After hours of watching Yellowstone elk herds through a spotting scope, Scott Creel noticed a few interesting things.
When wolves appeared, the elk turned skittish. They spent more time on alert – heads in the air, ears pricked – and less time eating. They also left prime winter range to take cover in forested areas, where less food was available.
Even when wolves were nearly two miles away, the vigilant behavior persisted, said Creel, a Montana State University ecology professor.
Creel and fellow researchers linked the altered elk behavior to lower calf production. As their body fat drops, cow elk have difficulty staying pregnant through winter. They grow emaciated and abort, the research concluded.
The work helps answer questions about low elk calf numbers in some herds in Yellowstone National Park, said Creel, the lead author of a study that appeared last year in the Proceedings of the National Academy of Sciences. It indicates that wolves affect elk populations in subtle but important ways beyond direct kills, he said.
“If you look at the observed rate that wolves are killing elk calves, there are too many calves missing,” Creel said. “We can’t account for them all through direct predation.”
As wolves regain their role as top predators in the West, Creel said, the study could have broad implications.
“Those things elk are doing to avoid being killed may carry high costs,” he added. “We know that elk reproduction is affected by the severity of the winter. Wolves could be a complicating factor that makes it more difficult to maintain a pregnancy through a period when they’re losing body fat anyway.”
The research comes amid intense public interest in wolf-elk interactions. In Idaho, elk hunters and environmentalists engage in fierce debate over which factor played the greater role in the once-famous Lolo elk herd’s decline: worsening habitat or wolf kills. In Wyoming, state Fish and Game officials are trying to understand discrepancies in pregnancy rates among 4,000 elk that live in herds between the city of Cody and Yellowstone. Drought, disease and wolves are possible influences.
“It’s a complicated set of questions that people badly want to know the answers to,” said Arthur Middleton, a University of Wyoming doctoral student involved in the study.
Other studies by the National Park Service haven’t established a correlation between wolf-elk ratios and the proportion of pregnant adult cows. Creel said he couldn’t comment on other researchers’ work, but he said differences in study methods could have yielded different results.
Creel’s work is intriguing because it links wolves, habitat and elk nutrition, said John Cook, a wildlife researcher from La Grande, Ore. For the past 20 years, Cook and his wife, Rachel, have studied Western elk herds and how nutrition influences reproduction. Their work indicates that pregnancy rates drop off when a cow elk’s body fat drops below 9 percent.
But scientific understanding of how wolves influence elk behavior is in its infancy, Cook cautioned: “There’s an awful lot that’s not known.”
Cook and his wife surveyed Yellowstone’s northern elk herd during 2001, 2002 and 2003. Pregnancy rates and body fat were high.
“Those animals were in pretty darn good shape,” Cook said.
During a four-year period, Creel and his research team collected 1,205 scat samples from cow elk. The samples were taken after March 15, when pregnant elk would be in their third trimester. The scat was tested for progesterone, a hormone needed to maintain pregnancy. In areas with high wolf-to-elk ratios, progesterone levels in the scat decreased fourfold, Creel said.
In some other projects, the elk were tested for pregnancy in late fall or early winter. Creel said his method looked at whether elk were still pregnant later in the season, when the nutritional demands of the pregnancy are the highest, and food is typically scarce.
“This isn’t a new idea in ecology,” Creel said. “When a predator is in the system, it affects the system’s dynamics.”
Aquatic invertebrates retreat to colder, deeper waters to escape their prey. They grow more slowly in colder waters and produce fewer eggs, Creel said.
But less is known about large carnivores, he said. For the past 2 1/2 years, Creel has been working on similar studies with lions and hyenas in southern Kenya. Those studies examine whether the presence of lions and hyenas affects reproduction in their prey species beyond direct kills.
Closer to home, Creel said his work could shed new light on the intricate interactions of predators and their prey. Yellowstone wolves rely on elk for 85 percent of their diet.
As a result, the evolutionary pressure on elk to be good at detecting wolves is “pretty strong,” Creel said. Unfortunately for elk, “you can’t eat and do that at the same time,” he said.