The Times-Independent
MOAB WEATHER

Fawn death spikes in Book Cliffs

‘We’ve never had anything like that’

Over his roughly 10 years of monitoring newborn fawns in Utah’s wild areas, Brock McMillan said their mortality rates and causes of deaths remained consistent.

Sedated fawns with their tracking collars. The extremely young offspring are known as “neonates,” having just been born.
Photos courtesy of Brock McMillan

Many years, the Brigham Young University professor would see mortality rates of about 20%. Typically, those fawns had been killed by predation — attacks from mountain lions, black bears or bobcats.

“That was not the case last year in the Book Cliffs,” McMillan said.

Among the roughly 50 fawns McMillan and his team tracked last June in the Book Cliffs, over two-thirds of them died within the first three weeks of life. And they hadn’t seemed to die of predation.

“It was whole carcasses that had just tipped over,” he said.

In fact, researchers still don’t know what happened to the fawns: multiple lab tests have all returned inconclusive.

“We just don’t know, because we’ve never had anything like that,” McMillan said.

A rugged region of cliffs and canyons in northern Grand County, the Book Cliffs features higher elevations and lower temperatures than Moab’s own environs.

The region is known for its abundant large wildlife, with not only deer, mountain lions, black bears and bobcats, but bighorn sheep, elk, coyotes and hawks.

According to McMillan, sportsmen started noticing a drop in deer and elk populations in the Book Cliffs about five years ago, enlisting the state to investigate why. So starting in 2019, McMillan and a team of researchers started tracking fawn populations, augmenting tracking he’s done in other parts of the state since 2012.

With most fawns born in early June, McMillan and researchers capture, sedate and collar fawns, attaching monitors that allow the team to track fawns and find them if they die.

“Almost always in all the previous years we’ve done, the primary cause of mortality during the first six months of life is predation,” McMillan said.

The Book Cliffs fawn monitoring, which started in 2019, typically involves 50 newborn deer.

This year, however, researchers could tell predators hadn’t killed most of the fallen fawns. According to Division of Wildlife Resources biologist Clint Sampson, who partnered on the research, fawns slain by predators might feature bite marks, lost limbs or evidence of caching.

The fawns that died last year didn’t show any of these signs.

Nor had the cause been malnutrition, Sampson said.

A fawn who died from lack of nourishment “looks like it just laid down somewhere; their heads are kind of tucked in by their sides,” he said.

These animals, however, had milk in their bellies and weren’t tucked away. They were “basically falling on the … trail behind their moms, and then just tipping over and dying,” Sampson said.

“And it was such a close proximity of time,” he added. “… All of a sudden we just started losing fawns. … For about six days, I think we lost the majority of the collar study.”

Researchers still aren’t sure what happened to the fawns; tests at multiple labs have all been inconclusive. Only one fawn showed signs of potential disease.

Right now, scientists’ best guess is heat exhaustion.

Indeed, June 2021 in the Book Cliffs, like in the rest of southeastern Utah, was “extremely hot,” in Sampson’s words.

Researchers in the Book Cliffs tag fawns for monitoring last year.

“At 8,500 feet, it was 90 degrees in June, which is unseasonably hot,” said McMillan. “It was just ridiculous. Those animals, I don’t think, have the ability to deal with that kind of heat.”

Sampson said many of the fawns’ ears were crinkled or wrinkly when they were found, a potential sign of dehydration.

“The heat is just brutal for these little animals, because their motors are just going so fast and they’re growing at such a fast rate,” he said. “Maybe they’re having such a hard time cooling down.”

“Females will walk away from offspring if it’s too hot,” McMillan added.

Heat exhaustion is, however, still just a hypothesis. McMillan said his team is analyzing other factors, such as time handled by researchers and conditions of the fawns’ birth, to dispute or give credence to the idea that their high mortality rate was temperature-driven.

McMillan, however, doesn’t think the brief handling time is a viable explanation on its own.

“We’ve been doing this for nine years, and we’ve never experienced this before,” he said. “But it could be a combination of the extreme heat and the handling time.”

“I’ve been studying fawns for nine years,” he later added. “This is the first year we’ve ever had newborn mortality like that.”