Plasticity and repeatability in spring migration and parturition dates with implications for annual reproductive success
Animals are faced with unprecedented challenges as environmental conditions change. Animals must display behavioral plasticity to acclimate to changing conditions, or phenotypic variation must exist within the population to allow for natural selection to change the distribution of trait values. The timing of migration and parturition relative to important annual environmental changes such as snowmelt and vegetation green-up and how they co-vary may influence reproductive success. We tested for plasticity and individual differences in migration and parturition timing as a function of the timing of snowmelt and green-up in a migratory herbivore (caribou; Rangifer tarandus, n = 92) using behavioral reaction norms. We tested whether timing of parturition, plasticity in parturition timing, or timing of green-up were correlated with calf survival. Migration and parturition timing were plastic to the timing of spring conditions, and we found moderate repeatability for migration timing, but no repeatability in timing of parturition. We detected a novel behavioral syndrome where timing of migration and timing of parturition were correlated. Our results suggest that observed shifts in caribou parturition timing in other populations are due to plasticity as opposed to an evolutionary response to changing conditions. We did not detect a correlation between annual reproductive success and either the timing of spring or plasticity to the timing of spring events. While this provides evidence that many populations may be buffered from the consequences of climate change via plasticity, we caution that a lack of repeatability in parturition timing could impede adaptation as climate warming increases. Significance StatementAnimals have evolved to reproduce when resources are abundant. Climate change has altered the timing of annual events, resulting in earlier peaks in resource abundance. Animals can cope with this change in two ways. Individuals can display plasticity and alter the timing of reproductive activities to match the change in the environment, or consistent differences among individuals can result in sufficient variation to drive an evolutionary response. We tested these two alternative hypotheses in caribou (Rangifer tarandus) using an individual-based modelling framework. Caribou displayed plasticity in both when they migrated and gave birth, suggesting they can acclimate to changing conditions, but we did not find evidence of differences among individuals that would be likely to result in an evolutionary response.