Foraging benefits promote fitness in migratory mule deer

Although migration is widespread among many animal taxa, including ungulates,¹ the fitness benefits associated with different migratory tactics have rarely been documented.^2,3 Here, we evaluated a 9-year dataset on a migratory population of mule deer in western North America to understand whether long-distance migration provides access to seasonal forage, which translates into demographic benefits. Mule deer that migrated over 50 km to high-elevation summer ranges accessed higher forage quality and thus gained around twice the amount of fat over the growing season compared with mule deer that remained year-round as residents within a desert ecosystem. Elevated levels of fat translated to ∼20% higher probability of adult annual survival than residents. Mule deer that remained year-round in the desert portion of the study area were so resource-limited that they raised fawns at the expense of their own survival. Due to their higher levels of fat, annual survival, and fetal rates, migrants showed more robust population growth (λ = 1.03) compared to residents, which exhibited projected declines in population size over time (λ = 0.95). These results support the notion that migration translates into demographic benefits and highlight the urgent conservation work necessary to sustain diverse ungulate migrations amid habitat alteration due to climate change and an expanding web of linear barriers to movement.^3,4,5.