From the vast movements of wildebeest across the Serengeti to the circumpolar movements of the arctic tern, migratory behavior has fascinated humans for centuries. Experimental work on birds and insects has demonstrated how geographical and celestial features, as well as the earth’s magnetic field are employed, in some cases via memory, as a navigational guide to migratory movements. For mammalian taxa, we know much less about how individuals migrate, and evidence suggests that migratory mammals may simply track resource waves that move predictably across space (e.g., the longitudinal/elevational wave of newly emergent vegetation in spring). Many mammals, however, migrate distances that greatly exceed their perceptual range – suggesting that memory plays a strong role. Using a longitudinal dataset of GPS-collared mule deer that migrate up to 500 km annually, we tested alternative predictions of a tracking- versus memory-based mechanism for migration and quantified the associated fitness benefits.
Results/Conclusions
Using a model of migratory movements, deer chose movements that resulted in higher green-up rate – supporting the resource tracking mechanism. Memory also strongly influenced migratory movement, providing navigational knowledge far beyond that of tracking resource waves. Variables representing spatial memory had 3-10 times the effect size of other variables. Indeed, migratory movements were remarkably consistent over time with individuals spending much of their 500 km migration on the exact same routes as the previous year. Moreover, we found evidence of attribute memory, where the strength of fidelity to remembered route segments was conditional on the quality of vegetation experienced locally on those same routes the previous year. Combining spatial and attribute memory allows migratory mammals to optimally navigate across large landscapes while also incrementally improving the quality of their migratory route and adapting to environmental change. Using simulation, we found that green-wave tracking could not recreate observed migratory trajectories, but adding memory did. These analyses suggest that memory – and to a lesser extent green-wave tracking – were key to the foraging benefits of migration. Memory can be viewed as the template for adaptive mammalian migration, providing a cognitive map of an animal’s seasonal ranges and migratory routes. Resource tracking is an adaptive mechanism for individuals to determine when to move within their cognitive map. The cognitive capacity for spatial memory likely evolved prior to, or in tandem with, migratory behavior.