Australian Geckos Do Not Display Diel Variation in Thermoregulatory Behavior

Body temperature (Tb) influences almost every aspect of a reptile's physiology and behavior (for reviews, see Huey, 1982; Lillywhite, 1987). For example, Tb can alter both auditory capacity (Werner, 1976, 1983) and locomotory performance (Bennett, 1980, 1990), thus facilitating or impairing escape from and defense against predators (e.g., Christian and Tracy, 1981; Hertz et al., 1982; Goode and Duvall, 1989), prey capture (Greenwald, 1974), and prey handling (Van Damme et al., 1991). Energy available for allocation to growth and reproduction is determined by digestive and metabolic processes, both of which are temperature-dependent (Bennett and Dawson, 1976; Waldschmidt et al., 1986; Van Damme et al., 1991). In viviparous species, the development of offspring depends on the Tb of the gravid mother (Bull, 1980). Thus, Tb can have both direct and indirect consequences for fitness. For many physiological processes (e.g., locomotion, feeding, digestion), the rate of function is maximal over some range of TbS and decreases rapidly above and below this range (Huey, 1982). Field studies of thermoregulation (e.g., Peterson, 1987; Christian and Weavers, 1996) indicate that many reptiles maintain relatively high and constant TbS during activity. Presumably, thermoregulatory behavior allows animals to function over a range of TbS that is conducive to the requisite physiological processes (e.g., locomotion, digestion). However, field TbS do not indicate temperature preference. For many reptiles, tropical environments may provide stable thermal climates such that a narrow range of TbS may be achieved with little effort (Shine and Madsen, 1996). In other cases, individuals may accept TbS that are higher or lower than preferred temperatures due to constraints on their thermoregulation imposed by the environment (Huey and Slatkin, 1976). Clearly, field TbS cannot be strictly interpreted as measures of Tb preferences in reptiles. Preferred body temperature (Licht et al., 1966) has been used as an estimate of body temperature preferences in reptiles. Preferred body temperature (T,) is the Tb maintained by an organism free from all ecological constraints and is measured in a laboratory thermal gradient that provides a full range of equally accessible thermal environments. Measures of T, are useful because they indicate the temperature that n ganism tries to attain during thermoregulation in nature (Hertz et al., 1993). Integrated