28th Annual Meeting and Symposium of the
Desert Tortoise Council, February 21-23, 2003
Abstracts

STUDENT PAPER

Solar Absorptance of the Carapace Appears to Have Little Influence on the Thermal Biology of Desert Tortoises

Kenneth Nussear and C. Richard Tracy
University of Nevada, Program in Ecology, Evolution and Conservation Biology, and Biological Resources Research Center, Reno, Nevada

There is great variation in the carapace color of individual tortoises, permitting animals to range from very light to very dark coloration. Coloration should influence the amount of the incident solar energy that is absorbed (i.e. the absorptance) by the animal, which should, in turn, influence the thermal energy balance, and therefore the behavior of the animal. To understand the relative importance this mechanism of energy exchange, we manipulated absorptance to solar radiation of desert tortoises by painting the carapace with very reflective or very absorptive paints (only non-toxic tempura paints were used). This caused the animals to have absorptances to solar radiation that were both greater than and less than that naturally occurring. Animals were placed in outdoor enclosures that provided full exposure to the sun, and a burrow, which could be used as a shelter from sun exposure. We compared the amount activity time, the times of day that animals were active, rates of heating, and body temperatures of treatment tortoises with light and dark paints to unpainted tortoises. Surprisingly neither light nor dark tortoises differed in the amount of time in which they were active relative to naturally colored tortoises. Neither did they heat or cool at different rates. In addition, all tortoises, regardless of their colors, experienced similar body temperatures. These results appear to be counterintuitive when considering only heat transfer between the animal and its environment. However, the explanation for this surprising result may be found in regulation of heat transfer within the animal. We suggest that the carapace may function as a regulatable insulator for heat transfer within the tortoise thus regulating warming due to incident solar radiation. We observed tortoises behaving in a manner consistent with this hypothesis. Tortoises with higher absorptances frequently covered their carapace with soil, which may act to shield the carapace from excessive solar radiation. These results suggest that tortoises likely have physiological and behavioral mechanisms to regulate heat exchange with the environment so as to hold body temperatures within tolerable limits. Coloration of the tortoise carapace may play a lesser role in the thermal ecology of this species than previously thought.