Hibernation and Circannual Rhythms of Food Consumption in Marmots and Ground Squirrels

Abstract

In order to understand better the evolution and adaptive value of hibernation, ecological aspects and experimental studies of closely related hibernators, the Marmotini, are examined. The central hypthesis is that annual changes in the environment integrate three or, perhaps, four physiological processes: torpor, reproduction, consumption of food, and metabolism. Reproduction occurs promptly after emergence from hibernation. For most species, the breeding season is very short. Although the experimental data are rather meager, no variation in external factors has consistently altered the season of reproduction. Consumption of food and change in weight increases until July or September and then decreases. The large members of the Marmotini store their energy as fat, but small species store their energy as seeds and nuts. Experiments to test the hypothesis that some aspect of the supply, such as fat content, might vary seasonally have produced negative results. Complex experiments on the length of the photoperiod on woodchucks and several species of ground squirrels failed to alter the annual cycle of consumption of food. Animals kept in constant conditions showed a cycle of about 11 months, but woodchucks sent to Australia changed their cycle in two years to match the seasons of the southern hemisphere. Experiments with temperature and torpor and castration did not alter the annual rhythm. Metabolism has an annual cycle of increase and decrease, even in animals that are not permitted to become torpid. Factors that might influence torpor have been extensively tested in laboratory experiments. Individuals kept at low temperature without food will enter torpor in any month of the year. In summer it is necessary to deprive an animal of food for 3 to 4 weeks to initiate torpor, but in winter, 3 to 4 hours may suffice. Various lengths of photoperiod or changes of photoperiod throughout the year have failed to cause animals to enter torpor or to arouse. A reduction of the consumption of food or absence of food was necessary for torpor. Deprivation of water seems to induce torpor. An annual cycle of consumption of food (and water) dominates the control of the occurrence of hibernation. In nature when the decline in consumption coincides with low temperatures, the animals begin hibernation. The consumption of food follows and annual rhythm which apparently becomes synchronized with the environment at about the age of two years. Endogenicity of the rhythm is demonstrated by several sets of evidence: (1) the rhythm runs freely at about 11 months duration; (2) the period is independent of the temperature; (3) entrainment has been demonstrated. As yet, no data demonstrating a phase response curve has become available. The adaptive value of the annual rhythm is apparent. Torpor is a splendid mechanism for conserving energy in the season of scarcity by lowering the metabolic functions. The existence of a biological alarm clock is necessary to arouse the individual. After arousal the individuals consume little food, but at the season of abundance, they consume large quantities. Interpretation of the results described in this review recognizes that some physiological processes are controlled by annual clocks and others by circadian clocks. For example, the analysis of the relation of photoperiod to the increase in size of gonads of many birds and mammals suggests that a circadian rhythm controls the cycle. But a woodchuck requires two years when individuals are transplanted to a different hemisphere to change their cycle of consumption. Thus, it is possible that the circannual rhythm controls some physiological functions, while a circadian clock controls other functions.