Body Size and Limits to the Daily Range of Body Temperature in Terrestrial Ectotherms

Abstract

A nonequilibrium heat-transfer model is used to calculate the extreme range of body temperatures available to ectotherms of different masses. At about 1 kg a transition occurs in the amplitudes imposed on a terrestrial ectotherm's potential range in body temperature by the daily cycling of the thermal environment. Animals smaller than 1 kg can choose a wide range of temperatures, whereas animals larger than 1 kg experience smaller ranges in Tb. The range in Tb that small insects experience is limited by the range in Ta, which, however, can be large in the boundary layer of perching surfaces. Data from the literature indicate that ectotherms in the range of sizes from 0.01 to 1 kg have limited ranges of Tb (about 30⚬ C). The model predicts that it could be as large as 55⚬ C, 15⚬ C greater than the range of lethal temperatures of reptiles. That the observed range is only about half of that predicted reaffirms our understanding of the importance of behavioral thermoregulation for these animals. Body size appears to limit the range of available Tb of ectotherms larger than 10 kg. The model is also used to calculate how much higher Tb could be than Ta for an animal exposed to high solar radiation and to low wind speed. For dry-skinned ectotherms, the difference increases about 4.5⚬ C for each 10-fold increase in body mass. For example, the maximum difference between Tb and Ta is 3⚬ C for a 10-6 kg ectotherm but 30⚬ C for a 1-kg ectotherm. Data from the literature generally support the quantitative predictions of the model. The size of a terrestrial ectotherm will determine which behavioral options are useful for controling Tb. Small insects (10 mg) cannot elevate Tb above Ta but they can control Tb by spatial movements. Ectotherms smaller than 1 kg can heat quickly to temperatures at which they can be active when the thermal environment is suitable for only short periods each day. When conditions are extreme these opportunists must retreat quickly. Because of their thermal inertia, ectotherms larger than 10 kg have a smaller range of body temperatures from which to choose. If the mean operative temperatures are appropriate, however, large ectotherms can use their thermal inertia to be active over longer time periods during the day and in more extreme environments for the same range of body temperature than can smaller animals.