Autumn conditions as a driver of spring phenology in a free-living arctic mammal

Abstract

Several studies have shown that birds and mammals are breeding earlier in response to earlier spring conditions. Delay in the onset of winter should also affect reproductive timing and may allow for breeding later instead of earlier in spring, if extended autumns lengthen the season that food is available to offspring. Using 4 years of fine-scale environmental and phenological data of annual timing of hibernation and reproduction in two free-living populations of arctic ground squirrels in Northern Alaska, we show that the onset of winter snow-cover may influence females’ spring phenology via its interaction with the timing of hibernation of young-of-the-year (YoY). At the Atigun site, snowmelt occurs 26 days earlier and snow-cover occurs 14 days later than at the Toolik site. Previously, we found that Atigun females emerged and bred earlier than those at Toolik; however, here we show that this shift is not equivalent in magnitude to the earlier timing of spring conditions. At Atigun, females emerged and bred 13 days prior to snowmelt, while those at Toolik emerged and bred 34 days prior to snowmelt. We also found, importantly, that YoY entered hibernation after a set amount of time from birth regardless of site or environmental conditions. This resulted in Atigun YoY entering hibernation 24 (females) and 12 (males) days before snow-cover, but those at Toolik entering hibernation 0 (females) days before and 9 (males) days after snow-cover.This may allow females at Atigun to breed later (relative to snowmelt) in spring but select for early emergence and breeding of females at Toolik to allow sufficient time for offspring growth and preparation for hibernation. These results indicate that autumn conditions may influence subsequent spring phenology and changes in spring conditions may be only one factor influencing phenological shifts associated with climate change. We suggest that in areas with harsh spring conditions, such as the Arctic, animals may not shift their phenology to match earlier, but still unfavorable spring conditions, but may delay their timing of spring breeding if climate change also delays the onset of snow-cover, allowing sufficient timing for offspring growth and preparation prior to winter.