Being fat or flexible in highly seasonal environments pay substantial dividends to survival

Kristin Denryter, Mary M. Conner, Thomas R. Stephenson, David W. German, Kevin L. Monteith

This is a plain language summary of a Functional Ecology research article which can be found here.

Animals living in seasonal environments have evolved various adaptations, including accumulation of body fat and migration, to cope with seasonal highs and lows in resource availability and quality. In populations wherein only some individuals migrate, the importance of body fat to survival, however, may vary. For example, animals that migrate to seasonal ranges with higher-quality and more abundant food supplies may be able to survive at lower levels of body fat than their counterparts that do not migrate and experience lower quality and abundance of food seasonally.

In this study, we explored how body fat and migratory tactic influence overwinter survival of bighorn sheep in the Sierra Nevada mountains of California, USA. Sierra Nevada bighorn sheep exhibit at least three migratory tactics, including residency (remaining on one range year round), traditional migration (making one seasonal round-trip movement between high- and low-elevation ranges), and vacillating migration (making multiple seasonal round-trip movements between high- and low-elevation ranges).

Fat reserves are important for overwinter survival of Sierra Nevada bighorn sheep, especially for individuals like these that traverse through deep snow at high elevations in the Sierra Nevada mountains of California, USA (credit: Steve Yeager)

Regardless of migratory tactic, animals with high levels of body fat experienced high rates of overwinter survival. Animals that remained residents at high elevations during winter fared worse at lower levels of body fat than either vacillating or traditional migrants. Vacillating migrants, which were the most flexible in terms of movements and winter habitat use, experienced high survival irrespective of body fat level.

Collectively, our results suggest that body fat provides an important buffer against harsh environmental conditions and that its buffering capacity varies depending on the environment. That is, in milder environments (i.e., those experienced by migrants), a small amount of body fat provides a greater buffer than the same amount of fat in harsher environments (i.e., those experienced by residents). These findings highlight that synergies between fat accumulation and migration carry potential fitness consequences for individuals and consequences to population growth rates, and are essential to providing a diverse set of options to support life in seasonal environments.


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