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They are few. They are fat. They are Western Wyoming’s deer

A decrease in competition for exceptionally lush, nutritious vegetation has created a population of exceptionally porky mule deer.

Judging by the fat levels of mule deer in the region, this fawn mule deer in Pinedale was in great shape going into the winter of 2023-'24, which has been mild through late January. (Mike Koshmrl/WyoFile)

Deer F219 was lucky. Now she’s fat.

The 7-year-old doe survived a harrowing winter that killed off more than 70% of the female deer that researchers were monitoring in the Wyoming Range. Deer F219 even raised two fawns for about a month last spring, which siphoned precious energy and fat reserves. 

Yet when University of Wyoming ecologists captured and immobilized the collared doe on her winter range in December, she was still in remarkably good shape. Some 22% of her body mass was fat, an “exceptional” number and just a few percentage points off of the fattest deer ever measured by the now 11-year-old research project.  

“These are the fattest deer that we’ve ever seen on a population level,” University of Wyoming ecologist and professor Kevin Monteith told WyoFile. “Animals were impressively fat,” across the board in the Wyoming Range, he said. 

Mule deer were exceptionally fat in December 2023 on both the north (NorthWR) and south (SouthWR)winter ranges, . (University of Wyoming/Monteith Shop)

UW’s research team found portly deer to the south, to the north, and they found fat deer across ages and sexes. Even lactating mothers “were as fat as we’ve ever seen them,” Monteith said.  

“We had a number of animals that were pushing the physiological limits of their ability to gain fat,” he said. 

The Wyoming Range’s deer, in other words, were just about as fat as captive deer that laze about feasting on rich, provided feed. 

A rotund research deer tracked by University of Wyoming moves her tush through the Wyoming Range foothills. (Rachel Smiley/University of Wyoming)

Like humans, fat deer don’t store their fat evenly across the body. Accumulation is not perfectly sequential, Monteith said, but it tends to start in the bone marrow, then around the organs. When they hit about 6% body fat, he said, the fat starts to layer around the rump.

“That’s where we measure it with an ultrasound,” Monteith said. “As it accumulates it kind of thickens, and it also moves forward on the animal toward its shoulders.” 

The chubby deer of the Wyoming Range do derive some benefits from those fat rumps. 

This winter has so far been mild, but if it turns into another severe one, they’d be likely to survive. 

“They’re set up for a knock-down, drag-out winter,” Monteith said. “Add to that the sagebrush growth, which was off the charts, and they’re prepared.”

This article was originally published by WyoFile and is republished here with permission. WyoFile is an independent nonprofit news organization focused on Wyoming people, places and policy.