Ecological plasticity of denning chronology by American black bears and brown bears

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Ecological plasticity may index the capacity of individuals, populations, and species to respond to stochasticity. Few studies have demonstrated mechanisms influencing expression of plasticity and less have addressed proximate and causal mechanisms at multiple scales. We used linear models and meta-analyses to investigate factors influencing among and within population plasticity in den chronology for American black bears (Ursus americanus) and brown bears (U. arctos) in North America. We found later mean den entrance date and longer mean duration of denning among populations for both species in response to apparent increasing food availability during the growing season and denning period, along with later onset and shorter duration of mean snow accumulations ≥10 cm. Greater variability in den chronology appeared to be in response to greater variability in snow accumulation and duration precluding access to forage. Observed patterns of plasticity in den chronology varied among age-reproductive groups with male bears denning later and for shorter duration than sub-adults, non-parturient females, and parturient females. This pattern supports expected allometric energetic demands and energetic expenditures required for reproduction. Furthermore, our results align with previously published works indicating that plasticity in den chronology in response to relaxed selective pressure of food availability is energetically advantageous under the cost-benefit hypothesis of energy conservation. Understanding how species respond to variation in selective pressures is central to predicting these effects and our understanding of evolutionary and ecological processes.