WYOMING RANGE—At nearly 14% body fat, mule deer F14 was plump in the rump, and expecting two fawns, going into this most recent winter.
The 8-year-old doe’s dual pregnancy was par for the course — F14 was prone to birthing twins. She was good at raising them, too. Four of her eight previous fawns had survived to independence.
“F14 was a really good mom,” University of Wyoming ecology PhD candidate Tayler Lasharr recalled.
No track record of success could save F14 and her peers from the brutal winter that was to come. She and most other collared does in the heavily studied Wyoming Range Mule Deer Herd died before spring thaw.
“Families have been either wiped out completely, like F14’s,” Lasharr said, “or a very big proportion of them got removed this winter.”
Only two of F14’s progeny — twin bucks born in 2020 — are still alive, but they don’t mirror mom’s home range and habits like her female offspring did.
Lasharr and the ecologists of the University of Wyoming’s Monteith Shop had captured F14 twice a year to take samples and measurements and followed her movements with a GPS collar since she was a wee fawn born on the slopes of the Wyoming Range in 2015. They’d studied her mother, too — F108 was one of the very first does captured for the now decade-old Wyoming Range Mule Deer Project, Lasharr said.
“We’ve been studying not just this animal, but we studied her mom and we studied her offspring,” Lasharr said. “We knew what the family was doing. Now they’re essentially gone from the population.”
It’s “sad,” she admitted, momentarily straying from the emotionless impartiality that’s typically expected of research scientists.
Such losses, however, have not been bereft of lessons. Having tracked F14’s and other mule deer families through the previous severe winters of 2016-’17 and ‘18-’19, Monteith Shop ecologists knew what was likely in store for their subjects as the winter grew ever colder and snowier. Most of the previous summer’s fawns would likely die. This spring’s fawn crop would also likely struggle after being born to mothers in terrible shape. More adult deer would die, too: In ‘16-’17 and ‘18-’19, over 30% of the research does perished.
Yet, the winter of 2022-23 ended up an altogether different beast. In its wake, researchers are left to make sense of off-the-charts new data sets and wrestle with an urgent new question: How does a deer herd recover from such devastation?
By the time UW professor Kevin Monteith, who leads the lab, briefed WyoFile in early June, roughly 70% of the herd’s collared does and 60% of collared bucks had been lost to winter. Every single collared fawn born in 2022 was gone. A single yearling had been hanging on, he said, but it too died on May 15, the opening day of the delayed shed antler hunting season.
Hard as it is to fathom, the death rate could have ended up much worse. Deer in the Wyoming Range went into winter pretty plump, averaging 12% body fat.
“They were prepared for winter,” Monteith said.
The herd enjoyed a much higher “nutritional plane,” he said, than it had going into the winter of 2016-17, when average body fat in the Wyoming Range was nearer to 8%.
“We would have lost almost everything, had those deer started at 8% fat” this year, Monteith said. “It puts survival down to like 10%.”
As it is, the Wyoming Range Deer Herd has been completely reshaped.
Wyoming Game and Fish Department wildlife biologist Gary Fralick assumed his current post in the early 1990s, just after the population hit its modern day high of nearly 60,000 mule deer. Numbers have ebbed and flowed during his three decades on the job, but generally have hovered around 30,000 animals since the bad winter of 2016-’17. Even that makes the Wyoming Range Herd, renowned for producing big bucks, one of the largest mule deer herds in the American West.
It’s impossible to say where the population stands today ahead of the state agency’s systematic post-hunt flight surveys, but some are predicting that the herd could dwindle to 10,000 deer. Fralick doubts the numbers will slump quite that low, and has stayed sanguine about the embattled herd’s prospects.
“This herd still has the capability to climb back to the objective of 40,000 deer,” Fralick said while on a drive through the herd’s La Barge-area winter range. “But you need good conditions. It’s all a gamble.”
The herd, he said, can grow at a 20% clip thanks to productive mom’s like F14, even when factoring for natural causes of mortality, like predation. Human hunters only target antlered bucks in the Wyoming Range, and killing mature males has almost no effect on the population trajectory. Winter severity remains the most important variable, and it’s the real wild card.
It may be spring, but one factor that’ll stymie the herd’s recovery is that the winter isn’t done killing Wyoming Range deer. The University of Wyoming research crew has learned that the dismal health of does following such a hard winter will plague fawns born in coming weeks for their entire lives.
Fralick emphasized just how rough shape the surviving does are in.
“It’s just extraordinary, I’ve never seen anything like it,” he said. “The angular hips, the crest of the spine sticking above the hide, the J-shape of the neck.”
