While a thick fur coat may be a chic way for fashionable ladies to keep warm, it is a vital necessity for mammals overwintering in cold environments. But what if your body temperature during hibernation is near freezing anyway?Does a fur coat confer any advantages? This question was recently addressed by the laboratories of Alexander Kauffman, Matthew Paul and Irving Zucker.

Small mammals such as rodents escape the rigors of winter by hibernating:reducing metabolism and numerous physiological functions and allowing body temperature to drop to within a degree or two of ambient temperature. The metabolic rate of hibernators may drop to less than 6% of normothermic values,conferring considerable energy savings and allowing the animal to survive the winter, utilizing stored body fat for fuel. Such hibernators, however, do not remain torpid throughout the winter but instead arouse periodically for approximately 24 h before returning to a hypometabolic state. The function of these periodic arousals is as yet unknown, although they are presumed to be critical events. The energy costs of such arousals are immense – up to 70% of a hibernating mammal's winter energy expenditure is consumed by these arousal events. This study examined the role of furry insulation on the energetics of hibernation in golden-mantled ground squirrels.

The team kept the hibernating squirrels in a cold chamber(5±1°C) and gave the animals a reverse Mohawk haircut, shaving and depilating the rodents during an arousal event and removing the hair from the entire dorsal surface except the head and paws. The recovered animals were also fitted with intraperitoneal temperature transmitters, and returned to the cold chamber where they re-entered the torpid state. Control animals were handled similarly but less than 10% of their pelage was removed from the ventral surface (which remained insulated while the animals hibernated, curled up in a ball).

Despite the lack of insulating fur, the authors found no differences between the shaved and control squirrels in the mean length of torpid bouts,the mean duration of the normothermic intervals or the food intake during these arousal periods. Nor were there differences in minimum body temperature between shaved and furry hibernators. However, significant effects of shaving were seen during arousal episodes. The shaved animals re-warmed more slowly and utilized far greater fat stores than did animals with intact pelages. Thus furless animals lost body mass more quickly; although both control and shaved animals had the same body mass upon their final arousal, this resulted only because shaved animals terminated hibernation at an average of 3.7 weeks ahead of the control group. During the first 3 weeks after the terminal arousal,body mass in shaved squirrels also increased more slowly than in intact animals, despite the fact that the food intake in shaved squirrels was significantly greater in the same period. This presumably reflects the greater energy expenditure required to remain warm when someone has stolen your fur coat!

It appears, then, that insulation is not critical to hibernators when their body temperature is close to ambient, but a nice fur coat is important during metabolically expensive arousal periods to decrease heat loss and reduce overall energy expenditures. Without these energy savings, ground squirrels`woke up' nearly a month earlier than their insulated counterparts, a potentially fatal consequence should food stores be limited and spring not yet sprung.

Kauffman, A. S., Paul, M. J. and Zucker, I.(
2004
). Increased heat loss affects hibernation in golden-mantled ground squirrels.
Am. J. Physiol. Reg. Integr. Comp. Physiol.
e-pub ahead of print 11.1152/ajpregu.00670.2003.