Variation in body temperature is related to ambient temperature but not experimental manipulation of insulation in two small endotherms with different thermoregulatory patterns

Maintaining a high and constant body temperature ( T b ) is thought to enhance performance in endotherms, but such a thermoregulatory pattern is energetically expensive. Thus, some variation in T b is probably universal among endotherms, and several recent attempts have been made to generalize the factors that should cause this variation. Two factors that may be closely tied to the thermoregulatory pattern expressed are the cost of thermoregulation and food availability. To test these predictions, we measured T b of eastern rock elephant shrews ( E lephantulus myurus ; a traditionally defined heterotherm) and N amaqua rock mice ( M icaelamys namaquensis ; a homeotherm) in the field and under semi‐natural conditions after experimentally reducing insulation by shaving a patch of dorsal fur. After accounting for ambient temperature ( T a ), there was no significant difference in the level of variation in T b between shaven and unshaven elephant shrews. In rock mice, the increase in heat loss paradoxically led to decreased variation in T b in the field, but no effect was evident in captivity. Furthermore, as predicted, both species displayed significantly less variation in T b under semi‐natural conditions when given food ad libitum and predation risk involved with obtaining that food was low. Our results show that small mammals, both homeothermic and heterothermic, are capable of altering their thermoregulatory patterns in response to ecological conditions (e.g. rapid changes in food availability). However, increasing the cost of thermoregulation, at least as it was done in this experiment, does not appear to affect the expression of T b as strongly as does T a .