Fawns born to those does are doomed to have smaller bodies and antlers, for life. And those are the lucky ones. It’s likely most 2023 fawns won’t even get the chance to live a stunted life.
“We probably won’t lose all of them, but fawn survival this coming year we anticipate being very low,” Monteith said. “I’d be surprised if [survival] was over 30%. I suspect it’s going to be well under. As we’ve seen before, the leading cause of mortality will probably be stillborns and abortion.”
On top of that, predation dynamics are poised to change — and not in the deers’ favor. Winter likely didn’t knock down predator numbers like it did their prey, Monteith said. Predation is normally a drop in the bucket in terms of Wyoming Range Herd population drivers, but it becomes more pronounced in the aftermath of severe winters.
“Their prey base just dropped to less than a third of what it was before,” Monteith said of black bears, mountain lions and coyotes. “It changes things so much.”
The Wyoming Game and Fish Department is examining whether to drive down lion and bear numbers through hunting seasons and coyotes through aerial culls. County predator boards equipped with record funding are doing the same, but past research out of Idaho suggests that even aggressive coyote killing has “marginal” short-term benefits and no appreciable long-term effect on mule deer population dynamics.
Yet another factor that could slow regrowth of the Wyoming Range Herd is that large swaths of prime habitat are likely to go unused in the near future because all the deer that knew to use them are dead.
Monteith Shop ecologists have learned that female yearlings select home ranges adjacent to their mothers, clustering near where they were born like the pedals of a rose. When winter wipes out an entire multi-generation family, like it did to F14’s, the remaining deer don’t branch out to all the viable habitat.
“If there are places on the landscape that are not used … we’re trying to understand what that means,” Monteith said.
Translocating deer to unoccupied swaths of rangeland could be one potential remedy, he said, but that intervention is also fraught with tricky questions: “Whether or not it would work? Whether or not we should? Whether or not we let natural recovery take place?”
Surely, Monteith said, the population has bounced back from winters of this magnitude before — but it happened before the science was mature enough to fully understand what recovery looks like.
A silver lining
The growing body of scientific literature ecologists are building in the Wyoming Range also suggests there are causes for optimism in the wake of the deadliest winter on record.
Because of the big precipitation year, survivors are likely to have access to better food on their summer range, which in turn will make them fatter and more resilient going into future winters.
And there’ll likely be more and better food on their winter ranges, too, said Troy Fieseler, a terrestrial habitat biologist with the Game and Fish’s Pinedale office. He anticipates well above average leader growth on sagebrush, bitterbrush and other mule deer food staples in 2023.
“These plants, they greatly benefit from annual precip and really the snowpack,” Fieseler said.
He spoke while standing near a fenced enclosure in the Wildcat Canyon area. Mahogany that grew inside, and were protected from deer, towered over those available for eating outside the fence. Muleys waiting out winter in the Wyoming Range can eat 100% of the fresh growth sprouting off of the most delectable deer foods like mahogany during some years, he said. With the deer population and browsing pressure now much lower, that’ll give those choice plants the chance to grow and perhaps take root in new places.
State wildlife officials made big investments into stimulating deer habitat in the Wyoming Range. They’ve sprayed to keep invasive cheatgrass at bay on tens of thousands of acres, mowed sagebrush to stimulate new leader growth and they’ve trimmed and burned in the high country to promote aspen growth on mule deer summer range. It’s unclear if all that work will influence the Wyoming Range Herd’s population dynamics, but it can’t hurt.
“The Wyoming Range is one of the very few places in the world where there’s been landscape-level habitat treatments and manipulation,” Monteith said. “Even if it’s like a fragment of the population, every little bit helps.”
Survivors on the Wyoming Range are likely to fatten up for other reasons, too. Researchers have previously detected up to 25% body fat content in the piggiest of deer. Does, more likely to be without fawns, will be “liberated” from the costs of reproduction, Monteith said, and able to devote more of their resources to packing on body fat. And there are fewer mouths out there competing for really good food.
“We anticipate them to be impressively fat this fall,” Monteith said. “They’re going to go from basically near rock bottom to some of the fattest deer that we’ve seen.”
Game and Fish spokesman Mark Gocke summed up his view of the path to recovery after documenting some of the tens of thousands of carcasses that littered the Wyoming Range in winter’s wake.
“Mother nature holds the cards,” Gocke said. “There is nothing, from a hunting perspective, holding that deer population back. Biologically, it’s like, ‘green light.’ They can grow and grow and grow.”