Temperature Regulation and Habits in Two Species of Jerboa, Genus Jaculus

Abstract

Moroccan samples of Jaculus orientalis, mean body weight 139.1 grams, and J. deserti, mean weight 74.5 grams, when subjected to ambient temperatures of 0 to 33° C maintained body temperature at levels reported for other placental mammals of similar size. In that ambient range, mean rectal temperatures were 36.2 to 37.4° C in orientalis and 36.9 to 38.0° C in deserti. At ambient temperatures above 33° in orientalis and above 35° in deserti, rectal temperatures increased. At 40° the animals exhibited heat exhaustion and some died after an exposure of one hour. At 0° and 5° C the animals shivered and reduced surface exposure. There was no torpor. Minimum rates of oxygen consumption obtained within ambient temperatures 28 to 33° in J. orientalis and 33 to 35° in J. deserti. Those standard metabolic rates, respectively 1.00 and 1.23 cc O2 (g hr)−1, accord with those predicted on the basis of body weight. Mean conductances for the ambient range 0 to 20° were 0.082 and 0.129 ml O2 (g hr °C)−1, respectively, in orientalis and deserti. Assuming 4.7 cal/ml O2, those amount to 0.385 and 0.606 cal (g hr °C)−1. These conductances were 95 and 109 per cent, respectively, of that predicted on body weight. Surface-specific insulation is greater in orientalis, which can tolerate lower temperatures with less metabolic cost. These species live in climatically rigorous environments in which water typically is scarce and food supply fluctuates. Air temperatures may reach 45° C in summer and in winter near 0° (deserti) or below 0° C (orientalis). The species avoid the intense heat by spending the day in burrows where temperatures typically are below levels critical to the species. Temperatures in burrows of orientalis in Morocco in winter were 9° or 10° C while air temperature varied from 25° to sub-zero. The species are not known to store food or to become torpid in response to low temperatures. Environments of Jaculus are similar to those inhabited by Dipodomys, and the two groups of species resemble each other in features of anatomy, habits and physiology. In species of Dipodomys surveyed, however, standard metabolic rates and conductances below thermoneutrality were less than predicted by weight.