Breeding phenology of free-living arctic ground squirrels in an early spring: Is autumn a back seat driver?

Background/Question/Methods

Animals attempt to maximize lifetime reproduction, and the timing of their breeding is essential for success. In seasonal environments, breeding in most animals coincides with availability of resources to support gestation and lactation while also allowing time for offspring growth before winter. But breeding too early under poor conditions can compromise adult survival. Ground squirrels adjust breeding time in response to local conditions, but also as part of an endogenous, circannual rhythm that is likely entrained by photoperiod. Given that global warming and climate change are occurring at rapid rates affecting seasonal timing, a potential exists for a mismatch between the active season of animals and the growing season of plants. In the Arctic, warming directly affects snowmelt and snow cover, length of growing season, and access to food. Previously, we found that arctic ground squirrels living in areas with earlier snowmelt emerged from hibernation and bred earlier. However, climate change not only affects snowmelt in spring but can also delay snow cover in autumn, prolonging the active season and time available for offspring growth and winter preparation. Here we investigate how changes in autumn conditions affect timing of hibernation and breeding in two arctic ground squirrel populations. 

Results/Conclusions

These populations are only 20 km apart but experience very different seasonal conditions. At Atigun, snowmelt occurs 27 days earlier and snow cover begins 17 days later than at Toolik. We found that in relation to date of 100% snowmelt, Atigun squirrels emerged and bred later than Toolik squirrels. At Atigun, males and females ended hibernation 23 and 12 days before snowmelt, respectively. At Toolik, males and females ended hibernation 41 and 27 days before snowmelt, respectively. The average birth date occurred 16 days after snowmelt at Atigun but only 1 day after snowmelt at Toolik. Thus, although squirrels at Atigun are emerging and breeding earlier in the calendar year compared to animals at Toolik, they are actually delaying emergence and breeding with respect to onset of spring conditions. Density and recruitment estimates suggest that delayed emergence may increase adult survival during the active season without compromising offspring survival. We argue that a prolonged autumn allows females to delay their spring emergence and breeding at Atigun, thus emerging to a more favorable environment in spring while still allowing time for their offspring to grow and fatten in preparation for hibernation.