Yellowstone Grizzly Bears Maintain Their Weight Amid Challenges

Loss of food sources and increase in population hasn’t resulted in skinnier, smaller bears, study finds

June 20, 2023

By K.L. McQuaid

Special to the Wyoming Truth (via AP Storyshare)

Despite a loss of key foods, a changing environment and an increase in their numbers over the past two decades, grizzly bears in and around Yellowstone National Park remain fat and happy.

Well, fat at least.

A new study by a consortium of government agencies that keeps tabs on ursus arctos horribilis – Latin for grizzly bear – concluded that the animals have adapted to changing conditions and maintained their body weights and fat compositions from 2000 through 2020.

The amount of bear fat is especially important for female bears, which require at least 20% of their body weight be in the form of fat to reproduce, according to an interagency study published earlier this month in the scientific journal Global Change Biology.

Male bears require significant fat stores, as well, to survive hibernations that last between three and seven months through Wyoming winters.

“Larger and fatter individuals usually have a higher probability of survival and a greater capacity to invest energy in reproduction,” the study stated.

Scientists from the U.S. Geological Survey and an Interagency Grizzly Bear Study Team, which includes officials from Wyoming Game & Fish and several other state and federal agencies, noted that the grizzly bears’ weight and fat levels have stayed steady despite the decline of high-calorie foods, such as cutthroat trout, elk and whitebark pine tree seeds.

Many of the trees, which earlier this year were designated as threatened under the Endangered Species Act, have been felled from wildfires and a beetle infestation in recent years.

In all, the scientists believe there are now about 1,000 grizzly bears in the Greater Yellowstone Ecosystem and about 2,000 in the Lower 48 states.

Three years ago, the ecosystem comprised both Yellowstone and Grand Teton National Parks, portions of five national forests and private land in Wyoming, Montana and Idaho. In all, it measures over 27,200 square miles – nearly the size of Vermont, New Hampshire and Massachusetts combined.

Since 1975, when grizzly bears were placed on the federal Endangered Species List as their numbers dwindled to about 250, grizzly hunting has been prohibited.

“This study shows the benefit of having a long-term data set in studying a species like grizzly bears,” Dan Thompson, the large carnivore section supervisor at the Wyoming Game & Fish Department and co-author of the group’s paper, said in an interview with the Wyoming Truth.

Scientists had hypothesized that as the bears’ population increased – and along with their numbers greater competition for food – the grizzlies’ fat composition would decrease.

To their surprise, however, the capture of over 400 bears determined that grizzlies had not lost fat stores.

“We know that some of their food supply has declined, especially over the past decade, but what the bears’ weight and fat composition is telling us is that they can still find the resources to build up an adequate fat supply to thrive,” Frank van Manen, an ecologist at the U.S. Geological Survey who co-authored the study with nine other researchers, told the Wyoming Truth.

A renewed debate on status

The grizzly bears’ success comes as renewed debate has swirled around their protected status.

In May, Wyoming sued the U.S. Department of the Interior in federal court for failing to meet a 12-month deadline to decide about delisting the grizzly bear from the Endangered Species List.

Gov. Mark Gordon and others contend the bears have sufficiently recovered and their future conservation and management should be left up to the state. Such a move could pave the way for the bears to be hunted again.

“Wyoming’s grizzly bear numbers have not only greatly increased, but we have exceeded population goals for years,” Gordon said in a statement announcing the lawsuit.

“It’s time for the delisting process to move forward.”

The U.S. Fish and Wildlife Service in February 2022 received petitions from both Wyoming and Montana seeking to delist the grizzly bear in the Yellowstone and Northern Continental Divide ecosystems, and the agency acknowledged that “credible information” had been provided.

But the federal agency has yet to render a decision on delisting, leading to the state’s lawsuit.

Grizzly bears in the Greater Yellowstone Ecosystem have shown signs of recovery since the late 1980s, van Manen said. Much of that rebound can be attributed to their protected status in and around Yellowstone.

The federal agency has taken steps to delist the grizzly bear previously. Six years ago, it concluded the bears could be removed from the endangered list, but the move was stopped by environmental groups that successfully sued the federal agency over the decision.

Most recently, prior to the state’s legal push, U.S. Rep. Harriet Hageman (R-Wyo.) sponsored a bill seeking both to delist the grizzly bear and prevent the action from being reversed by litigation. The legislation has thus far cleared the U.S. House’s Natural Resources Committee.

A Lot of Food

Van Manen’s team determined that grizzly bears have been able to maintain their body fat levels despite the gains in their population in recent years because they eat a wide variety of foods.

In all, the study found that omnivorous grizzlies consume at least 260 types of food.

“Grizzly bears are very flexible and adaptable,” Thompson said.

In place of the whitebark pine seeds, for instance, the bears have increasingly fed on the carcasses of dead animals to build up fat stores, especially in the fall months prior to hibernation.

From June 1 through Oct. 31, female grizzly bears gained 15.3% body fat, while male grizzly bears gained nearly 17% body fat. The bears’ physiology apparently allows them to “prioritize” fat storage over the development of lean body mass, the scientists concluded.

Female bears typically reach physical maturity at age seven, while males don’t mature until age 14. Grizzly bears’ average lifespan exceeds 25 years, in part because they have no natural predators except humans.

While the study concluded that grizzly bears have ample food supplies at present, van Manen and his colleagues caution that continued climate change, droughts and increased human impacts, such as real estate development, could impact grizzly bear populations, their food supplies and their ability to reproduce in the future.

Scientists found that more food for the bears existed in the first decade of the study than in the second.

Additionally, the Yellowstone ecosystem has undergone “profound warming” over the past 20 years, the study found.

For now, the bears are thriving despite the challenges.

“They’re really resourceful animals,” van Manen said. “They’ve shown a lot of adaptability and resilience, and that bodes well for the immediate future of this population.”