The secret life of ground squirrels: accelerometry reveals sex-dependent plasticity in above-ground activity.

Cory T Williams,Victor Zhang,Jeanette Moore,Kathryn Wilsterman,C Loren Buck,Brian M Barnes

doi:10.1098/rsos.160404

Abstract

The sexes differ in how and when they allocate energy towards reproduction, but how this influences phenotypic plasticity in daily activity patterns is unclear. Here, we use collar-mounted light loggers and triaxial accelerometers to examine factors that affect time spent above ground and overall dynamic body acceleration (ODBA), an index of activity-specific energy expenditure, across the active season of free-living, semi-fossorial arctic ground squirrels (Urocitellus parryii). We found high day-to-day variability in time spent above ground and ODBA with most of the variance explained by environmental conditions known to affect thermal exchange. In both years, females spent more time below ground compared with males during parturition and early lactation; however, this difference was fourfold larger in the second year, possibly, because females were in better body condition. Daily ODBA positively correlated with time spent above ground in both sexes, but females were more active per unit time above ground. Consequently, daily ODBA did not differ between the sexes when females were early in lactation, even though females were above ground three to six fewer hours each day. Further, on top of having the additional burden of milk production, ODBA data indicate females also had fragmented rest patterns and were more active during late lactation. Our results indicate that sex differences in reproductive requirements can have a substantial influence on activity patterns, but the size of this effect may be dependent on capital resources accrued during gestation.

Highlights

For each year and site, we examined the effects of sex and environmental conditions on time spent above ground each day and mean daily overall dynamic body acceleration (ODBA) using mixed models with ID included as a random effect
Differences between the sexes in how and when energy is allocated towards reproduction have led to substantial sex-related variation in the seasonal onset and termination of heterothermy [47]. We anticipated these sex differences in reproductive requirements would influence time spent above ground each day across the active season
In addition to the energetic costs of producing milk during late lactation, females have higher activity-specific energy expenditure than males during this interval

Summary

Introduction

Males and females differ profoundly in how and when they allocate time and energy towards reproduction, and this is expected to influence daily patterns of movement and energy expenditure across the annual cycle. Females can respond to the energetic demands of reproduction by increasing the absolute amount of time spent foraging, often at the expense of risk-aversive behaviours [3,4,5]. Semi-fossorial animals face a direct trade-off between their ability to forage, which only occurs on the surface, and the risk of predation, which can be reduced by remaining below ground Given this trade-off, energy demands of particular life-history stages may dictate the amount of time spent in the riskier above-ground environment. Males may engage in riskier behaviours (such as being above ground) if it affords them increased reproductive opportunities, even if energy demands are not substantially higher during the mating season [10]

Methods

Discussion

Conclusion