In the midst of the long Northern Hemisphere winter more than a few of us wish that we could hibernate, shutting off the outside world for the duration of the cold winter. This is the fate of many small mammals, which rather than decide to remain active throughout the cold, barren winters, retreat to hibernacula, emerging only once temperatures have warmed and the spring ushers in fresh food sources.
Hibernation, as it turns out, is about much more than just avoiding harsh environmental conditions, as suggested by a recent study on the edible dormouse by Claudia Bieber and her colleagues at the University of Veterinary Medicine in Vienna, Austria. While most previous research into hibernation has focused on the interactions between the environment and hibernation patterns, this study looked at how energy reserves and predator avoidance affect hibernation. Presenting the results of 4 years of study on both wild-caught and captive females, the study, in press at Functional Ecology, looked at the amount of fat the animals stored before they retreated to their burrows for the winter. This was then evaluated against the amount of time spent in hibernation, mass at emergence in the spring and a number of other factors including reproductive success and age.
During hibernation, small mammals enter a state of hypometabolism, called torpor, during which body temperature is lowered to levels nearing that of the environment. In this state many bodily functions are reduced or stopped completely. Although prolonged torpor can result in significant energy savings, it also comes at a price, resulting in a diminished capacity to fight off infections, as well as reductions in basic maintenance – such as protein synthesis – and even brain function. However, these adaptations also incur risks and they are the likely reason why torpor in colder climates never lasts throughout the entire winter. Instead, hibernators frequently warm up to active body temperatures, usually for less than 24 h. This leads to a trade-off between increases in energy savings resulting from lower temperatures during torpor and the physiological costs of prolonged exposure to low body temperatures.
What Bieber and her colleagues discovered was that females with larger fat stores were able to maintain higher body temperatures and rewarm more frequently than their thinner counterparts, lessening the potential costs. They also managed to emerge from hibernation with larger fat stores, which could be used to fuel early reproduction. What is surprising about their results is that even though the fatter females could have emerged earlier, they remained underground for similar periods of time to thinner females, prolonging inactivity well past the point when food became available and spending as long as 8 months in hibernation. It therefore seems that hibernation in these dormice is about more than just saving energy. These results, as well as previous work showing low rates of mortality during hibernation in a number of species, led the researchers to suggest that this prolonged period of hibernation also serves to avoid predation. These results suggest that when predicting how species will respond to environmental conditions, we need to consider not just their physiology but also other aspects of their ecology.