How wolf predation shapes elk antler evolution

University of Montana researchers and their partners recently published a study in Nature Ecology and Evolution chronicling an evolutionary tie between wolves and when bull elk shed their antlers.

The authors were UM doctoral candidate Matt Metz, along with UM co-authors Doug Emlen and Mark Hebblewhite, Dan Stahler and Doug Smith of the National Park Service, and Dan MacNulty of Utah State University.

They discovered that wolves in Yellowstone National Park preferentially hunted bulls who already had shed their antlers over those who still had them during late winter. The finding suggests that antlers are used for more than just competing for cows -- that they help deter predators, too -- which could help explain why bulls shed their antlers long after the rut.

"Because wolves often prefer elk in these systems, male elk uniquely keep their antlers for much of the winter," Metz said. "Other species, say moose in our study system, shed their antlers beginning in December. We believe elk evolved to keep their antlers longer than any other North American deer because they use their antlers as an effective deterrent against wolf predation."

Antlers evolved to improve breeding success. The bigger the antlers, the more likely a bull is to successfully breed cows during the fall. In many species, these types of weapons also serve secondary purposes, but those reasons are not always well understood, as is the case for elk.

Bulls shed their antlers beginning at the end of each winter and immediately start growing another set. Getting rid of antlers as soon as possible removes a cumbersome burden and gives individual bulls a jumpstart at growing antlers for next year's rut.

But as a whole, elk shed their antlers months later compared to other North American deer species, and shedding is staggered over a two month period beginning in March, suggesting there might be other reasons to keep antlers around a little longer.

"Antlers are the product of sexual selection, where males are competing over breeding opportunities in a short time window in the fall," Metz said. "Here we show that the evolution of antlers was also influenced by other things in an elk's environment, like wolf predation, and that a secondary function also helped to shape the characteristics of this structure, such as when antlers are shed."

Metz, a longtime Yellowstone wolf researcher, analyzed over a decade's worth of data from the Yellowstone Wolf Project, a long-term study recording interactions between wolves and their prey.

Wolves in Yellowstone often kill bull elk during the winter months, but Metz and his co-authors found that wolves strongly preferred to kill individuals who had already shed their antlers -- even though they were often in better condition than bulls who still had their rack. The results showed that antlers are indeed an important predatory deterrent for elk -- a secondary function that could help explain variation in antler retention time across species in temperate climates.

"These males that shed their antlers first are more vulnerable to being killed by wolves despite being in better nutritional condition," Metz said. "The individuals who are in the best condition are the first to drop their antlers to get a leg up on growing larger antlers for the next season and therefore gain the greatest reproductive success. Wolves mostly target individuals who are very young, old or in poor nutritional condition, which are characteristics that make them vulnerable. Here we identified a new, unexpected vulnerability -- shedding antlers early."

The study highlights an evolutionary Catch-22: Weapons come with both benefits and costs. Bulls who drop their antlers early may grow relatively larger antlers in the upcoming year, winning more cows, but they're also at greater risk to become dinner first.

For younger bulls unlikely to be successful in the upcoming rut anyway, the risk is too high. These males are among the last to cast their antlers.

Metz earned a B.S. in wildlife biology from UM and has studied predator-prey dynamics in Yellowstone since 2002.

"The study first came about through observations that I had when I worked as a seasonal field technician for the Yellowstone Wolf Project in the early 2000s," he said. "We usually see antler-casting starting at the tail-end of March. But the winter of 2004-05 was milder, and bulls were shedding their antlers a little earlier."

One day he and another researcher spotted a wolf-killed elk.

"We hiked into it, and it was an antlerless male in early March," Metz said. "We hadn't seen any living bull elk without antlers yet on the landscape. We thought, 'That's kind of interesting.' We just started to notice this relationship where we would observe wolves encountering male elk, and they seemed to be focusing on those who had already shed their antlers. It took 13 years of data to actually test this idea."

Along the way, Metz pulled in other co-authors, including UM professors Hebblewhite and Emlen.

"To me, one of the coolest things about this project is how it started," said Hebblewhite, a wildlife biology professor in UM's W.A. Franke College of Forestry and Conservation who focuses on large-mammal predator-prey systems. "It started with Matt just being observant, noticing something in nature, and then diving into it over a decade to find out why. This is the heart of the scientific process."

Hebblewhite has studied wolves and their prey for a quarter century, but this is the first time he'd stopped to consider what makes antlers antlers. "A lot of the time, the reasons we study nature are to control or manage it, to make it do something we want for us," he said. "This is a case of wonder at its finest."

Emlen, an evolutionary biologist and professor in the Division of Biological Sciences, said what was most fun about the project was the nature of the collaboration, which crossed departments and disciplines.

"Matt is a wolf biologist. And Hebblewhite is an elk biologist, but he doesn't focus on antlers," