.

control traps. Because only two snakes were capturedin control traps, the control treatment was removedfrom the SVL analysis. Snakes captured with livemice lures i= 92.8 2.7 [SE] cm, range = 70-109cm) were larger than those captured with quartereddead mice i= 81.5 - 3.3 cm; range 58-103, F,,, =6.99, P = 0.012; Fig. 1). We did not detect a differencein the attractiveness of live or dead lures to male orfemale snakes (x2= 0.45, df = 2, P = 0.50).

In the second study, we captured 152 snakes. Snoutvent length of snakes captured with visually apparentlures (N = 113, 2 = 93.8 1.2 cm) were not signifi-cantly larger than snakes caught with only odor lures(N = 3 9 ; i = 90.8 2.0;F,,,, = 1.7,P = 0.20).How-ever, snakes captured with dead mice (N = 74, i =90.0 1.6, range = 51-120) were smaller than thosecaptured with live mice (N = 78,2 = 95.7 1.7; range= 58-119, F, = 3.8, P = 0.05). No interaction be-tween visual and odor or odor only cues and live ordead lure types was apparent (F,,,,, = 0.24; P = 0.62).Using the Mantel-Haeszel test, male and femalesnakes did not show differential attraction to any luretype (x2= 1.08; df = 1; P = 0.30).

In regard to carrion foraging, male and femalebrown tree snakes appear to have similar preferences,because we did not detect a difference in capture rateswhen using live prey versus carrion, nor did we detecta difference in proportions captured with live or deadand visually apparent or visually obscured prey.These results are consistent with previous brown treesnake studies on Guam that did not detect a differencein male and female food habits (Savidge, 1988).In contrast, we found evidence that lure type (i.e.,live versus carrion) differentially affected capture rateas a function of snake size. Snakes captured with car-rion lures were smaller than snakes captured with livemice lures. Various snakes, including the brown treesnake, show ontogenetic shifts in feeding habits (Sav-idge, 1988; Greene, 1989, 1997; Arnold, 1993), and our

data suggest the behavioral mechanism for these ob-servations. Our observed difference in snake size in-dicates prey preference (i.e., a higher degree of moti-vation to enter a trap that contains a particular preytype). Further studies in the laboratory should be per-formed to examine this phenomenon more thoroughlybecause differential prey preference will influence theeffectiveness of inanimate lures.

Even though brown tree snakes may eat a wide w-riety of prey and be considered opportunistic, it isinteresting to note that individual variation occurswithin the brown tree snake population on Guam.Throughout their range, brown tree snakes may eat awide variety of prey, but individual snakes and sizeclasses may be much more specialized and less op-portunistic.

We have, in two separate experiments, demonstrat-ed that live or dead lures are biased to catching dif-ferent sized snakes. In our studies, snakes capturedwith live mice were 23% and 8% longer than thosecaptured with dead mice, relative to the range of thesnakes captured. The mean size of captured snakes isinfluenced by the lure used as well as by the under-lying distribution of snakes sampled (Rodda andFritts, 1992). Given current trapping methods, it willbe difficult to estimate the actual numbers of snakesin each size class because different size classes havedifferent susceptibilities to different trap lures. Ironi-cally, because the size distribution of local populationscannot be determined accurately with present trap-ping methods, we are unable to fully interpret theecological significance of the observed difference incapture rates by size class.Researchers performing population analyses onbrown tree snakes should use sampling methodolo-gies that account for the possible undersampling ofsmall snakes (Rodda and Fritts, 1992; Rodda et al.,1992). However, differential attractiveness of inani-mate lures may enable development of artificial luresthat target specific size classes of brown tree snakes.For control programs, it may be important to targetcertain age classes of brown tree snakes. Therefore,future studies should report the size classes of snakescaptured with various lure types in order to under-stand potential biases, and lure developers should de-velop lures that target specific size classes of snakes.

