Factors controlling the phenology and limits of hibernation in a sciurid

Abstract

Animals that live in seasonal environments have a variety of adaptations to survive periods of low to no food availability. One such adaptation is hibernation, which is characterized by profound decreases in activity, metabolic rate, and in most cases, body temperature. Among animals that hibernate, only two species are known to maintain low tissue temperature while defending significant temperature gradients, and the best studied of these is the Arctic ground squirrel (Urocitellus parryii). In the first chapter, we determine the lower ambient temperature limit of hibernation for an arctic ground squirrel (-26°C), and that a maximum torpid metabolic rate exists (0.37 mL O₂/g*h). This maximum torpid metabolic rate allows animals to defend a ~26°C temperature gradient between their core and their environment. In this chapter we also demonstrate that another, temperate, hibernating species, the golden-mantled ground squirrel (Callospermophilus lateralis), is capable of continuing hibernation at sub-freezing temperatures and can defend a temperature gradient of at least 9°C. Due to the extreme environment that Arctic ground squirrels inhabit, they have a very short growing season (~3-7 months) during which they must reproduce, grow, and accumulate energy stores prior to hibernation onset. In the second chapter we investigate the roles androgens play in hibernation phenology and male aggressive behavior. We use plasma samples collected from free-living animals and radioimmunoassays to determine circulating androgen levels. We then match the peaks in androgens to the timing of the two periods of male-male aggression (testosterone in the spring and dehydroepiandrosterone in the late summer/fall). We also present evidence to support testosterone as the main factor determining the timing of spring euthermy and emergence among reproductively mature males. In the third chapter we utilize captive animals to determine the importance of a cache to male reproductive development. Using three separate experiments, we show that while the accumulation of a cache in the late summer/fall may increase the likelihood of a male undergoing reproductive development, it alone may not be enough to ensure reproductive development. Additionally, we demonstrate that simply having access to ad libitum food in the spring is not enough to ensure reproductive development, nor is a restricted spring ration enough to prevent it.