Where an animal moves and spends its time depends on its environment. Factors, such as presence of resources, weather, and risk from predators all factor in. But the role of memory in determining movement is still up for debate. Is movement based on sensory cues in the moment, seen fresh in each season? Or does the past experiences of animals play a strong role in determining their future movements?
These questions become even more profound when we consider some of the largest movements of animals – long distance migrations. Previous research has mostly assumed migrations to be driven by the tracking of resources across scales. In this case, large-scale migration by an herbivore might be made up of the sum of a series of small-scale decisions about where to forage. However, when animals migrate long distances, exceeding their ability to sense trends across the landscape, memory likely plays an important role.
Up to recently, the memory hypothesis has only been explored in two mammal species – zebras and blue whales. A recent paper adds to this collection by determining the role of memory and resource availability the movements of mule deer. These important herbivores perform yearly migrations across the Mountain West, with timing closely matching spring greenup, and the appearance of winter weather in the fall.
The study used collar tracking data for 91 mule deer across 3 years of migration. Across the migratory routes, the researchers broke habitat up into units, and assigned each unit memory and environmental variables. The first year of data was used to represent the memory for each individual in the second year, and the second year served as memory for the third. Memory was measured as how far the animal was moving from its previous year’s migration, and how far it was moving away from the center of its previous range. To characterize the environment, they collected data on environmental variables that might impact movement, including snow depth, forage availability, slope, and distance to roads. They then tested models based on either the memory, environmental, or combination data to see which could best recover patterns of movement.
The findings indicate that memory plays a strong role during both spring and fall migrations in mule deer. While they did find evidence that environmental variables impact movement, memory had the strongest effect, ranging from 2-28 times stronger than any environmental variable. When simulated deer were only able to rely on environmental data, they were unable to replicate migratory routes in their models. Simulated deer relying on memory were able to recreate migratory routes and access higher quality resources, compared to those only relying on resource availability.
For wildlife and land managers, this study argues that the definition of habitat should be updated to not only include the interactions between an animal and its environment, but also the knowledge possessed by an animal of a place. For habitat conservation, if populations of an animal are lost, their memory of suitable habitat might be lost as well, and might no longer be utilized. In restoration, if newly restored habitats are not discovered by populations, they will not have the memory necessary to drive individuals to use these new habitats. These findings also become important for animal relocation; for example, it may take several years after introduction for a population to fully begin to utilize the resources in their new range.
Merkle, J. A., Sawyer, H., Monteith, K. L., Dwinnell, S. P. H., Fralick, G. L., and Kauffman, M. J. 2019. Spatial memory shapes migration and its benefits: evidence from a large herbivore. Ecology Letters 22(11): 1797-1805.