Thermal Dependence of Locomotion and Aggression in a Xantusiid Lizard
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Herpelologicn, 48(3), 1992, 271-279 O 1992 by The Herpetologists' League, Inc. THERMAL DEPENDENCE OF LOCOMOTION AND AGGRESSION IN A XANTUSIID LIZARD WILLIAMJ. MAUTZ', CHRISTOPHERB. DANIELS~,~, AND ALBERT F. BENNETT' 'Department of Community and Entiironmental Medicine and *Department of Ecology and Euohtionary Biology, University of California at Irtiine, Irvine, CA 92717, USA ABSTRACT: The thermal dependence of sprint speed, exertion distance and time, and aggressive response was examined in the island night lizard, Xantusia riuersiana, over a body temperature range of 10-35 C. Critical thermal limits were 6.6 and 39.0 C. Thermal optima for sprint speed and distance approximated preferred body temperature (28.3 C) and declined at higher or lower body temperatures. Exertion time was largely independent of body temperature, and lizards ran for about 70-90 s. Speed during exertion sprinting declined exponentially with increasing time and distance covered. Maximum oxygen consumption at 30 C (V,,,,,) occurred immediately following burst sprinting and was greater than values recorded for other species of lizards of similar body mass; factorial aerobic scope relative to standard V,, was 29. Maximal aggressive responses occurred at low body temperatures which are suboptimal for sprinting, but aggressive behavior was always preceded by a sprint to exhaustion. Temperature and exercise state appear to be interacting factors influencing aggressive responses. Xantusia riversiana is a diurnal but reclusive lizard with relatively low preferred body temper- ature and low temperature optimum for sprinting. In the dense vegetative scrub and rocks that the lizards inhabit, sprint escape from predators is the initial response at all activity body temper- atures. Sprint performance also represents capacity for struggle against restraint, and the increased aggressive response of exhausted lizards at low body temperatures may be a defensive response compensating diminished capacity for struggle at low body temperatures. Key words: Aggression; Lizard; Oxygen consumption; Speed; Sprint performance; Temperature; Xantusia riversiana ALTHOUGH active defenses of ecto- (Bustard, 1968; Daniels, 1984, 1985). The therms against predation are influenced by thermal dependence of individual defen- body temperature, maximum perfor- sive tactics thus varies such that o~timal mance of defensive actions may not be thermal conditions for one mechanism do realized at or near mean preferred body not necessarily overlap those for another, temperatures (T,) (Bauwens and Thoen, and the complete repertoire of defenses 1981; Bennett, 1980; Daniels, 1984; Hertz provides effective responses to attack over et al., 1982). For example, lizards at cold a broad range of body temperatures. body temperatures have reduced loco- The purpose of this study was to ex- motor capacity for escape, but they may amine the effect of body temperature on display more aggressive defensive behav- maximum sprint speed,-exertibn capacity ior, such as open mouth threats and biting (distance and time run to exhaustion under (Crowley and Pietruszka, 1983; Greene, pursuit), and aggression in the xantusiid 1988; Hertz et al., 1982), or take flight at lizard Xantusia riversiana. Xantusiid liz- greater predator approach distances (Rand, ards are secretive and confine activities to 1964). Tail autotomy is another important dense shelter. They are generally active by anti-predator defense in lizards, and tail day (Lee, 1974; Mautz, 1979a; Mautz and autotomy frequency is greater at body Case, 1974), sprint ably when exposed in temperatures both above and below T, their retreats, and bite furiously when grasped (Greene, 1988). Compared to oth- JP~~s~~~ADDRESS: Department of Physiology, er species of lizards that are active abroad School of Medicine, Flinders University of South Aus- in open habitats, xantusiids have relatively tralia, Bedford Park, S.A. 5042, Australia. low T, = 26-33 C in thermal gradients 272 HERPETOLOGICA [Vol. 48, No. 3 (Brattstrom, 1965; Kaufmann and Ben- tained on an LD 12:12 photoperiod at 28 nett, 1989; Mautz, 1979b; Mautz and Case, C by day and 20 C by night. Choice of 28 1974; Regal, 1968). The thermal depen- C was based on T, of X. riversiana in a dence of sprint speed in xantusiids might thermal gradient during the daytime ac- therefore be expected to facilitate perfor- tivity period (Mautz, 1979b).Lizards were mance at lower body temperatures. Fur- provided with water ad lib. and fed larval thermore, xantusiids have standard rates Tenebrio once a week and two days prior of oxygen consumption (v,,) which are to any experiments. All animals were re- only one-half to one-third those of lizards leased in excellent condition at the capture of other families (Mautz, 1979~).We mea- locality at the conclusion of the study. sured v,, during activity to examine Two types of locomotor tests were