Spring emergence schedules and vernal behavior of Richardson's ground squirrels: why do males emerge from hibernation before females?

Summary

Schedules of emergence from hibernation were recorded for adult (≥1-year old) Richardson's ground squirrels each spring from 1979 to 1983 in southern Alberta, Canada. In all years and regardless of the calendar dates of emergence, males emerged from hibernation, on average, 8–16 days in advance of females. On the median date of emergence by males, 87–100% of females still remained in hibernation. On the day the last male appeared, the proportion of females still in hibernation ranged from 27% in 1979 to 89% in 1980. As females typically bred 1–4 days after resuming aboveground activity, late emerging males found that females that had been active several days had already been impregnated by earlier emerging males. Thus, late emergence by males resulted in reduced breeding opportunities.

Male Richardson's ground squirrels differed from females in a constellation of behaviors related to male intrasexual competition for breeding opportunities. In the breeding season, males moved longer distances, occupied larger ranges, expended less time on burrow maintenance, engaged in more injury-producing fights, and lost more body weight than did females. Early emergence from hibernation by males relative to females is viewed as one of several behaviors exhibited by males to promote individual reproductive success by maximizing the number of potential mates available.

The sex difference in time of vernal emergence by Richardson's ground squirrels is interpreted as an outcome of intersexual selection and is an example of the phenological strategy of the mate-limited sex maximizing reproductive success by being on the breeding ground by the time the mate-limiting sex appears.