Climate, habitat interactions, and mule deer resource selection on winter landscapes

Mule deer (Odocoileus hemionus) are a valued game species across their range in North America. At higher latitudes mule deer inhabit the Northern Forests ecoregion, where the winter season poses challenging conditions. We studied resource selection during winter by mule deer in northwest Montana, USA, focused on the effects of snow and canopy cover on deer behavior. Specifically, we assessed the effects of land cover type, terrain characteristics, snow accumulation, percent canopy cover, solar radiation, and forage biomass on resource selection by 126 mule deer captured during 2017-2019 in 3 partially migratory subpopulations. We collected pellet samples and conducted diet analyses to assess forage species by individual and study area. Deer displayed a shift from nitrogen-rich forbs and shrubs in summer to nutrient-poor evergreens like conifers in winter. Deer avoided areas with deeper snow and sought more closed canopy forests for their snow-intercept advantages. Weather and vegetative variation among and within study areas resulted in disparities in selection for attributes such as land cover. Similarly, mule deer diets varied spatiotemporally as a function of snow, with some deer favoring coniferous species in deep snow, whereas others less constrained by snow had more diverse diets with more shrub species. Additional analyses revealed annual individual survival correlated with changes in snow depth, suggesting fitness consequences as snow accumulated. These differences suggest locally adapted behaviors as mule deer reflect energetic constraints of winter landscapes.