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

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

Insects are the major prey of frillneck lizards (Shine and Lambeck 1989), and during the dry season insects are less abundant than during the wet season (Churchill 1992). By lowering their midday [T.sub.b]'s, frillneck lizards lower their daily metabolic expenditure and water flux (Christian and Green 1994). If the lizards were to maximize the energy and water savings during the dry season they would remain in deep shade throughout the day in order to maintain the coolest [T.sub.b]'s possible. However, Fig. 2 suggests that they do not stay as cool as would be possible if they remained in the shade all day. The midday [T.sub.b]'s during the dry season average 4.9 [degrees] C warmer than [T.sub.e] in full shade. This could represent an inability of the lizards to remain in full shade given that the canopies of the Eucalyptus trees they inhabit are not lush, and the lizards may be partially exposed to solar radiation even if they attempt to stay on the shady side of a tree throughout the day. However, closer inspection of the [T.sub.b]'s and [T.sub.e]'s during the dry season [ILLUSTRATION FOR FIGURE 2 OMITTED] suggests that the animals are carefully thermoregulating at low, but not the lowest possible, [T.sub.b]'s during this season. For the first 2-3 h after sunrise their [T.sub.b]'s increase rapidly and on the same trajectory as [T.sub.e] in the sun, indicating that the animals bask rather than sit on the shady side of the tree. The early morning pattern of warming is similar for both seasons [ILLUSTRATION FOR FIGURE 2 OMITTED] except that the rate of warming slows from a linear increase in [T.sub.b] between the hours of 900 and 1000 during the dry season after the lizards have reached 30 [degrees] C, and the rate of warming decreases between 1000 and 1100 in the wet season after they have reached a temperature of [approximately equal to]34 [degrees] C. This difference in [T.sub.b]'s at which intensive basking stops is the same difference between the seasonal midday [T.sub.b]'s. The pattern of late afternoon cooling is also similar in the two seasons, and the [T.sub.b]'s during the last 2 h before sunset indicate that the animals are basking on the sunny side of the tree receiving the near-normal rays of the setting sun.

In addition to thermoregulating at a [T.sub.b] [approximately equal to]4 [degrees] C lower during midday in the dry season, the set-point range (as measured in the laboratory) also shifts downward during this season. Consequently, the midday [T.sub.b]'s in the field during each season tend to fall within the set-point range of the respective season [ILLUSTRATION FOR FIGURE 2 OMITTED].

The amount of time that an animal spends within the bounds of its set-point range expressed as a percentage of the time available for them to achieve [T.sub.b]'s within the set-point range can be used as an index of thermoregulation (K. A. Christian and B. W. Weavers, unpublished manuscript). This index represents the extent to which an animal exploits the available thermal environment, and it can be obtained from the information in Fig. 2 by comparing the amount of time [T.sub.b]'s and [T.sub.e]'s intersect the set-point range (see Huey et al. 1989 for a similar analysis). Once the early morning [T.sub.e] has reached the set-point range, the animal can remain within the range by shuttling between sun and shade until the late afternoon when the [T.sub.e] in sun falls below the set-point range. A high degree of exploitation indicates careful thermoregulation in an environment in which there are many sites that would result in sub-optimal [T.sub.b]'s. This would be the case in any relatively open habitat in which [T.sub.e] in the sun is greater than the set-point range, such as the savanna woodland inhabited [TABULAR DATA FOR TABLE 1 OMITTED] by frillneck lizards. This index alone would not indicate thermoregulation in a homogeneous thermal environment in which [T.sub.e]'s of most sites were within the set-point range, such as the heavily shaded forests inhabited by Anolis gundlachi (Huey 1982, Hertz 1992a, Hertz et al. 1993). The exploitation of the available thermal environment by frillneck lizards in the wet season is 96% assuming the animals bask in a horizontal position or 77% assuming they bask perpendicular to the sun. During the dry season the exploitation is 95 and 84%, respectively, for the two postures. These values indicate that in both seasons the lizards are thermoregulating carefully.