Abstract 192 – The ultimate driver of parturition timing in grizzly bears: synchronized cues or energetic tradeoff?

Cecily Costello, Lori L. Roberts, Daniel D. Bjornlie, Matthew D. Cameron, Justin G.
Clapp, Mark A. Haroldson, Grant V. Hilderbrand, Kyle Joly, Wayne Kasworm, Jeremy Nicholson,
Tom Radandt, Mathew Sorum, Justin Tiesberg, Frank T. van Manen, Milan A. Vinks

In grizzly bears, timing of parturition during hibernation has been explained by ancestral traits
(delayed implantation, altricial young, and obligate maternal denning), but the ultimate driver
for more precise timing has not been fully explored. Capitalizing on latitudinal and denning-
period variation among 4 populations in interior North America, we tested two alternative
hypotheses. First, we hypothesized that birth timing results from a physiological cue that
synchronizes implantation with the onset of hibernation, allowing females to forgo
reproduction should they lack adequate fat stores. Alternatively, we hypothesized that
parturition is optimally timed relative to den exit to balance the energetic trade-off between
minimizing lactation time to protect the mother’s fitness and maximizing developmental time
to increase cub survival. Based on anomaly detection in activity data, we predicted 114
parturition dates and classified females according to litter survival at first visual observation:
58% successful (with cubs); 22% unsuccessful (alone); and 20% unknown (not observed).
Estimated birth dates were 27 Dec–28 Feb. With each increasing degree of latitude, model-
predicted day of birth increased by 1.00 and number of days between den entry and birth
increased by 2.48 (P < 0.001), but number of days between birth and den exit showed no trend
(P = 0.43). Implantation dates were not centered on den entry dates (P < 0.001; range -15 to 91
days). The period from birth to den exit averaged 103 days for successful females but only 76
days for unsuccessful females, owing to later births and earlier emergence by unsuccessful
females. Evidence supports that birth timing is governed by an energetic trade-off to maximize
lifetime fitness in this long-lived species. We conclude that natural selection has favored a
relatively consistent number of days between birth and den exit under normal body conditions,
and a shift toward later births under poorer body conditions.