Hibernation timing in ground squirrels is shaped by predation risk, food availability, and reproductive opportunities

Austin Allison, Courtney Conway, Alice Morris

This is a plain language summary of a Functional Ecology research article which can be found here.

Hibernation is usually assumed to be an adaptation allowing animals to survive weather extremes and food scarcity. But, some animals hibernate for long periods even when weather is suitable and food is abundant. This fact is perplexing because hibernation adversely affects vital physiological functions. Hence, we asked: what other factors influence when to hibernate and for how long?

We examined this question in the federally threatened northern Idaho ground squirrel (Urocitellus brunneus), which inhabits a limited range in west-central Idaho. Hot, dry summers and cold, snowy winters characterize this region along the Idaho-Oregon border, and squirrels spend ~9 months (mid-July to early April) hibernating underground each year—among the longest annual hibernation periods of any animal. However, hibernation entry and exit timing varies within and among populations of these squirrels.

We tested four potential explanations for ground squirrel hibernation timing: 1) temperature sensitivity, 2) food availability, 3) predation risk, and 4) reproduction. To do so, we used small data loggers that recorded light and temperature to determine precisely when squirrels entered and exited hibernation. We then quantified the influence of individual traits (fat stores, sex, and age) and environmental conditions (weather and food availability) on these hibernation behaviors.

A wild adult male northern Idaho ground squirrel equipped with a radio-collar and a geolocator is released back into the study area where it was captured (credit: Austin Allison)

All squirrels hibernated through the harshest winter weather, and squirrels exited hibernation later when wintry conditions lingered into spring. However, individual traits best explained differences in hibernation entry and exit timing among squirrels. Heavy squirrels—that could survive longer without food—entered hibernation earlier and hibernated longer than lean squirrels. The detrimental physiological effects of hibernation should deter squirrels from hibernating unless underground hibernation provides protection from aboveground predators. By reducing predation risk via extended hibernation, heavy squirrels increase their likelihood of surviving to reproduce in spring. This explains why squirrels use fat stores to lengthen hibernation. Reproductive-aged male squirrels exited hibernation in spring well before squirrels of other ages and sexes, demonstrating that reproduction also influences when squirrels hibernate. Our study illustrates that animals hibernate for several reasons and balance the costs and benefits of hibernation by choosing when to hibernate based on a combination of individual and environmental circumstances.