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Updated: Jan 24, 2026

Preparation and Reactivity of Gasless Nanostructured Energetic Materials
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An Energetic Tradeoff Best Explains Parturition Timing in Grizzly Bears.

Cecily M Costello1, Lori L Roberts1, Daniel D Bjornlie2,3

  • 1Montana Fish, Wildlife and Parks Kalispell Montana USA.

Ecology and Evolution
|January 23, 2026
PubMed
Summary

Grizzly bear birth timing balances maternal energy reserves and cub survival, with later births for females needing more time and earlier births for those with ample energy. This flexible strategy aids climate change resilience.

Keywords:
Ursus arctosbirthbody conditionbrown bearcub survivaldelayed implantationdenning chronologyhibernationreproduction

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Area of Science:

  • Wildlife Ecology
  • Reproductive Biology
  • Animal Behavior

Background:

  • Grizzly bear parturition timing during hibernation is linked to ancestral traits but the primary driver remains unclear.
  • Observed latitudinal variation in denning duration suggests environmental influences on reproductive timing.

Purpose of the Study:

  • To test hypotheses explaining grizzly bear parturition timing: physiological cue vs. energetic tradeoff.
  • To investigate the relationship between birth timing, den exit, and litter survival in relation to latitude and maternal energy stores.

Main Methods:

  • Utilized accelerometer data to predict parturition dates for 115 female grizzly bears (Ursus arctos).
  • Classified females based on litter survival (successful, unsuccessful) post-den exit.
  • Analyzed the duration between birth and den exit, and its correlation with latitude and maternal condition.

Main Results:

  • Parturition timing was not synchronized with den entry, supporting the energetic tradeoff hypothesis.
  • Successful females had a longer interval between birth and den exit (103 days) than unsuccessful females (77 days).
  • Later births and longer denning periods were observed at higher latitudes, indicating adaptation to local conditions.

Conclusions:

  • Grizzly bear birth timing is driven by an energetic tradeoff, balancing maternal energy reserves with cub development needs.
  • Flexible parturition timing allows adaptation to varying environmental conditions and maternal physiological states.
  • This adaptability may enhance population resilience to climate change and explain variations in natality and cub survival.