Sexes in sync: phenotypic plasticity, sexual selection, and phenological synchrony between the sexes in a wild hibernator

Desynchrony of phenological responses to climate change is a major concern in ecological communities. Potential uncoupling between one of the most fundamental divisions within populations, males and females, has not been well studied. To address this gap, we examined sex-specific plasticity in hibernation phenology in two populations of Columbian ground squirrels (Urocitellus columbianus). We find that both sexes display similar phenological plasticity to spring snowmelt dates in their timing of torpor termination and behavioural emergence from hibernation. As a result of this plasticity, the degree of protandry (i.e., males’ emergences from hibernation preceding those of females) did not change significantly over the 27-year study. Earlier male behavioural emergence, relative to females, improved the likelihood of securing a breeding territory and increased annual reproductive success. Sexual selection favouring earlier male emergence from hibernation may maintain protandry in this population but did not contribute to further advances in male phenology. Older males also tended to emerge earlier and secure territories, and so it is unclear whether selection acts indirectly through age. Together, our results provide evidence that the sexes should remain synchronised, at least in response to the weather variation investigated here, and further support the role of sexual selection in the evolution of protandry in sexually reproducing organisms.

Methods
Long-term dataset monitoring populations of Columbian ground squirrels in Alberta, Canada.