From "Listening in the Dark," by Donald R. Griffin (1958)--a study of echo-location in bats and other animals--p. 37:

Bats are not alone in being intermediate in their metabolic personalities between the poikilotherms and homotherms [i.e., cold-blooded and warm-blooded animals]; as everyone knows, certain other mammals are also able to pass the winter in a torpid state...Bears do not leave their dens for months, and eat no food during that time, since their stores of fat are ample. For a long time little was known about the body temperature of hibernating bears--and for obvious reasons. But recently R. J. Hock has liad the curiosity and the courage to crawl with rectal thermometers into the dens of several hibernating black bears.