Biology and Conservation of North American Tortoises

4 Thermoregulation and Energetics of North American Tortoises

4 The basic thermal biology of tortoises has been known for more than 120 years. Hubbard (1893) described the summer foraging of Gopherus polyphemus in Florida and noted that they emerged in midday between 1100 and 1400 at temperatures around 33°C, and foraged for an hour before retreating to their burrows “in the moist, cool sand.” In winter, “the gopher very rarely quits its burrow, and comes forth to feed only on the very hottest days at noon.” Hubbard also described the burrow and reported that temperatures were stable, generally remaining above 23°C in winter and below 26°C in summer. Cowles and Bogert (1944) quantified thermal requirements of desert reptiles; and in the 1960s and 1970s, a new wave of studies in physiological ecology examined thermoregulation in tortoises (e.g., McGinnis and Voigt 1971, Voigt 1975, Voigt and Johnson 1977, Douglass and Layne 1978) and other reptiles. One hundred years after Hubbard, Zimmerman et al. (1994) quantified thermoregulation of G. agassizii and described the relationship between operative temperature (Te), body temperature (Tb), and microhabitat utilization. BioPhYsiCaL eCoLoGY It is the interaction of the physical aspects of the environment and the size and physical characteristics of the tortoise that determine its thermal ecology and thermoregulation. All tortoises have the same basic shape, shell type, and thermal characteristics. Desert tortoises (the literature has heretofore not recognized the two proposed species as different, and the life-history information presented here largely amalgamates data and analyses from desert tortoises in all southwestern deserts of North America) live in hot deserts and have a rounder and more dome-shaped shell. Gopherus polyphemus lives in eastern pine-oak forests, beach scrub, oak hammocks, or pine forests, and has a flatter shell. Desert tortoises also live in Sinaloan thornshrub and deciduous forest in Mexico, where they encounter lower temperatures in summer and higher temperatures in winter than relatives in the Mojave and Sonoran Deserts (Germano et al. 1994). In Sinaloan ecosystems , G. morafkai may experience similar thermoregulatory environments to G. polyphemus in Florida. Gopherus ber­ landieri is intermediate in size and lives in habitats ranging from near-desert to brush grasslands. Gopherus flavomarginatus is the largest North American tortoise (as large as 400 mm carapace length), but has the most restricted range, limited to a small area in north-central Mexico. It lives in semidesert or dry savanna up to 1400 m. All tortoises face similar physical constraints, avoiding heat during the middle of the day in the hottest portion of the year and retreating to burrows or surface refuges to survive both heat and cold. Tortoises control Tb by balancing heat absorbed from the environment in the form of solar and thermal radiation, with heat loss and gain from conduction with the substrate and convection (wind) from the atmosphere, and heat loss from evaporation. Porter and Gates (1969) and Spotila et al. (1972) discuss these processes. Different-sized tortoises are more or less affected by these avenues of heat exchange, because animal mass buffers transients in heat flux. For example, hatchling tortoises are active above ground on warm sunny days in winter because they heat up quickly. They are constrained from operating on the surface during midday on sunny summer days, however, because they will rapidly overheat. At the opposite extreme, large adult Gopherus agassizzi extend surface activity time by anticipating retreat to cooler microclimates in the morning and by exploiting their thermal inertia to dampen heating rates in the afternoon (Zimmerman et al. 1994). Little is known about thermoregulation by G. flavomar­ ginatus in nature, but we expect that large adults exploit body size to buffer heat gain and loss (Bonin et al. 2006). At higher elevations, individuals can avoid the heat stress typical of hot, dry desert areas at the foot of Cerro San Ignacio in Durango. By measuring microclimatic variables and using models of tortoises painted to have the same solar absorptivity (68.0– 73.2%) as living tortoises, Zimmerman et al. (1994) deterJames R. spotila thomas a. Radzio Michael P. o’Connor Thermoregulation and Energetics of North American Tortoises James R. spotila, thomas a. Radzio, and Michael P. o’Connor Thermoregulation and Energetics of North American Tortoises 31 population activity of juvenile desert tortoises and was able to predict potential time that juveniles could spend above ground to forage. The use of Te models is simple for small ectotherms like lizards because the physical model is a thin copper shell painted to have the same solar absorptivity as the...