con- whether maximum V,, was also reduced ducted. Sprint speed measurements were and possibly limiting locomotor perfor- made in a linear track 0.1 x 1.5 m floored mance. with plastic lawn (Astroturf) and ruled with Xantusia riversiana inhabits three of the 0.25 m marks. Lizards were chased down Channel Islands off southern California. the track by gently prodding their tails. Although these lizards are not readily ob- Sprint trials were videotaped, and speed served abroad in the field, they move over each 0.25 m interval was determined among thickets of low scrub vegetation from slow motion and stop-action replays. with peak activity centered at midday We ran lizards two times in succession, (Fellers and Drost, 1991). They actively allowed 1 h to rest, and then ran them seek sources of heat (Regal, 1967, 1968), twice again. The fastest speed over any have a T, of 28.3 C in a thermal gradient, 0.25 m interval was used in subsequent and in the field on sunny days they have analyses. We measured exertion running body temperatures ranging from 15-32 C capacity in a circular track 0.20 m wide that are elevated an average of 5.7 C over with a lap distance of 3 m. Lizards were retreat site air temperatures (Mautz, chased around the track by an investigator 1979b). Xantusia riversiana ranges up to standing at the center and using a blunt 109 mm body length and 37 g (Goldberg prod. Endpoint was determined as failure and Bezy, 1974), and on Clemente Island, of the lizard to run further after 10 rapid individuals fall prey to native kestrels and taps on the tail with the prod. We mea- shrikes. Feral cats and pigs present on San sured time and distance run to this end- Clemente Island within the past century point. At the end of the exertion runs, X. also prey on these lizards, and X.riversiana riversiana frequently assumed defensive comprised 40% of the dietary items in a postures similar to those adopted by other sample of 10 cats from this island (R. Wil- species of lizards in advance of, or instead son and W. Mautz, unpublished observa- of, running (Hertz et al., 1982). The post- tions). The thermal dependence of escape run behaviors of the lizards were cata- behavior and aggressive response is likely logued and scored as follows: head up (1) an important component of predator de- or down (0), mouth open (1) or closed (0), fense for the lizards, and predator defense attempt to bite (1)or not (0). We summed is an important aspect of conservation bi- scores for the three possible conditions for ology for this narrowly distributed and each lizard. threatened species of Xantusia. Locomotor tests were performed in a temperature controlled room. On the day MATERIALSAND METHODS of an experiment, lizards were brought into Eleven X. riversiana were collected from the room (at test temperature) 4 h before San Clemente Island, Los Angeles County, running. Body temperature of lizards was California, (Memorandum of Understand- measured following each run. Tempera- ing, California Fish and Game, to the se- ture was set at a selected level for two days, nior author) and were returned to the lab- and sprint tests were done on Day 1 fol- oratory. They were held in individual cages lowed by exertion tests on Day 2 (Bennett, in a temperature controlled cabinet main- 1980). Initially, sprint performance was September 19921 HERPETOLOGICA 273 tested at 28 C on consecutive days to es- Statistical analyses were performed us- tablish a baseline of repeatable perfor- ing paired t-tests, repeated measures anal- mance. Xantusia riversiana did not exhib- ysis of variance, and regression analysis it a significant difference in performance (Dixon, 1985). Thermal performance between the first two days at this temper- breadths, B,, (the temperature range for ature. We randomly ordered five succes- performance exceeding 95% of maximum) sive test temperatures for sprint and ex- and B,, (temperature range for perfor- ertion runs (25, 10,20, 30, and 15 C), and mance exceeding 80% of maximum), and we made final sets of measurements at the optimal temperature, To (the midpoint warmest temperature, 35 C, and then again temperature of B,), for sprint speed, ex- at 28 C. On the final exertion test at 28 C, ertion distance, and exertion time (Hertz the time to complete each consecutive 3.0 et al., 1983) were determined by the meth- m lap was recorded for each lizard. For od of convex polygons (van Berkum, 1986). determination of thermal performance breadth and optimum, we measured crit- RESULTS ical thermal maximum (CT,,,) and mini- Xantusia riversiana ran immediately mum (CT,,,) from separate groups of upon release in the linear and circular race freshly captured lizards on San Clemente tracks at all temperatures tested, and the Island. Lizards were held in a plastic con- lizards did not engage in defensive or ag- tainer set in a water bath and warmed or gressive displays before running. In trials cooled predominantly by conductive and conducted at 28 C at the beginning and convective exchange at 1-2 C/min.