Ack11ou~1dgments.-We thank M. A. Linnell, M. E.Pitzler, D. Rodriguez, H. Hirsh, and D. S. Vice for lo-gistical assistance on Guam. This project was con-ducted with funds provided to the National WildlifeResearch Center and the Department of Biology, Col-orado State University, through the U.S. Departmentof Defense Legacy Project Number 1281 Develop-ment of Chemical Control Methods for Brown TreeSnake Management . All trapping was performedwith the approval of National Wildlife Research Cen-ter and Colorado State University animal care and usecommittees.

ARNOLD, . J 1993. Foraging theory and prey-size-predator-size relations in snakes. 111 R. A. Seigeland J T. Collins (eds.), Snakes Ecology and Behav-ior, pp. 87-115. McGraw-Hill, Inc., New York.

CHISZAR, ., K FOX,AND H. M. SMITH.992. Stimuluscontrol of predatory behavior in the brown tree

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336 SHORTER COMMUNICATIONSsnake (Boiga irregularis) IV. Effect of mammalianblood. Behav Neural Biol. 57:167-169.

- M. DUNN, N D H. . SMITH.993. Responseof brown tree snakes (Boiga irregularis) to humanblood. J Chem. Ecol. 19:91-96.

- M. DUNN, N D H. M. SIIITH.Predatory be-havior of brown tree snakes (Boign irregularis): lab-oratory studies of chemical attractants. 111G. Rod-da, Y Sawai, D. Chiszar, and H . Tanaka (eds.),Problem Snake Management Habu and BrownTree Snake Examples, pp. 26G282. Cornell Univ.Press, Ithaca, New York. 111 press.

- W. LUKAS,ND H. M. SMITH.997. Responseto rodent saliva by two species of rodentiophagoussnakes. J. Chem. Ecol. 232329436,

F ~ m s , . H. HISZAR,., N. J SCOTT JR.,A N D B. E.SMITH. 989. Trapping Boiga irregularis on Guamusing bird odors. J Herpetol. 23:189-192.

GREENE,. W. 1989. Ecological, evolutionary, and con-servation implications of feeding biology in oldworld cat snakes, genus Boiga (Colubridae). Proc.California Acad. Sci. 46:193-207.

1997. Snakes: the Evolution of Mystery in Na-ture. Univ. California rress, Berkeley.

KIRBY,. N. 1993. Advar.ced Data Analysis with Sys-tat. Van Nostrand Reinhold, New York.

LINNELL,. A,, R. M. ENGEIIAN, . E. PITZLER,. 0WATTON, G. F WHITEHEAD,N D R. C. MILLER.11press. An evaluation of two designs of stampedmetal trap flaps for use in operational trapping ofbrown tree snakes (Boign irregularis). The Snake.

O n , R. L. 1993. An introduction to statistical methodsand data analysis. Duxbury Press, Belmont, Cali-fornia.

RODDA, . H. 1992. Foraging behaviour of the browntree snake, Boiga irregularis. Herpetol. J 2:110-114.

- T. H. F ~ l n s .992. Sampling techniques for anarboreal snake, Boiga irregularis. Micronesica 25:2340.

- A N D P. J CONRY.992. Origin and populationgrowth of the brown tree snake, Boign irregularis,on Guam. Pacific Scl. 46:46-57.

- D. CHISZAR,N D T. H. FRITTS. 997. The dis-appearance of Guam's wildlife. Bioscience 47:565-574.

- T. H. FRITTS, . J MCCOID, N D E. W. CAMP-BELL,11. 111 press. An overview of the biology ofthe brown tree snake, Boign irregularis, a costly in-troduced pest on Pacific Islands. 111 G. Rodda, YSawai, D. Chiszar, and H. Tanaka (eds.), ProblemSnake Management Habu and Brown Tree SnakeExamples, pp. 74-125. Cornell Univ Press, Ithaca,New York.

