Amphibolurus Muricatus):Anagamidlizard with Temperature-Dependent Sex Determination
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DISPLAYRATEANDOPPONENT ASSESSMENT INTHE JACKYDRAGON (AMPHIBOLURUS MURICA TUS): ANEXPERIMENTALANALYSIS by TERRY J. ORD1,2) and CHRISTOPHER S.EVANS 3,4) (1 AnimalBehaviour Laboratory, Department of Biological Sciences, Macquarie University; 3 Departmentof Psychology,Macquarie University) (Acc.28-VIII-2003) Summary Honestsignals allow animals to assess an opponent without the injury risk of ghting.Play- backexperiments have shown that call rate is an important parameter in theacoustic signals ofseveral taxa. Here we describe an analogous study of a visualdisplay. Male Jacky drag- ons, Amphibolurusmuricatus ,respondto intruding conspeci cs witha stereotypedseries of motorpatterns, including a push-up.These displays are typically given in bouts and there issubstantial individual variation in inter-bout interval. In a recentvideo playback experi- ment,we found that the display rate depicted in two sequences appeared to in uence the typeof signalresponse evoked. The present study was designed to systematically examine theimportance of thiscue, while controlling opponent morphology and all other aspects of signalstructure. Male lizards were presented with vedigital video stimuli, each depicting thesame male conspeci c, but with inter-bout intervals that varied over the full natural range. Theduration of aggressivepush-up responses changed signi cantly as a functionof stimulus 2) Correspondingauthor: Department of Biology, 1001 E. 3 rd Street,Jordan Hall 142, Indiana University,Bloomington, IN 47405,USA; e-mailaddress: [email protected] 4) WethankR.A. Petersfor assistance in animal capture and husbandry, W .McTeggand N.Lambertfor additional support in animal care and A.J. T aylorfor statistical advice. T .J.O. wassupported by a MacquarieUniversity postgraduate award, the Macquarie University postgraduateresearch fund, the Australian Geographic Society, a SigmaXi Grant-in-Aidof Researchand Rufunsa Technology Services; and C.S.E. byanAustralian Research Council grant.The research described here was covered by Animal Research Protocol No. 98036, approvedon 9 th October1998 by the Animal Care and Ethics Committee of Macquarie University.Collection and housing of animals complied with NSW NationalParks & Wildlife Serviceguidelines under permit B1920. Research was conducted in partial ful lment of a doctoralthesis for T .J.O.at MacquarieUniversity. © KoninklijkeBrill NV ,Leiden,2003 Behaviour140, 1495-1508 Alsoavailable online - 1496 ORD & EVANS properties.Males tracked playback display rate from unusually slow to the population mean, butdisplayed less to stimuliwith unusually fast rates. The interval between display bouts is hencesalient to conspeci cs, independent of othercharacteristics. These ndingsare consis- tentwith the idea that energetically-expensive dynamic visual signals play a rolein opponent assessment. Introduction Inmany taxa, there is intense competition overterritories, mates, andother resources.When contests degradeinto physical combat,the risk ofinjury canbe high.However, most encountersdo not escalate to this level (May- nardSmith &Price, 1973;Parker, 1974) because there are mechanisms for opponentassessment that allow animals to avoid ghtingwhen they are un- likely to win(Zahavi, 1977; Clutton-Brock & Albon,1979). Physical attributes suchas the overall size ofanopponent, or the dimen- sions ofweapons such as hornsor antlers, canplay a role in determining contest outcome.These properties also providecues forestimating poten- tial ghtingability ( e.g. Geist, 1987;Johnson, 1988). The structure ofsome signals similarly facilitates opponentassessment —the inverse relationship betweenbody mass andfundamental frequency is afamiliar example(Davies &Halliday,1978; Arak, 1983; Ryan & Brenowitz,1985; Robertson, 1986; Given,1987; Wagner, 1989; Gouzoules & Gouzoules,1990; Hauser, 1993). Inanuran amphibians, potential opponentsproducing low-frequency calls are morelikely to beavoided (Davies &Halliday,1978;Arak, 1983; Robert- son,1986). Other signals, suchas colourpatches orbadges,can reveal the dominancestatus ofa rival andhence provide an indirect indication oftheir ability to competefor resources (Rohwer,1975, 1977; Rohwer & Ewald, 1981;Jä rvi &Bakken,1984; Hover, 1985; Mø ller, 1987;Rø skaft &Ro- hwer,1987; Hö glund & Lundberg,1989; Thompson & Moore,1991; Pryke et al., 2001). Dynamicsignals are likely to beparticularly useful indicators of ght- ingability becausethey can reveal moment-to-momentvariation in condi- tion andmotivation. High signal rates haveconsistently beenfound to corre- spondwith highlevels ofaggression(Carpenter, 1961, 1963; Clutton-Brock et al.,1979; Stewart &Rand,1991; Alberts et al.,1992; Adhikerana & Slater, 1993;Langemann