Evolutionary trade-offs in dormancy phenology

Abstract

Seasonal animal dormancy, hibernation or diapause, is widely interpreted as a physiological response for surviving energetic challenges during the harshest times of the year. However, there are other mutually non-exclusive hypotheses to explain the timing of animal dormancy over time, that is, entry into and emergence from hibernation (i.e. dormancy phenology). Other survival advantages of dormancy that have been proposed are reduced risks of predation and competition (the “life-history” hypothesis), but comparative tests across animal species are not yet available. Under this hypothesis, dormancy phenology is influenced by a trade-off between the reproductive advantages of being active and the survival benefits of being in dormancy. Thus, species may emerge from dormancy when reproductive benefits occur, regardless of the environmental conditions for obtaining energy. Species may go into dormancy when these environmental conditions would allow continued activity, if there were benefits from reduced predation or competition. Within a species, males and females differ in the amount of time and energy they invest in reproduction. Thus, the trade-off between reproduction and survival may be reflected in sex differences in phenology of dormancy. Using a phylogenetic comparative method applied to more than 20 hibernating mammalian species, we predicted that differences between the sexes in hibernation phenology should be associated with differences in reproductive investment, regardless of energetic status. Consistent with the life-history hypothesis, the sex that spent the less time in activities directly associated with reproduction (e.g. testicular maturation, gestation) or indirectly (e.g. recovery from reproductive stress) spent more time in hibernation. This was not expected if hibernation phenology were solely influenced by energetic constraints. Moreover, hibernation sometimes took place at times when the environment would allow the maintenance of a positive energy balance. We also compiled, initial evidence consistent with the life history hypothesis to explain the dormancy phenology of ectotherms (invertebrates and reptiles). Thus, dormancy during non-life-threatening periods that are unfavorable for reproduction may be more widespread than previously appreciated.