The Thermal Dependence of Lizard Behaviour

New techniques were utilized to measure burst speed and distance running capacity in six species of lizards and to examine the thermal dependence of these behavioural capacities. These behaviours were repeatable among groups of animals within a species, within a group during the experiments, and among individuals of a group. Burst speeds averaged 130 to 150 m/min for active species, with some individuals exceeding 200 m/min. Total distances run during 2 min averaged 60 to 70 m in species with the greatest stamina. Thermal dependence of these behaviours was low or absent (Q l o < 1.5) over the normal range of active body temperatures, and performance was not necessarily maximal a t normally experienced body temperatures. These processes have much less thermal dependence than do most physiological processes and suggest adaptations to maintain functional behavioural capacity over a broad range of body temperature. Behaviour is the result of complex and integrated physiological processes. These involve numerous individual events in the central and peripheral nervous systems and in the skeletal musculature, all of which are dependent upon biochemically controlled rate processes. A primary external factor controlling these physiological processes is the temperature at which they occur. Most biochemical and physiological reactions have a pronounced thermal dependence and double or triple in rate whenever temperature is increased 10 C (written Q I o = 2-3) (Precht et al. 1955; Prosser 1973). If quantitative aspects of behaviour were similarly dependent upon body temperature, the consequences for performance capacities would be striking. An animal with a body temperature of 20 C would be able to run only 10 to 25 % as fast as an animal with a temperature of 40 C. Homeothermic animals, by physiologically stabilizing body temperature, avoid these thermal effects. Poikilotherms, however, must contend with varying temperatures. The influence that thermal variation exerts on behavioural performance in these animals has received little attention, in spite of the fact that it might be expected to be a significant factor. In the absence of data, predictions concerning the thermal dependence of behaviour in poikilotherms are not particularly easy. On one hand, the general thermal dependence of most physiological reactions suggests that behaviour should have a similar dependence. On the other, one might hypothesize that the selective importance of appropriate behaviour is great enough to have resulted in the evolution of adaptations that minimize the influence of body temperature on behavioural processes. In the latter case, the performance capacities of a poikilothermic animal would be relatively constant over a range of body temperatures. Experiments addressing the thermal dependence of behaviour would seem to constitute a rewarding line of investigation. Lizards are particularly interesting animals in regard to an examination of the influence of body temperature on behaviour. Some species live and forage under cover and experience little diurnal temperature variation. Others bask in the sun during the day and regulate their body temperatures at high levels by behaviour (Cowles & Bogert 1944; Cloudsley-Thompson 1971). These latter animals may undergo daily cycles in body temperatures of 25 C or more. In addition, they must expose theniselves when body temperature is still low in order to heat to preferred thermal levels. Consequently, lizards as a group exhibit considerable diversity in thermoregulatory pattern, and interspecific examinations may be expected to reveal the extent to which thermal compensation of behaviour is possible and appropriate. Another feature of saurian thermobiology of interest is the functional significance of preferred body temperature. Behaviourally regulated temperatures in sun-basking animals are maintained with considerable precision and are fairly constant within genera (Cowles & Bogert 1944; Brattstrom 1965). Given the stability of these thermal levels, it has generally been assumed that optimization or maximization of function occurs at preferred or field active body temperatures in lizards. These functional correlates have been elusive in laboratory experiments (see reviews by Dawson 1967, 1975). A central question for the thermobiology of reptiles is the extent to which behavioural capacities are maximized a t body temperatures occurring during normal activity. 753 BENNETT: TEMPERATURE AND ACTIVITY IN LIZARDS The behavioural capacities of lizards are strongly limited by their metabolic support systems. They cannot attain or sustain high levels of oxygen consumption. Consequently, any activity greater than a slow walk must be fuelled by non-sustainable anaerobic metabolism (Bennett & Licht 1972; Bennett & Dawson 1976; Bennett 1978). They exhaust readily at even moderate speeds and frequently undertake only burst activity of short duration. Rapid escape behaviour is characteristic of these animals when pursued. The following experiments measure performance during two modes of running behaviour: burst speed and distance capacity. Burst speed as examined here is the average velocity attained by running from a standing start over a distance of 2 m. This measure is realistic in approximating an escape attempt under natural conditions, in which an animal runs to cover over comparably short distances. Distance capacity is measured as the total distance run during 2 min of constant pursuit. It is used as a measure of work capacity over a relatively short but ecologically relevant time period to indicate stamina during long pursuit, territorial defence, or courtship. These are measurements of the greatest capacity for activity that these lizards possess and consequently set an upper limit on the quantitative aspects of the behaviour of these animals. The thermal dependence of these factors and their correlation with preferred body temperature are measured in six species of small lizards of diverse thermal preferenda. Materials and Methods The following lizards were used in this study. All are adult animals of mixed sexes, except as indicated. Date indicates month of running experiment. Cnemidophorus murinus; 14 animals, 2 mass = 49.4 g (24.8-82.7 g), November. Dipsosaurus dorsalis, 20 animals (juveniles and adults), 2 mass = 29.8 g (12.7-57.0 g), August; 40 animals, 2 mass = 38.3 g (27.3-50.4 g), October. Eumeces obsoletus, 6 animals, 2 mass = 25.6 g (20.0-33.4 g), June. Gerrhonotus multicarinatus, 12 animals, 2 mass = 15.3 g (6.0-30.4 g), October. Sceloporus occidentalis, 17 animals, 2 mass = 9.2 g (6.4-12.5 g), June; 14 animals, 2 mass = 9.5 g (6.613.1 g), July; 40 animals, 2 mass = 13.8 g (10.9-18.4 g), October; 12 animals, 2 mass = 11.3 g (5.6-17.9 g), December. Uma inornata, l l animals, 2 mass = 19.3 g (9.1-32.8 g), September. Cnemidophorus is an actively foraging herbivore in an open thorn scrub habitat and has a high body temperature characteristic of the genus. Dipsosaurzis is also a high-temperature herbivore, living with Uma in sandy deserts. Sceloporus is a common insectivorous lizard in oak-woodland and chapparal habitats. Eumeces and Gerrhonotus are reclusive species, the former living under rocks in the Great Plains area and the latter foraging under the leaf litter and in vegetation. Cnemidophorus were originally collected in Bonaire, Netherlands Antilles, and were maintained in the laboratory for several months prior to running. Eumeces were collected in Kansas and all other species were collected in Southern California (California Scientific Collectors Permits No. 1181-1977 and 1067-1978). Body temperatures selected under laboratory conditions and obtained from animals of these species active in the field are reported in Table I. Data on the Eumeces and Gerrhonotus used in this study were obtained in a radiant-heated sand Table I. Body Temperatures of Lizards in Thermal Gradients and Active in Natural Environments