Photograph by Robbie George, National Geographic Creative
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Bighorn sheep, like this ram photographed in Montana's Yellowstone National Park, must learn the best migration strategy from their mothers.

Photograph by Robbie George, National Geographic Creative

Sheep Teach Each Other How to Migrate Long Distances

Bighorn sheep migrate on long journeys, following a wave of green as plants come to life. Instinct doesn't teach this, but culture does.

Large migrations are some of nature's greatest spectacles. Wildebeest and zebra chase the rains through the Mara ecosystem every year, monarch butterflies trace a path from Mexico to Canada and back, and tiny songbirds fly nonstop for days at a time. And now scientists are starting to figure out how they know where to go, and when.

Some of these animals, they’ve found, have their migration pathways written into their genes. A songbird hatched in a laboratory, having seen nothing of the natural world, still attempts to begin migration at the right time of year and in the right cardinal direction.

But large mammals like bighorn sheep and moose are a different story. Wildlife researchers have long suspected that they require experience to migrate effectively, that their annual journeys are the result of learning from one another, not of genetic inheritance. A new study, published Thursday in the journal Science, suggests that those hunches may be correct—some animals must learn how to migrate.

The existence of collective information and knowledge, that can be passed from older animals to younger ones, is a form of “culture,” researchers explain. And when animals learn as a result of social interaction and the transfer of this information, that’s a type of cultural exchange—as opposed to genetic.

On the mountains and plains of North America, massive herds of ungulates—hoofed creatures, like caribou, elk, moose, and bighorn sheep—migrate from high altitude breeding grounds to warmer, lower altitudes during the harsh winter, following the growth of new greenery. Ecologists call this “surfing the green wave,” and the new study finds that bighorn sheep and moose have to learn how to surf.

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Mother Knows Best

Due to market hunting and disease transmission from domestic sheep, bighorn sheep populations began to plummet in the late 1800s. Starting in the early 1970s, wildlife officials and hunting groups began to re-establish wild sheep populations by moving individuals from surviving herds into areas that were once part of the species’ historical range.

The history of these translocations, combined with newly available GPS tracking technology, allowed University of Wyoming ecologist Matthew Kauffman and his team to trace the development of migration behaviors. Led by graduate student Brett Jesmer, the team affixed GPS collars to 129 bighorn sheep from established populations at least two hundred years old, along with 80 sheep and 189 moose that had recently been moved.

“With ungulates, the thought was that there is no genetic program. Instead they just have to learn how to do this,” says Kauffman. And if that’s the case, he says, the animals that have been moved shouldn’t migrate, because they wouldn’t yet have learned a new migration route.

That's exactly what they found.

“With bighorn sheep and moose, and this is true for deer and elk as well, the young are highly dependent on their mothers. For pretty much the first year of life, they're basically following mom around,” says Kauffman. “So they're developing this spatial memory of the migration route from mom.”

And of the 80 translocated sheep, only seven even attempted a migration, and these were individuals that were integrated into pre-existing herds of several hundred migrating sheep. That suggests that migration knowledge can be transferred horizontally, among adults, not just vertically, through generations.

This isn't to say that ungulates couldn't have some innate motivation to seek out new opportunities. The issue is knowing how to do so while staying safe. “Knowing how to get from A to B usually involves crossing some habitats where there is a higher risk of predation, where maybe forage conditions aren't very good, so animals need to know where to go,” says University of Sherbrooke biologist Marco Festa-Bianchet, who was not a part of the study. "That's the part that needs to be learned."

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Bighorn sheep can form groups, which facilitate the transfer of cultural knowledge. This group was photographed scaling a mountain in Yellowstone National Park.

Learning to Surf

An optimal migration would precisely line up with the peak of the green wave, with animals moving into new areas as soon as vegetation starts growing, while also avoiding risky areas full of predators. The new study shows that sheep and moose do learn to optimize new routes over time; the longer a population survives in a new habitat, the more effectively its members can surf.

Those animals who chance upon a more efficient migration strategy survive longer and leave behind more offspring. Those young learn how to migrate from their mothers, and acquire new knowledge along the way to even further refine their migration strategies. Taken together, this means that ungulate migration is a form of cumulative culture—says Kauffman, a system of behaviors passed from generation to generation, each cohort building upon the knowledge of their predecessors.

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Moose, like this male photographed in Alaska's Denali National Park, lead more solitary lifestyles, which means it takes longer for knowledge to propogate through a population.

There's just one catch: for a newly translocated herd of bighorn sheep, it could take 50 or 60 years before half the herd becomes proficient at surfing the green wave. For moose, perhaps because they are more solitary creatures with fewer opportunities for social learning, it could take a century, or longer.

“Wildlife reintroductions, for all sorts of different species, often fail. This gives us a lens into why they fail,” says Kauffman. “They fail, in part, because animals don't have knowledge about how to exploit new landscapes.”

So when a road, a fence, or a new housing development disrupts an established mammal migration pathway, there are consequences not just for the herd's survival, but for its collective knowledge. And if mitigation strategies, like wildlife-friendly fences and highway overpasses and underpasses, aren't implemented quickly enough, it could take decades or even a century before a population can recover.