How to regulate temperature

The microclimate of the subterranean ecotope is rather stable. Particularly in the nest chamber, which in giant Zam-bian mole-rats (Cryptomys mechowi) is usually 23.6 in (60 cm) (in some cases, even 6.6 ft [2 m]) below ground, there are minimal daily or seasonal fluctuations in temperature and humidity. This constant temperature enables a lower basal rate of metabolism. In the thermally buffered environment of the underground "incubator," it is possible to abandon complex and complicated morphological and physiological mechanisms of thermoregulation. Indeed, subterranean mammals tend to hypothermia (lowering the body temperature—on average 89.6-96.8°F [32-36°C]). Body temperature is partly de-

The European mole (Talpa europaea) uses its long claws to dig its burrows. (Photo by Hans Reinhard/OKAPIA/Photo Researchers, Inc. Reproduced by permission.)

pendent upon the ambient temperature. This relaxed thermoregulation (heterothermia) is most pronounced in the smallest (and the only hairless representative) among the subterranean mammals, the naked mole-rat. High relative humidity (about 93%) in underground burrows results in a relatively low vapor pressure gradient and low rate of water loss through exhalation or through the skin. This is beneficial for water balance as these animals do not drink free water, but instead obtain water from the food they consume.

However, high humidity and relatively high temperatures, which can occur on sunny days in shallow foraging burrows, may result in thermoregulatory problems. In the absence of evaporative and convective cooling, overheating and thermal stress would seem to be inevitable, since burrowing is energetically demanding and most mammals can tolerate dry, warm climate better than humid, warm climate. Subterranean mammals living in warmer environments have high thermal conductance, which means that the animals may exchange heat (cool or warm themselves) relatively easily through direct physical contact between themselves and the soil. As in poikilothermic reptiles, behavioral thermoregulation is of particular importance in heterothermic mammals. Thus, the animals can adapt timing and duration of their working activity to ambient temperatures in shallow burrows. Comparative and experimental physiological research of thermoregulation and energetics has a long tradition since McNab in 1966 first compared the metabolic rate of five subterranean rodent species and emphasized their shared adaptive convergence syndrome: low resting metabolic rate (involving also lower

The lips of the valley pocket gopher (Thomomys bottae) close behind its teeth to keep dirt out. (Photo by Tom McHugh/Photo Researchers, Inc. Reproduced by permission.)

ventilation and heart rates than would be expected on the basis of body size), low body temperature, and high thermal conductance. Since then, additional physiological data have been obtained on diverse species of subterranean mammals supporting the earlier conclusions by McNab.

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