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

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

A thermal gradient was established in an air-conditioned room (4 x 4 m) with a series of heat lamps concentrated in one corner and one lamp near the center of the room. This provided a continuous gradient of floor temperatures that ranged from 22 [degrees] to 50 [degrees] C. The animals were placed in the gradient one at a time in the late afternoon. The lights, connected to an automatic timer, switched on at approximately the time of sunrise the following morning. Measurements of [T.sub.b] were made every 15 min in the manner described for field measurements from a laboratory [approximately equal to]30 m from the thermal gradient room.

Microclimate

Microclimate data were collected simultaneously with field telemetry measurements. Solar radiation was measured with a Middleton pyranometer in conjunction with a Middleton integrator. Environmental temperatures (air, soil surface in the sun, soil surface in the shade, tree trunk surface in the sun, and tree trunk temperature in the shade) were measured hourly. Air temperature was measured at a height of 2 m with a 30 gauge type K thermocouple with its sensing tip painted white to reduce radiation errors (Christian and Tracy 1985a). Surface temperatures were measured with a Model PM3 (Raytek, Santa Cruz, California, USA) infrared thermometer. Wind speed was measured with a miniature cup anemometer (Model RAMI, Rimco, BWD Precision Instruments, Mulgrave, Victoria, Australia) on some days and was qualitatively described on other days. The reflectivity of ground surfaces was measured by inverting the pyranometer and expressing the inverted measurement as a fraction of the upright reading (Christian and Tracy 1981).

Energy exchange model

Microclimatic measurements and lizard characteristics were combined in an energy exchange model used to calculate operative temperatures ([T.sub.e]) (Bakken 1981, 1992, Bakken et al. 1985), adapting a model designed for lizards that has been previously described and applied in studies of other lizards (Christian and Tracy 1981, Tracy 1982, Christian et al. 1983, 1984). Because the model has been detailed in these earlier reports, we will only describe the specific configurations used in this study.

[T.sub.e] values were calculated at hourly intervals for the two most extreme habitats: full sun and deep shade (Christian and Tracy 1981). Calculations in full sun were made for two conditions: assuming that the animals were horizontal (as if lying on the ground or on a horizontal branch), and assuming that the animals were normal to the sun. The solar radiation incident on an animal normal to the sun was obtained by adjusting the hourly pyranometer readings by Lambert's cosine law (Gates 1980). Lizards would be normal to the sun early and late in the day if they were perched vertically on the sunny side of a tree trunk. By shuttling and selecting sites of partial shade, we assume the lizards could achieve any [T.sub.b] between the extremes (Christian et al. 1983). The hourly means of the microclimatic data collected over several days of telemetry measurements in each season were used in the biophysical model.