et al.,2000). Signal productionis oftenenerget- ically costly (Rand& Rand,1976; Clutton-Brock et al.,1979;Bennett et al., DISPLAY RATEANDOPPONENTASSESSMENT 1497 1981;T aigen& Wells, 1985;Marler &Moore,1988; V ehrencamp et al., 1989;Cherry ,1993),can increase predationrisk (Green,1990; Jakobsson et al.,1995)and reduces time available forother activities suchas foraging (Radesäter et al.,1987)or thermoregulation (Rand & Rand,1976), so only individuals better able to incursuch costs canafford to signal at highrates. Wetherefore expect signal rate to providehonest information about condi- tion.Similar argumentshave been proposed to explainstrong correlations betweensignal rate andfemale mate choice(Gibson & Bradbury,1985; Got- tlander,1987; Radesä ter et al.,1987;Alatalo et al., 1990 Passmore et al., I 1992;Cherry, 1993; Collins, 1994;Kodric-Brown & Nicoletto, 1996;Y ang &Greeneld, 1996; Gray, 1997). Comparativeanalyses suggest that iguanianlizards haveevolved com- plexvisual signals to improveopponent assessment underconditions of high male-male competitive intensity (Ord et al.,2001).The Jacky dragon, Am- phibolurusmuricatus ,is amediumsized, semi-arboreal Australian agamid. Communicationis primarily visual andinvolves push-up, arm-wave and tail displays (Carpenter et al.,1970;Peters et al.,2002).The presence of male- biased sexual size dimorphism(Harlow & Taylor,2000), evidence of territo- riality in the eld (Harlow& Taylor,2000) and the formationof dominance hierarchies in captivity (Carpenter et al.,1970)all suggest that opponentas- sessment is likely to beimportant. Ina recent videoplayback study (Ord et al.,2002),male Jackydragons wereshown two sequences depicting male conspecics, whichdiffered in bothbody size andaggressive display rate. Thevideo male with the higher display rate elicited signicantly moreslow arm-waves,which are putative appeasementsignals, eventhough the male depictedwas the smaller ofthe two (Ord et al.,2002).Subjects also trackedstimulus display rate, producing moreslow arm-waveswhen they saw the videoopponent display vigorously, andswitching to aggressive push-upswhen the videolizard displayedless frequently(Ord et al.,2002).These observations imply that Jackydragons are sensitive to variation in display rate andthat this attribute might beim- portantin opponentassessment. Inthe present study,we evaluatedthe importanceof display rate directly byusingvideo playback to makeprecisely-controlled changesin the interval betweensuccessive push-upbouts. This approacheliminated variation in otheropponent attributes andallowed us to isolate the effects ofsignalling behaviour.Manipulations werebased upon estimates ofnatural variation 1498 ORD & EVANS in signal behaviourand were scaled in standarddeviations ofinter-bout interval. Wepredictedthat subjects wouldattempt to match stimuli with slow oraverage display rates, butthat this wouldnot be possible with the stimuli depictingunusually high display rates. Methods Subjects Subjectswere 36 sexually-mature male Jacky dragons. Lizards were wild-caught between March1999 and November 2000 in urban bushland surrounding Botany Bay, Sydney, Aus- tralia.They ranged in size from 18.0 g, 83mm SVL (snout-ventlength) to 46.5g, 95 mm SVL. Eachanimal was de-wormed shortly after capture and then held in aglassaquarium (36 cm wide 92 cm long 38cmhigh) for a minimumof 4weeksto habituate to captiv- £ £ ity.Housing was designed to match as closely as possible characteristics of the natural habitat. Aquariahad sand substrates and contained branches and foliage to providerefuges and places forbasking. Cardboard sheets were placed between adjacent tanks to screen males from nearbyconspeci cs. Lizards were maintained on a lightregime of 14 :10h light: darkcy- cle(lights on at 06:00), corresponding to midsummer.In addition, heat lamps (125W Philips Spotone)and UV lamps(300W Osram Ultra-V italux)were suspended above the aquaria. Theseallowed behavioural thermoregulation and ensured access to UV lightto prevent vita- minde ciency. Room temperature was maintained at approximately26 ±C,andthere was a temperaturegradient from this value to 30-34 ±Cmeasuredon perchesdirectly beneath heat lamps. Jackydragons are insectivores and will consume only live prey. Lizards were fed twice weeklywith live crickets dusted with vitamin supplements (RepCal); this also provided en- vironmentalenrichment, allowing animals to hunt invertebrates as they would in the wild. Waterwas available ad lib ina smallbowl and aquaria were lightly sprayed to provide addi- tionalmoisture when humidity was low. All lizards were healthy at theend of the experiment andwere retained for further study. Oneweek prior to experiments,lizards were moved into large pens (64 cm wide 150 cm £ long 120cm high) constructed of aluminium frame and rigid plastic sheeting. The sides £ andback