Effect of body mass on hibernation strategies of woodchucks (Marmota monax).

The benefits of mammalian hibernation have been well documented. However, the physiological and ecological costs of torpor have been emphasized only recently as part of a hibernation-optimization hypothesis. This hypothesis predicts that hibernators with greater availability of energy minimize costs of torpor by less frequent utilization of torpor and by maintaining higher body temperatures (T(b)) during torpor. In order to further examine the relationship between body mass and other parameters of hibernation, we present data, collected over a 12-year period, on the hibernation patterns of free-living woodchucks (Marmota monax) in southeastern Pennsylvania. Body mass was positively correlated with T(b) and negatively correlated with percentage of the heterothermic period spent in torpor. Thus, woodchucks with greater mass exhibited less time in torpor as a proportion of their heterothermic period and at higher T(b) than those with lesser mass. This strategy potentially enhances the physiological and physical ability of woodchucks to defend territories, avoid predation, find mates, and complete the reproductive cycle upon emergence from hibernation. Our results further support the hibernation-optimization hypothesis by demonstrating the relationship between body mass and characteristics of torpor and contributing toward a fuller understanding of this concept.

Latitudinal differences in the hibernation characteristics of woodchucks (Marmota monax).

There is little information on the phenotypic flexibility of hibernation characteristics within species. To address this issue, we observed differences in hibernation characteristics of three free-ranging populations of woodchucks (Marmota monax) distributed along a latitudinal gradient from Maine to South Carolina. Data from free-ranging animals exhibited a direct relationship between latitude and length of the hibernation season. As expected, woodchucks in the northern latitudes hibernated longer than those in the southern latitudes. Also, the length of interbout arousals decreased with increase in latitude, whereas the length of torpor bouts and the number of arousals increased. Thus, we observed phenotypic plasticity in hibernation characteristics based primarily on latitudinal temperature differences in each population. Further analysis revealed a direct relationship between latitude and total time spent in torpor. Maine animals spent 68% more time in torpor than South Carolina animals. However, total time spent euthermic did not differ among the three populations. The "cost-benefit" hypothesis of hibernation may help to explain these results. It assumes that hibernators avoid the physiological stress of torpor by staying euthermic as much as possible. Woodchucks in each population maximized time spent euthermic, utilizing torpor only at the level needed to survive winter hibernation and to commence reproduction in the spring.

Maintenance of biological rhythms during hibernation in Eastern woodchucks (Marmota monax).

We undertook a study to determine presence of circadian rhythms during woodchuck hibernation using continuously monitored body temperatures. Males had shorter torpor and longer euthermic periods than females. Circular statistics revealed a significant mean vector for males entering into torpor (10:21 h), but not for females. No significant mean vector was found for male or female arousal from torpor. A contingency test was applied to the torpor bout durations. All 7 males tested had significant tau's between 24 and 26 h, while 6 of the 13 females tested had significant tau's with a range of 22-27 h. These results implicate a free-running circadian clock during torpor bouts. Overall, the data support the existence of biological rhythms during hibernation in woodchucks, especially for males during arousals. Since entries into torpor appear to be synchronized for males, arousal periods may be used to resynchronize their circadian system. The persistence of biological rhythms during hibernation may help to insure successful mating in the spring after emergence.