NUSSEAR, K.E.*; TRACY, C.R.; University of Nevada, Reno; University of Nevada, Reno: Influence of animal absorptance on the thermal biology of desert tortoises

Individual desert tortoises have markedly different shell coloration and pigmentation. Those differences can affect the animal�s absorptance to solar radiation across the entire solar spectrum (i.e. the UV, Visible, and near Infrared). Differences in absorptance result in different amounts of radiant energy absorbed by the animal�s carapace, which will influence the energy balance and behavior of the animals. To understand this energy exchange mechanism and the resulting influence on tortoise thermal biology and behavior, we manipulated absorptance to solar radiation of desert tortoises by painting their carapace with very reflective and very absorptive paints. This transformed the properties of animals, to have absorptances above, and below natural coloration. Animals were placed in semi-natural outdoor enclosures with identical thermal environments. We compared the amount activity time (vs. time spent in burrows), the times of day that animals were active and rates of heating, and body temperatures among animals in the different treatments with naturally colored animals, and operative temperature models in the same thermal environment. Surprisingly neither treatment group differed in the amount of time in which they were active, nor did they heat or cool at different rates. Tortoises experienced similar body temperatures, and were frequently observed covering their shells with soil. 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.