A buck mule deer in southwest Wyoming. (Tom Koerner/USFWS)

Countless studies have chronicled the importance of Wyoming’s mule deer migrations. They’re some of the longest in the world, stretching, in some instances, across hundreds of miles. 

Scientists know from GPS collar data, trail-camera images and tireless work that those migration paths are so precisely defined that animals often skirt the same bluff, travel the same ridgeline and cross the road at the same spot year after year. 

The fidelity evidenced in these well-worn patterns may be causing deer more harm than good in the face of a landscape changed by humans, according to a new paper out of the University of Wyoming and several other universities

For some species, in fact, this loyalty to what once were the places with the best food and nesting sites might actually be maladaptive. 

“Mule deer, especially, are impressively faithful and maybe somewhat unique in that matter compared with other ungulates,” said Kevin Monteith, a UW professor and longtime mule deer researcher. “And their world has been literally changing under their feet for decades, and we’re observing the consequences of that based on the population counts over the years.”

The paper, published Jan. 11 in Frontiers In Ecology and the Environment, is authored by UW professors Jerod Merkle and Anna Chalfoun, along with Western Ecosystem Technology’s Hall Sawyer and several others. 

Human-caused climate change altering habitats is bad news for wildlife, but it doesn’t guarantee mass die-offs, the authors say. Given enough time, some species may be able to find new places with food and breeding sites by following “innovators” — individuals that are more inclined to try new things. 

Mule deer cross a waterway in the cover image of “Ungulate Migrations of the Western United States: Volume 1.” (Mark Thonhoff/BLM Wyoming/FlickrCC)

But for others, like mule deer, fidelity to migration routes and seasonal ranges is often greater than their ability to adapt — making it all the more important to conserve critical habitat.

Dependable returns

Merkle has thought about where animals move for years. A decade ago, as a Ph.D student in Saskatchewan, Canada, he studied site fidelity in a free-ranging herd of bison. 

“Those bison had a circuit,” he said. “They moved through meadows the same way, and they showed up at the same week every year.”

When he came to Wyoming and learned about mule deer doing the same things over and over again, he said, “it struck me that animals are relying nearly 100% on their past experiences.”

Whether mule deer learn from their mothers, from others in the herd or some basic instinct, researchers still aren’t sure. But they do know from millions of data points that mule deer largely follow the exact same paths, around the same time, each year. 

And that consistency has long served as an evolutionary boon. The longest migrators leave winter range early and follow greening plants to take advantage of the choicest bits of nutrition. Other deer may leave a little later and not go quite as far. Yet more will only go a dozen or so miles. All of those experiences mean herds like the one in the Wyoming Range can support tens of thousands of animals by taking advantage of the variability on vast stretches of wild land. 

Winged migrations

Many migrating birds return to the same places year after year — a fact that likely stems from a practical reason: If a bird knows a certain tree has just the right nesting hole, or a certain acre of sage brush has just enough food, it doesn’t need to spend valuable time and energy looking around every year for a new location. 

Birds nesting in Wyoming are less studied than species like mule deer, said Chalfoun, who is also an assistant unit leader at the USGS Cooperative Fish and Wildlife Research in Laramie, but the trends are still worrisome. She can’t pinpoint exactly where, say, the Brewer’s sparrow goes in the winter when it migrates to the southwestern U.S. and northern Mexico. But she can say that in western Wyoming, the small songbird tends to return to the same region, even the same acre, to nest each spring. 

For the most part, migration as a survival strategy has worked. Species thrived and evolved slowly with the landscape until the last couple hundred years, and particularly the last half century. 

A Brewer’s sparrow incubates eggs. (Anna Chalfoun/USGS Wyoming Cooperative Fish and Wildlife Research Unit)

“Mule deer could literally go up to their summer range, and while they’re gone, a 2-acre well pad could be put in a place they used to forage in the past,” Merkle said. “When they come back, they would say ‘OK, there’s a new thing that’s where I used to live’ and for an animal that bases its life on where it used to live, that’s a hard thing to digest. They’re not used to trying new things and going new places.”

Birds like lesser snow geese continue returning to traditional feeding grounds even when food resources decline significantly and brood sizes drop, according to the paper.

It’s likely the same for the Brewer’s sparrow when well pads harboring predatory deer mice encroach on nesting areas. 

Some species simply adapt better than others. Elk, for example, will sometimes follow similar routes, but if one year their winter range is altered, they’ll go somewhere else, Monteith said. 

Mule deer, however, just don’t. 

A 2017 study by Sawyer showed that the Sublette mule deer herd declined by about 40% over 15 years. They “made fine scale-behavioral changes to avoid habitat close to infrastructure, yet they continued to return to the same winter range year after year,” the paper reads. Less winter range meant less food, which meant fewer animals could survive. 

That’s why some proposed solutions, such as off-site mitigation for energy development, may not work, the researchers say. 

The best solution, Merkle said, is to conserve existing migration routes for future generations.

Christine Peterson has covered science, the environment and outdoor recreation in Wyoming for more than a decade for various publications including the Casper Star-Tribune, National Geographic and Outdoor...

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  1. I think more than the re-introduction of wolves 27 years ago, the introduction of Europeans nearly 300 years ago has done more to depress the numbers of wildlife of all kinds. Humans do it much more surreptitiously than killing them outright these days. If you look around while driving up the North Fork of the Shoshone, the areas around Dubois, the expansion of subdivisions around Pinedale and secondarily the Pinedale Anticline and the Jonah Field you will see that we have moved into the wildlife’s winter home and left them little alternative. Drive from Powder River to Shoshoni and take note of the wellheads and interconnecting roads on both sides of the highway. There are lots of us and we are taking up more and more space on the earth and consequently removing that space from wildlife habitat. We can blame predators all we like, but the one we ignore looks back at us in the mirror every day. Great balanced article. Thank you.

  2. We live in the South Okanagan Falls area where mule deer have a protected habitat. However the landscape changes and there is development and the mule deer and their young continue to return and adapt.

  3. Not one comment about wolves in this article, amazing ! I lived in Wyoming most of my life , never a habitat problem till the wolves were introduced. We never wanted them, and now there is a population problem. Da

  4. Animals across the world have been adapting to natural climate fluxuations that have been occurring since God made this planet. And they will continue to adapt, including the mule deer that I proudly hunt every year. They never needed your input ir help in the past, so find something relevant to study with the millions of dollars the government gives you, like how to reduce the wolf & cougar populations that decimate deer hurds. Or better yet, retire.

  5. The same scientists are also finding out that mule deer both graze and browse. Elk are predominately grazers. When both species eat the same stuff, the muleys lose out. Mule deer were in the past always thought of as browsers. Elk numbers are rising, and have been as long as muley numbers continue to drop. Maybe G&F should liberalize elk seasons even more?! G&F is ALWAYS complaining because they’re not getting the elk kill they want! The deck is stacked against mule deer. Migration routes are perhaps THE most important component of mule deer survival. But there are a myriad of other obstacles in a mule deer’s life, too. Preserving migration routes are key!

  6. Poor management has impacted the population more than anything in various states. They have only handed out more tags year after year in Idaho and Nevada. Do your research on that, and you will find the actual culprits. Brutal winters used to be a problem, and now it’s climate change. This is far from the fundamental truth on why populations are low In many states. It is more related to profitability than healthy populations, which is sad for many to see the continued decline of various animal populations that find themselves managed.

  7. Would a 500-turbine wind farm on the mule deer winter range in Carbon County count as a disruption? Wyoming Game & Fish has some good charts showing the proposed Chokecherry wind project smack dab on mule deer winter range.