Unpacking the interplay between spatial and social drivers of movement in a large herbivore along a latitudinal and life-history cline

Background/Question/Methods

Direct comparisons of spatial and social drivers of movement remain relatively rare. Here, we examine how life history — and in particular, breeding season and birth pulse duration — mediates trade-offs between social and spatial movement drivers through a detailed study of bighorn sheep along a latitudinal cline. The bighorn sheep breeding season, which is defined by occurrence of female estrus, ranges from approximately four weeks in duration in the Northern Rockies to over six months in the Mojave desert. This gradient offers us a unique opportunity to examine trade-offs in landscapes where spatial and social elements of the environment vary in utility. We characterize movements along this gradient through a hierarchical analysis of habitat selection and behavioral state-switching coefficients. The unique social dynamics of the bighorn system allow us to compare space use and movement decisions surrounding both a depletable (estrus females) and a nondepletable (membership in a nursery group with similar-aged juveniles) attribute of the social environment.

Results/Conclusions

Preliminary analyses suggest that male bighorn movements (both behavioral state-switching and space use) were powerfully driven by a depletable social resource (estrus females) when this resource was available for only a limited period of time. As the window encompassing female estrus events expanded, selection for estrus females declined in strength. Selection for the nondepletable attribute (nursery groups with similar-aged juveniles) was weaker, and often confounded with selection for escape terrain. The patterns we describe promote a richer view of movement dynamics, which encapsulates both spatial and social aspects of the environment.