Wyoming is known for its vast landscape. For years, the capability of large grazing mammals such as deer to confidently navigate this great space during their seasonal migrations has puzzled naturalists and scientists. There are two hypotheses about how they manage to do it.
Green Wave Surfing
The first hypothesis is that animals observe and follow cues in their immediate environment to determine where to move to. For example, the so-called green-wave surfing occurs when animals migrate to areas with better, greener forage, or come down from mountains covered in deep snow.
The central idea is that migratory behavior is dictated by the availability of the food, plus other environmental factors affecting the herd.
Herd Animals Might Use Mental Maps
The second school of thought claims that animals don’t simply live in the moment by following their immediate needs, but actually develop a memory of their habitats and use this information to guide them through the landscape. Further on, herd animals could be able to pass down this knowledge from generation to generation.
The paper from the University of Wyoming, published in the journal Ecology Letters, seems to support the latter idea – it found that memory can explain most of the deer migration behavior.
Mule deer (Odocoileus hemionus) is a species native to the American West, known for its markedly long ears. The University’s research suggests that mule deer navigate in spring and fall predominantly by using the knowledge of the past migration routes and the seasonal ranges of their habitats.
The team found that the past year’s migratory route and summer range had 2-28 times more influence on the choice of the current year’s migration path than the potential environmental cues and triggers – spring greening, snow depth, or topography.
Jerod Merkle, the lead author of the study and the assistant professor and Knobloch Professor in Migration Ecology and Conservation in the Department of Zoology and Physiology at the University of Wyoming, explains that “these animals appear to have a cognitive map of their migration routes and seasonal ranges, which helps them navigate tens to hundreds of miles between seasonal ranges.”
The UW study shows that animal migration is not reduced to the simplistic “where you find resources, you will find species that exploit them” idea. Rather, it supports the notion it is a complex phenomenon includes both the memory of the landscape and the current state of the environment.
“It appears that green-wave surfing helps them determine when to move within a kind of ‘map’ in their brain. The timing of spring green-up determines when an animal should migrate, but spatial memory determines where to migrate.”, Merkle says.
The migratory routes we see today were optimized for green-wave surfing in large landscapes across many generations of animals. Without the possibility of landscape memory to guide the animals between their seasonal ranges, the long-distance trails of the Green River Basin, which in some cases exceeds 300 miles, wouldn’t be able to exist.
The paper reinforces the findings of another UW-led study published in Science in 2018 – that translocated bighorn sheep and moose with no memory of the landscape can take anywhere from several decades to an entire century to learn how to migrate to vacant habitats.
The findings could have important conservation implications because it implies that the loss of a migratory population automatically erases the herd’s collective memory and mental maps of the landscape. “This is critical for conservation, because it tells us that, to conserve a migration corridor, we need to conserve the specific animals who have the knowledge necessary to make the journey,” says Matthew J. Kauffmann, the leader of the Wyoming Cooperative Fish and Wildlife Research Unit, where the research was conducted.
The concept clashes with the notion that original populations can be restored by ex-situ breeding programs and translocation of a fresh group of animals. The “lost memory” could make the restoration of migration routes very difficult, and the patches of potentially prime seasonal habitat would most likely go unused. The research points out that migrating animals can obtain greater forage benefits when they use the memory of the landscape than the migrants that rely on the local foraging cues only.
It could also have obvious implications for game management strategies, which commonly propose that the removal of the mature “trophy” animals which have gone past their peak reproductive years does the population a favor by making room for younger, stronger, more fertile individuals.
The study makes a strong case that scientists and other habitat managers cannot optimally manage populations by relying on the environmental information and range quality (the “cues”) only. It shows that it is difficult to establish sound conservation efforts without directly gathering movement data that reveals how animals interact with their habitat through complex multi-generational knowledge.
Besides the practical implication, the Wyoming mule deer migration study inevitably forces us to look at deer and other mammals as something beyond biomachines driven by hunger – it adds to the mounting evidence that they are well-capable of actions we could call reasoning.
Merkle, J. A., Sawyer, H., Monteith, K. L., Dwinnell, S. P., Fralick, G. L. and Kauffman, M. J. (2019), Spatial memory shapes migration and its benefits: evidence from a large herbivore. Ecol Lett. doi:10.1111/ele.13362
UW Study: Migrating Mule Deer Don’t Need Directions. The University of Wyoming. August 23, 2019. http://www.uwyo.edu/uw/news/2019/08/uw-study-migrating-mule-deer-dont-need-directions.html
Study Reveals Big-Game Animals Must Learn to Migrate. The University of Wyoming. September 6, 2018. https://www.uwyo.edu/uw/news/2018/09/study-reveals-big-game-animals-must-learn-to-migrate.html