SAVIDGE,. A. 1987. Extinction of an island forest avi-fauna by an introduced snake. Ecology 68:660468.- 1988. Food habits of Boiga irregularis, an intro-

duced predator on Guam. J. Herpetol. 22:275-282.SHINE, . 1991. Strangers in a strange land: Ecology

of the Australian colubrid snakes. Copeia 1991:120-131.SHIVIK,A. 1998. Brown tree snake response to visualand olfactory cues. J Wildl. Manage. 62:105-111.

- A N D L. CLAR K. 997. Carrion-seeking inbrown tree snakes: importance of olfactory and vi-sual cues. J. Exp. Zool. 279:549-553.

- A N D 111 press. The development of che-

mosensory attractants for brown tree snakes. 111R.E. Johnston, D. Miiller-Schwarze, and P. Sorensen(eds.), Advances in Chemical Signals in Verte-brates. Plenum Press, New York.

U. S. DEPT. F AGRIC. 996. Brown tree snake controlactivities on Guam. Environmental assessment,U.S. Department of Agriculture, Washington D.C.

Accepted: 20 January 1999.

journal o Heryetiiloxq Vol. 33 KO.2 pp. 33633 9 1999Copy r~gh t 999 Sooety for the Study of Amp hib~ ans nd Reptiles

Reproductive C ycle o the TropicalNight Lizard epidophyma pajapanensisrom Veracruz Mexico

FAUSTOR. MENDEZ-DEA CRUZ, ' MARICELAVILLAGRAN-SANTARUZ,~WALDO ERNANDEZ-GA-LLEGOS,~OKMA . MANRIQUEZ-MoRAN,~N D FELIPEJ RODR~GUEZ-ROMERO~,Laborntorio fi Herpetologin, De-partarrlerlto r Zoologin, lnst itu to de Biologia, UlliiersidadNacional Autd nom a de MPxico, A.l? 70-1 53, C.l? 045 10,Mixico 20, D. F M h i c o 2Laboratorio Biologia de In Re-produccidn A~limal,Focultad de Ciencins, U~liiersidadNa-cionnl Autdrlorrla de Mh i c o , A.l? 70- 515 , C.l? 04510, M4.r-ico 20, D. E M h i c o .

The family Xantusiidae is represented by the vivip-arous genera Xantusia, Lepidoplynla, and Crlcosaura(Crother et al., 1986). Only the reproductive biologyof Xnntusin ziigilis (Miller, 1948; Zweifel and Lowe,1966) and X. riiersinna (Goldberg and Bezy, 1974) havebeen studied in detail. In both species, reproductiveevents (gametogenesis, mating, ovulation, gestation,and parturition) occur during spring and summer, butslightly earlier in X. riiersiana. There is no informationon the reproductive biology of Cricosaura, and infor-mation about the reproductive biology of Lepidoylynlnis limited. According to Telford (1970) only parthe-nogenetic females of L. fimimnculnturr~were found ina population from Panama. Gestation takes place inJanuary and females give birth from April to earlyJune; litter sizes varied from one to five young. InMexico, Alvarez-del Toro (1982) found that L. fimli-rr~aculatumgive birth from five to eight young in Juneor July. In L. tuxtlm, gestation is initiated in February,parturition occurs by late June or July, and litter sizevaries from three to six (Greene, 1970; Castillo-Ceronand Lopez-Gonzalez, 1990). The present study de-scribes the reproductive biology of Lepidoplyrna yaja-p m m s i s , a night lizard that inhabits the Mexican trop-ics, and that exhibits asynchrony in the reproductivephenology of females and males.

The study area, Estacion de Biologia Tropical LosTuxtlas , is located in Veracruz, Mexico (95 04'W,18 36'N), between 300 and 400 m, and the vegetationis a tropical rain forest (Gonzalez-Soriano et al., 1997).Temperature variation through the year is low: the dryseason occurs in the spring, and the rest of the yearis wet (Fig. 1).A total of 62 adults (29 females and 33 males), and