The Metabolism of Fossorial Rodents: A Study of Convergence

Abstract

Temperature regulation and rate of metabolism were studied in five species of fossorial rodents: Geomys pinetis, Spalax leucodon, Tachyoryctes splendens, Heliophobius kapeti, and Heterocephalus glaber. The burrows of these rodents are in well—drained soils of poor water—holding capacity. Burrow atmospheres have an oxygen concentration of 15 to 20%, a carbon dioxide concentration of 0.5 to 2.0%, and limited temperature fluctuations; they are saturated with water vapor. Body temperature are somewhat low (35 to 37°C) in four species; temperature regulation is good in these species at ambient temperature down to 5 or 10°C. Heterocephalus, however, has a much lower body temperature (about 32°C) and the poorest capacity for thermoregulation of any known mammal. Fossorial rodents have low basal rates of metabolism, high conductances, and high ranges of thermoneutrality. Lethal ambient temperatures are inversely proportional to the normal levels of body temperature. Analysis of basal rates of metabolism (M), conductances (C), and body temperature suggests that the first two are determined in relation to environmental characteristics, and that body temperature, "goodness" of thermoregulation, midpoint of thermoneutrality, and lethal ambient temperature are dependent upon the ratio M/C. It is concluded that the lowest body temperature in homoiotherms compatible with good thermoregulation is about 35°C. A comparison of Heterocephalus and lizards illustrates the energetic similarities and differences between endotherms and ectotherms. The reduction of M and the increase of C in fossorial rodents are directly related to the mean burrow temperatures. These modifications reduce the probability of overheating in an environment where evaporative and convective cooling are greatly reduced. Tachyoryctes and Heliophobius use forced evaporative cooling for emergency thermoregulation, in spite of the saturated burrow atmosphere. Heterocephalus facilitates heat loss during heat stress by increasing peripheral circulation over its naked body; Geomys does this by increasing the circulation to its naked tail and, possibly, naked feet. Geomys may lose up to 30% of its heat production via its tail. As a result of these modifications, the lethal ambient temperature is proportional to the mean burrow temperature. Geomys responds to geographic variation in heat loading either by a decrease in body size or by an increase in tail length. The smaller size of nonpregnant, female gophers helps to maintain their heat production during pregnancy below the limits established by the environment for males. Gopher distribution appears to be limited to soils with high rates of gas exchange. Interaction among species of gophers has, to some extent, a physiological basis. It is suggested that the physiological characters concerned with energetics are readily modified, in an evolutionary sense, to conform to the requirements imposed by the environment.