Effect of Body Mass on Hibernation Strategies of Woodchucks (Marmota monax)

Abstract

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.

Metabolic Flexibility: Hibernation, Torpor, and Estivation

“…Intraspecifically among hibernators, larger (and, presumably, fatter) animals spend less time torpid, spend that time at higher T b (313), and arouse more frequently (21). Contrary to the prediction that heterotherms should maximize energy conservation, these patterns suggest that, if energy stores allow, heterotherms minimize the amount time spent torpid.…”

“…In animals that hibernate at low T b (i.e., near 5 • C), the duration of torpor phases depends on body mass within (313) and among species (104). In woodchucks (313) and edible dormice (21), larger, fatter animals spend less time torpid, and have higher torpid T b . These results are at least analogous to the reduced frequency and depth of torpor in hibernating chipmunks with large, supplemented food caches ( Fig.…”

Modelling mammalian energetics: the heterothermy problem

“…Studies on the T b patterns and thermoregulation of freeranging animals have illustrated the effects, and sometimes confounding influences, of a number of factors including predation risk [150], presence of conspecifics [145], food availability [130,161], competition [162] and extreme events [50,51,153,163] on T b . Yet, robust predictions of responses to climate change require an in-depth understanding of how animals exist in the wild [4,8].…”

The Hibernation Continuum: Physiological and Molecular Aspects of Metabolic Plasticity in Mammals

“…14); dormice engaging in daily torpor, hibernation, and estivation (93); highly variable T b patterns of ground squirrels preparing for hibernation during fall (71); seasonal changes in torpor bout duration and frequency of arousal that leads to reproducibly longer periods of torpor mid-hibernation season (23); and variable patterns of woodchuck hibernation depending on environmental conditions (97); taken together with the intermediate torpor utilization patterns of the Patagonian opossum (24) and tenrecs described above are consistent with the view that the metabolic plasticity observed among mammals represents a continuum of possible phenotypes rather than fixed discrete patterns. It is reasonable to infer that the apparent clustering of features, such as minimum T b and metabolic rate during torpor or maximum torpor bout length, into apparently distinct pattern groups for daily torpor and hibernation (70) may reflect optimal conditions for energy conservation given specific conditions of ambient temperature and body size taken together with strategies for avoiding predation and enhancing reproduction (79,96).…”