Migration in a changing world: Plasticity and consistent individual differences in migratory behavior of ungulates

Background/Question/Methods

Migration is a behavior used by animals to increase resource acquisition. For example, many ungulate species “surf the green wave”, a behavior in which animals track the flush of highly nutritious emergent vegetation as it greens-up along a gradient from winter to summer ranges. Tracking emergent vegetation requires individual animals to be plastic with regards to the timing of spring migration to match the timing of green-up in any given year. Additionally, consistent differences among individuals allows some individuals to be better adapted for different conditions across years, which could have implications as climate changes. Despite the adaptive ability of animals to track vegetation, climate change has altered the timing of resource availability, decoupling consumers from their resources during migration. To survive, migrants must acclimate or adapt to increased inter-annual variability associated with climate change, by being plastic to changes in the environment to avoid phenological asynchrony, or via differences among individuals in the population that may drive an evolutionary response. We examined whether individuals were plastic in the timing of spring migration as a function of the inter-annual variation in spring green-up and quantified among-individual differences in migration timing in seven populations of four ungulate species in Wyoming, USA.

Results/Conclusions

Timing of migration was plastic to timing of spring green-up and repeatable (e.g., some individuals consistently migrated early and others migrated late). However, individuals that were plastic to inter-annual variation in spring green-up with regards to the timing of migration were not better at selecting for high-quality forage during migration. Selection for high-quality forage was not repeatable, providing no evidence that certain individuals are better at selecting patches of high quality forage compared to others. Early springs did not result in reduced selection for forage, which would have been predicted if early springs represent cases where climate change has disrupted animals’ ability to match their behavior to environmental conditions. These results suggest that migration timing could be transmissible across generations via genetic effects or cultural/parental learning, but provide little evidence that plasticity in migration timing affects the selection of forage during migration. Our results are overall good news for migratory ungulates as they appear to have the capacity to respond directly to changing environmental conditions via plasticity and have sufficient natural variation in the timing of migration to potentially result in a population-level evolutionary response to changing conditions.