Daily foraging activity of an imperiled ground squirrel: effects of hibernation, thermal environment, body condition, and conspecific density

Abstract

Food acquisition is among the most important tasks faced by free-ranging animals. Predation and thermal risks, however, can make foraging a costly endeavor and foraging can preclude other important activities. Moreover, seasonal life cycle events such as hibernation impose energetic thresholds and time constraints on foraging. These factors interact with an animal’s endogenous state to influence foraging behavior. We tested a suite of predictions based on foraging theory to explore the effects of thermal environment, body condition, and conspecific density on aboveground activity (which is primarily foraging activity) of the northern Idaho ground squirrel (Urocitellus brunneus), an imperiled rodent that hibernates for 9 months each year. We took advantage of the squirrels’ semi-fossorial lifestyle to document daily aboveground activity by attaching geolocators to squirrels. We modeled squirrel activity with generalized linear mixed-effects models to document the relative importance of thermal environment, body condition, and conspecific density for daily aboveground activity. Aboveground activity by northern Idaho ground squirrels increased throughout their active season and leaner squirrels increased their activity more than heavier squirrels as residual foraging opportunities diminished. Thermal conditions also influenced squirrel activity: squirrels spent less time above ground during extreme temperatures and on days with significant precipitation. Aboveground activity of northern Idaho ground squirrels largely adhered to predictions of risk-sensitive and state-dependent foraging theory. Management actions that enhance forage will likely improve the probability of recovery for this federally threatened species by minimizing trade-offs squirrels need to make to acquire sufficient food to survive hibernation and reproduce in subsequent years.

Significance statement

Acquiring food is a vital task for wild animals, but foraging can be dangerous. Hibernation imposes annual energetic requirements animals must meet within a short time when food is available and the animal is active. Hibernating species, therefore, must navigate trade-offs among foraging, predation risk, and thermal intolerance. We investigated how these pressures influence daily foraging activity of the northern Idaho ground squirrel (Urocitellus brunneus), a federally threatened species. Ground squirrels forage above ground during the active season and retreat to burrows to avoid predation and extreme weather but accept greater risks to forage as hibernation approaches. Additionally, lean squirrels with high energetic needs forage more than heavy squirrels, exposing lean squirrels to higher predation risk. Improving forage may improve recovery odds for this imperiled species by allowing squirrels to reduce their mortality risk.