Hibernation of the Big Brown Bat

Relatively few of the some 1300 species of bats live in the temperate regions of the world. Many of the species found in this northern region live through the winter by hibernating. But hibernating bats, in contrast to other hibernating mammals such as the hedgehog and ground squirrels, are essentially poikilothermous (Kayser, 1940). Their metabolism is proportional to their body temperature, and in the inactive state they show no evidence of a thermoregulatory mechanism (Hock, 1951). Hence in bats there appears to be no real distinction between sleep and hibernation. The state of hibernation for bats can be defined, then, only as a relatively long period or series of periods when the animals are subjected to low environmental temperatures, become inactive, and have a very low metabolic rate correlated with a low body temperature. There is some disagreement in the literature as to the requirements for successful hibernation in bats. Rysgaard (1942), for instance, felt that in the more northern areas it was necessary for bats to carry out a limited amount of feeding during hibernation, while Bailey (1928) was of the opinion that bats not only do not feed but that they remain motionless during the entire period. Observations by Guthrie (1933) and others agree with our observation that bats move periodically during hibernation, as demonstrated by the subsequent locations found occupied by banded specimens (Beer, 1955). However, activity during winter months does not necessarily imply that the active bats are able to find food during a Minnesota winter (in some of the caves they could obtain water to drink). This suggestion that activity does not necessarily imply feeding is supported by the fact that we have held big brown bats in the cold (4–5°C) in laboratory cages from the middle of November after they had been in hibernation for a short time until the first of May. These individuals showed no apparent weakness or other ill effect from the prolonged period of known fasting. Clearly, it is not necessary for the big brown bat to feed during a hibernation period of about six months at 5°C.