Seasonal changes in thermoregulation by the frillneck lizard, Chlamydosaurus kingii, in tropical Australia

Ecology, Jan, 1995 by Keith A. Christian, Gavin S. Bedford

In addition to the studies listed in Table 1, investigations have revealed additional explanations for seasonal shifts in [T.sub.b]'s other than preference shifts or environmental limitation. Thermoconformity results in seasonal differences in the [T.sub.b]'s of Anolis gundlachi (Hertz 1992a, Hertz et al. 1993) simply as a passive result of seasonal differences in [T.sub.e]'s. Other Puerto Rican anoles also have [T.sub.b]'s that are slightly lower in January compared to August (Hertz 1992a, b, Hertz et al. 1993). The January [T.sub.b]'s are not limited by the thermal environment in an absolute sense, as evidenced by some [T.sub.b]'s in that month being above the set-point range ([ILLUSTRATION FOR FIGURE 2 OMITTED], Hertz et al. 1993). However, the availability of microhabitats with warm [T.sub.e]'s is lower in January (Hertz 1992a, b, Hertz et al. 1993), suggesting that the cost of thermoregulating (Huey and Slatkin 1976) at higher [T.sub.b]'s in the cooler season may result in lower [T.sub.b]'s. Thus, the relative availability of space and/or time needed to achieve the higher [T.sub.b] in the cooler season may result in thermoregulation at a lower [T.sub.b] although the thermal environment is not limiting in the absolute sense (Tracy and Christian 1986).

In addition to the lizards in Table 1, a few other reptiles are known to have seasonal differences in field [T.sub.b]'s. The tortoise Gopherus berlandieri is active at lower [T.sub.b]'s in spring compared to summer (Judd and Rose 1977), but whether or not the spring [T.sub.b]'s are limited by the thermal environment is not known. Seasonal shifts have been documented for snakes from the laboratory (Scott and Pettus 1979) and from the field (Lillywhite 1987 and references therein, Shine and Lambeck 1990). As previously noted (Lillywhite 1987, Peterson 1987), most of the field studies indicating seasonal differences in snake [T.sub.b]'s have not distinguished between preference shifts and limitations of the thermal environment. Furthermore, some presumed seasonal differences in snakes may be an artifact of sampling animals that are cooler than equilibrium conditions because exposed, immobile snakes are more easily captured (Lillywhite 1987, Peterson 1987). However, during years of low food availability, the Australian blacksnake Pseudechis porphyriacus has summer [T.sub.b]'s that are significantly cooler than during spring (Shine and Lambeck 1990). This represents a preference shift given that the lower [T.sub.b]'s are in the warmer season.

The freshwater crocodile Crocodylus johnstoni exhibits seasonal differences in [T.sub.b]'s, and biophysical analysis indicates that they are not limited by the thermal environment during any season (Seebacher 1993). All of the species of lizards that definitely have a seasonal shift in preferred [T.sub.b] in the field, as well as the freshwater crocodile, are tropical species. However, the lack of information for temperate species (Table 1) precludes a conclusive comparison between tropical and temperate species at this time with respect to seasonal shifts in field [T.sub.b]'s.