Every winter, thousands of mule deer eke out a living in southwest Wyoming’s Red Desert, a vast, rolling, sagebrush-covered landscape. Come spring, some of the deer in the Sublette herd will stay put, just as their mothers, grandmothers and generations before them did. Others will wander about 70 miles into nearby foothills. And the rest will head out on an ungulate odyssey, follow greener pastures up to 150 miles into the mountains, stopping for munchies along the way.
Now, a team of researchers at the University of Wyoming and Wyoming Game and Fish Department have conclusively determined that the deer who walk the farthest end up the fattest and live the longest. The farther they walk, the more calories they find as they reach lush mountain meadows teeming with high-protein groceries. And because they get fatter, they carry more fawns to term.
“The migrating mule deer are the engine” for the whole population, said Anna Ortega, lead author on a recent paper published in the journal Current Biology. “If you sever that migration, you will have far fewer animals.”
The conclusion is another remarkable finding from an even more remarkable study, one that required researchers to use helicopters to capture and study hundreds of deer every spring and fall for nearly a decade. The scientists measured the animals’ fat levels and pregnancies and followed them in the fall to document their fawns’ survival rates.
It’s scientific evidence backing up what many in the region already knew: This ability to surf the green wave, as the researchers call it — following good food as it creeps up into the mountains over the spring and summer — is critical to mule deer survival, said Brandon Scurlock, a Wyoming Game and Fish wildlife biologist who wasn’t involved in the paper. But reaching that conclusion scientifically required a lot of work — year after year of studying the same herd, following them through hard winters and dry summers.
“Long-distance migrants have a lot of risk associated with them, with fences, vehicles and predator assemblages,” said Scurlock, who helps manage the Sublette herd. “But is the risk worth it? It seems like it is.”
Tanner Warder/Wyoming Cooperative Fish and Wildlife Research Unit
At some point, the herd likely flourished using all three strategies, said Matt Kauffman, a U.S. Geological Survey researcher at the Cooperative Fish and Wildlife Unit at the University of Wyoming, who started the study. When early and harsh winters hit the mountains, the long-distance migrants may have had a hard time making their way back to the desert in the fall. During those years, the deer that stayed in the desert likely fared better. But now, only the medium- and long-distance migrants are thriving, as climate change makes the West hotter and dryer. The changing climate means there’s less food in the desert year-round, and, likely as a result, the desert-dwelling portion of the herd is dwindling; it may even blink out in the next 50 years.
Mule deer — unlike elk or pronghorn, which can wander based on food and weather conditions — tend to stick to the path they were born into. “Mule deer are really hard-wired,” Kauffman said. “They’re not like, ‘How are conditions this year? Is this a good year to stay or go?’ They have a strategy and do the strategy.” That means the desert residents are unlikely to decide to become long-distance migrants, he said.
As a result, it’s vitally important to keep pathways open so deer can continue accessing green pastures and better food. The Sublette deer migration received some protection under a 2020 executive order, but threats persist, said Joey Faigl, co-founder and president of the Muley Fanatic Foundation based in southwest Wyoming.
“There are so many things they face every year. It can either be housing or oil and gas or highway crossings,” he said. “There is always going to be something. And if people stop fighting for them, populations will continue to decline.”
