From the vast movements of wildebeest across the Serengeti to the circumpolar movements of the arctic tern, migratory behavior has fascinated humans for centuries. Earth is home to remarkable annual migration distances, including over 80,000 km for the arctic tern, over 5,000 km for desert locusts, and over 5,000 km for barren ground caribou. Such distances have led many biologists to ask the question: how are these extraordinary feats accomplished?

My research focuses on two mechanisms that aid mammals in migrating to distant landscapes.

  1. Surfing the green wave. Due to the seasonality of earth, resource waves (pulses of ephemeral food that propagate across a landscape over time) are ubiquitous. The most well-known resource wave is the flush of high-quality vegetation in spring that occurs along elevational and latitudinal gradients worldwide. Herbivores that track or ‘surf’ such waves of green-up are maximizing their exposure to tender young plants that represent the highest quality food of the year.  By tracking these resource waves across large landscapes, the resulting animal movements will resemble a migratory pathway. My work has helped extend this idea to ungulates (Merkle et al. 2016, PROC B), as it was originally developed to explain bird migration. We show how mule deer, bighorn sheep, bison, and moose select habitat patches when at peak forage quality, providing strong evidence that ungulates surf the green wave. My collaborators and I are also working on identifying the determinants of individual variation in green-wave surfing as well as it fitness benefits for ungulates.
Male bison grazing on the tender young shoots of spring green-up.
  1. Memory. 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 (including ungulates), we know much less about how individuals migrate. Although mammalian migration distances are generally shorter than avian and insect taxa, many mammals migrate distances that greatly exceed their perceptual range – suggesting that memory plays a strong role in these movements. Indeed, simulations of animal foraging suggests that memory should play a strong role in the emergence of animal distributions resembling migratory behavior. My work is showing how ungulates use memory during migration and how important memory is in allowing these animals to migrate such long distances. Along the same lines, my collaborators and I are also working on quantifying how site fidelity (based on memory of previously visited locations) constrains the movements of ungulates, with strong implications for conservation of migratory routes and offsite mitigation efforts.
Red Desert to Hoback mule deer migration routes of up to 250 km (one way), represented by GPS collar data from 2010-2012. Migration routes depict clear recurrent movements (i.e., high site fidelity and use of memory) bewteen spring and fall migration (A) and between consecutive years of monitoring (B).