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

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

Lizard characteristics used in the model included a mass of 400 g, and a snout-vent length of 23 cm. This corresponds to a large female or a near-average male. The absorptivity of short-wave solar radiation was measured using an integrating reflectance sphere (Christian and Tracy 1981, Porter and Tracy 1983). The mean values of absorptivity for the dorsal surface of frillneck lizards was 79% (K. A. Christian, S. T. Shannahan, and G. S. Bedford, unpublished data).

RESULTS

Microclimatic conditions during the wet and dry seasons are shown in Fig. 1. The dry season is cooler, particularly overnight. Although cloud cover is greater during the wet season, the seasonal sun angles resulted in higher midday levels of solar radiation for the days measured during this season [ILLUSTRATION FOR FIGURE 1 OMITTED]. However, the clear dry season days nevertheless receive considerable solar energy, which results in high midday temperatures for soil and tree trunks in the sun. Both soil surface temperatures and tree trunk temperatures are given in Fig. 1 to illustrate seasonal differences in the thermal environment, but only tree trunk temperatures were used in the calculations of [T.sub.e]'s.

Fig. 2 shows the mean [T.sub.b]'s, as measured by telemetry, as well as the [T.sub.e]'s for lizards in full shade, full sun on a horizontal surface, and in full sun normal to the incident radiation for the wet and dry seasons. The figure shows that [T.sub.b]'s during the dry season (from 13 lizards) are lower than those during the wet season (from 9 lizards). Although this was expected for overnight, early, and late periods of the day, the pattern continues during the midday period (defined as the hours between 1100 and 1500) when animals could reach the highest [T.sub.b]'s. During midday of the wet season the frillneck lizards had a grand mean [T.sub.b] of 36.7 [degrees] C, and during the dry season they had a grand mean [T.sub.b] of 32.8 [degrees] C. The wet season [T.sub.b]'s are similar to those previously reported for frillneck lizards during this season (Shine and Lambeck 1989). A repeated-measures ANOVA was used to compare [T.sub.b]'s during the midday hours with the repeated measure being the temperatures recorded across the midday period for each individual. This analysis indicates that midday [T.sub.b]'s during the dry season are significantly lower than those of the wet season ([F.sub.1,17] = 35.3, P [less than] 0.0001).

A comparison of the measured [T.sub.b]'s to the calculated [T.sub.e]'s for the dry season [ILLUSTRATION FOR FIGURE 2 OMITTED] indicates that lizards during this season could easily, by being in the sun, achieve [T.sub.b]'s higher than those that were measured. Patches of sun are bigger and more numerous during the dry season because of the semi-deciduous nature of the trees in the savanna woodland (Brock 1988; G. Duff, unpublished data). Thus, although environmental temperatures are cooler in the dry season, there are more and bigger patches of sun, and the thermal environment does not limit the animals to lower [T.sub.b]'s during this season.