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

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

INTRODUCTION

Some reptiles exhibit a seasonal shift in body temperatures both in laboratory thermal gradients (Ballinger et al. 1969, Mayhew and Weintraub 1971, Patterson and Davies 1978, Scott and Pettus 1979) and in the field (Mayhew 1963, McGinnis 1966, Pianka 1971, Deavers 1972, Cogger 1974, Huey and Pianka 1977, Judd and Rose 1977, Christian et al. 1983, Shine and Lambeck 1990, Magnusson 1993). An integrated approach involving both laboratory and field data is needed to determine the ecological and physiological significance of seasonal shifts in thermoregulation by ectotherms (Hutchison and Maness 1979). Field observations of body temperatures ([T.sub.b]) alone do not allow a distinction between a seasonal shift in thermal preference or the possibility that environmental conditions during a given season may force animals to be active with [T.sub.b]'s outside their preferred range (Christian et al. 1983, Peterson 1987, Van Damme et al. 1987). Thus, although some species of lizards have lower mean [T.sub.b]'s in the field during cooler seasons, this could be interpreted as either an inability of the animals to attain [T.sub.b]'s as high as their preferred temperatures during warmer seasons (McGinnis 1966), or as a shift in thermal preference. Seasonal shifts in thermal preference could be due to an acclimatization response (Mayhew and Weintraub 1971) to environmental temperatures and/or photoperiod, a response to reduced food availability, or the result of seasonal hormonal cycles (Heatwole and Taylor 1987). Biophysical models, in conjunction with microclimatic data, were used to determine the range of body temperatures that Galapagos land iguanas could attain during different seasons in order to explore an observed seasonal difference in midday [T.sub.b]'s (Christian et al. 1983). This species actively thermoregulates at a lower mean [T.sub.b] during the coolest season despite the possibility of attaining, for several hours of the day, the same mean [T.sub.b] as during the warmer seasons (Christian et al. 1983). Biophysical models were used to demonstrate that seasonal differences in field [T.sub.b]'s of male Lacerta vivipara were due to limitations of the thermal environment during cool spring months. A similar approach has been used to examine altitudinal differences in the thermal biology of the lizard Podarcis tiliguerta (Van Damme et al. 1989) and latitudinal differences in thermal preferences of the ornate box turtle Terrapene ornata (Ellner and Karasov 1993). As in the case with the Galapagos land iguana, both of these studies found that during midday the physical environment did not prevent the animals from attaining high [T.sub.b]'s.

Basic questions in respect to seasonal shifts in the field [T.sub.b]'s of ectotherms include: (1) Are the observed differences in [T.sub.b] due to active thermoregulatory shifts or to environmental limitations, and (2) If there is an active thermoregulatory shift, does it represent an acclimatization response or a response to the immediate environmental conditions? In addition to seasonal changes in the thermal environment, factors such as food and water availability, photoperiod, humidity, etc. also change simultaneously. Acclimatization may be a complex process that cannot be explored completely by a single experiment or set of observations, but a first step in understanding the nature of the acclimatization response is to determine whether or not seasonal shifts in [T.sub.b]'s manifest themselves in a simple thermal environment, such as a laboratory thermal gradient. If animals that are taken from the field during different seasons are immediately placed in a laboratory thermal gradient and show seasonal differences in selected [T.sub.b], this demonstrates a shift in preferred [T.sub.b] in the absence of other environmental cues. Although other factors may have been involved in the acclimatization process, such a result would at least indicate that the thermal preference is a product of an acclimatization process rather than a response to the immediate environmental conditions.

After establishing that a species exhibits different body temperatures during different seasons, then additional data must be collected from the field and the laboratory in order to address these two basic questions about the nature of the seasonal changes, An answer to the first question requires either biophysical models and microclimatic data (Porter and Gates 1967, Tracy 1982, Christian et al. 1983, Bakken 1992) or physical models of the animals that serve as operative temperature thermometers (Bakken and Gates 1975, Bakken et al. 1985, Peterson 1987, Bakken 1992, Ellner and Karasov 1993). The second question requires that animals be collected from the field during different seasons and their preferred [T.sub.b]'s measured in a laboratory thermal gradient. The first question related to seasonal shifts in the field [T.sub.b]'s of ectotherms was answered definitively for the Galapagos land iguana, Conolophus pallidus, but these iguanas could not be brought into the laboratory to address the second question. Lacerta vivipara did not show a seasonal shift in thermal preference in the laboratory, but they were unable to attain the preferred [T.sub.b]'s in the field during cool months (Van Damme et al. 1987). Before the ecological and physiological significance of seasonal shifts in field [T.sub.b]'s can be determined, more complete investigations from the field and laboratory are needed.