Energetic Trade-Offs Explain Birth Timing in Grizzly Bears

Abstract

In grizzly bears (Ursus arctos) and other temperate-zone ursids, the seasonality of births during winter hibernation is best explained by ancestral traits of delayed implantation, altricial young, and obligate maternal denning. But drivers determining parturition timing within the 3- to 6-month hibernation period are unknown. Capitalizing on latitudinal and den timing variation among four grizzly bear populations, we tested two alternative hypotheses. The first was that birth timing is temporally associated with den entry because of proximate cues linking implantation and hibernation physiologies. The second was that birth timing is associated with den emergence to balance the energetic trade-off between minimizing lactation time (to protect the mother’s lifetime fitness) and maximizing developmental time (to increase offspring survival) during the winter fast. Based on activity sensor data, we estimated 91 parturition dates using anomaly detection methods and associated first and last dates of dormancy using change point analyses. Observed birth dates ranged from 31 Dec to 27 Feb and day of year increased with latitude. Days between first day of dormancy and birth were positively correlated with latitude, but days between birth and last day of dormancy were not. Implantation dates (birth date - 56 days) ranged from 41 days before to 83 days after the first day of dormancy. Evidence supported the energetic trade-off hypothesis. Although the high variation observed may have been partially explained by individual age- or body condition-related adjustments to the trade-off, it also signaled that optimality may rarely be achieved given the stochastic nature of annual green-up.