Whatever moves you: Choice of migratory strategy shows plasticity in response to individual and environmental covariates in partially migratory bighorn sheep

Background/Question/Methods

Behavioral plasticity may provide long-lived species a means of responding to changing or variable environmental conditions at time scales faster than evolution allows. However, a species ability to alter its behavior in response to environmental stimuli may be constrained. Understanding the proximate determinants of behavioral plasticity is essential to determining the extent of a species’ ability to adapt behaviorally to changing conditions. One extreme example of a plastic behavior with major life-history implications is the selection of migratory strategy (i.e. resident vs. migrant) by individuals within partially-migratory populations (Chapman et al. 2011). Sierra Nevada bighorn sheep (Ovis canadensis sierra) are a federally-endangered subspecies in which migratory behavior has never been previously described.  Sierra sheep currently exist in a number of demographically-isolated sub-populations along the eastern crest of California’s Sierra Nevada mountains. We first quantified the occurrence and timing of migration in Sierra sheep using non-linear modeling methods applied to location data from global positioning system (GPS) collars deployed on ewes (N=53).  Second, we tested the ability of individuial and environmental covariates from used locations to explain selection of migratory strategy and variation in migratory behavior across a five-year period using cure-rate models.

Results/Conclusions

We provide the first quantitative documentation of individual bighorn sheep switching migratory strategy between years. Furthermore we show that migration in this subspecies is a conditional strategy. Both the selection of migratory strategy (migrant vs. resident) and the timing of migration are significantly explained by individual (i.e. age, nutritional condition, reproductive status) and environmental (i.e. available forage, snow cover, temperature) covariates. We also demonstrate population-specific differences in migratory tendency that may be the result of genetic differences. Finally, we discuss implications of the relative importance of environmental covariates in determining migratory strategy in light of available down-scaled climate-projection models.