# Seasonal body mass dynamics mediate life‐history trade‐offs in a hibernating mammal

## TL;DR

This study shows how seasonal changes in body mass in ground squirrels affect their survival and reproduction, with implications for conservation.

## Contribution

The study provides new insights into how seasonal body mass dynamics mediate life-history trade-offs in hibernating mammals.

## Key findings

- Reproductive females gain fat later in the active season compared to males and non-reproductive females.
- Squirrels with better pre-hibernation body condition have higher survival rates due to early hibernation immersion.
- Colder temperatures and lower conspecific density reduce body condition, affecting resource allocation and survival.

## Abstract

Energetic acquisition and growth are key traits that affect demography and life‐history strategies. Many animals that live in seasonal environments in which food availability fluctuates store energy endogenously as fat in anticipation of food shortage. Fat‐storing mammalian hibernators are an extreme example of this strategy wherein the optimal resolution of resource allocation trade‐offs is essential to survival. Hence, these species provide an opportunity to test potential causes and consequences of seasonal body mass dynamics.We used a 12‐year dataset of 8753 body mass records from 3351 individually marked northern Idaho ground squirrels (Urocitellus brunneus)—a federally threatened hibernator—to meet three objectives: (1) document seasonal body mass changes by sex, age and reproductive status, (2) test ecological hypotheses to explain spatiotemporal variation in body mass and (3) document fitness consequences of pre‐hibernation body condition via condition‐dependent overwinter survival.Squirrels varied substantially in seasonal body mass dynamics. The magnitude (36–155%) and onset (late May to early July) of rapid active‐season mass gain varied among demographic groups. Reproductive females acquired the necessary fat stores to survive hibernation later in the active season than did males and non‐reproductive females. Moreover, squirrels with better pre‐hibernation body condition were more likely to survive to the subsequent year, potentially because they allocated excess energetic reserves to prolonging hibernation via early immergence and thereby reduced predation risk. These results suggest a direct trade‐off between current and future reproduction mediated by resource acquisition and allocation, as predicted by life‐history theory.Colder active‐season temperatures and lower conspecific densities negatively influenced squirrel body condition, possibly via reductions in foraging activity associated with those conditions. These ecological effects on body condition constrain resource allocation and demographic outcomes. As such, our results can help guide research and conservation strategies to benefit hibernating animals.

Energetic acquisition and growth are key traits that affect demography and life‐history strategies. Many animals that live in seasonal environments in which food availability fluctuates store energy endogenously as fat in anticipation of food shortage. Fat‐storing mammalian hibernators are an extreme example of this strategy wherein the optimal resolution of resource allocation trade‐offs is essential to survival. Hence, these species provide an opportunity to test potential causes and consequences of seasonal body mass dynamics.

We used a 12‐year dataset of 8753 body mass records from 3351 individually marked northern Idaho ground squirrels (Urocitellus brunneus)—a federally threatened hibernator—to meet three objectives: (1) document seasonal body mass changes by sex, age and reproductive status, (2) test ecological hypotheses to explain spatiotemporal variation in body mass and (3) document fitness consequences of pre‐hibernation body condition via condition‐dependent overwinter survival.

Squirrels varied substantially in seasonal body mass dynamics. The magnitude (36–155%) and onset (late May to early July) of rapid active‐season mass gain varied among demographic groups. Reproductive females acquired the necessary fat stores to survive hibernation later in the active season than did males and non‐reproductive females. Moreover, squirrels with better pre‐hibernation body condition were more likely to survive to the subsequent year, potentially because they allocated excess energetic reserves to prolonging hibernation via early immergence and thereby reduced predation risk. These results suggest a direct trade‐off between current and future reproduction mediated by resource acquisition and allocation, as predicted by life‐history theory.

Colder active‐season temperatures and lower conspecific densities negatively influenced squirrel body condition, possibly via reductions in foraging activity associated with those conditions. These ecological effects on body condition constrain resource allocation and demographic outcomes. As such, our results can help guide research and conservation strategies to benefit hibernating animals.

We tested a suite of ecological hypotheses to explain variation in seasonal body mass dynamics of a fat‐storing mammalian hibernator. We further demonstrated that pre‐hibernation mass gain in ground squirrels mediates an annual allocation trade‐off between current and future reproduction as the squirrels forage and rear young under predation risk.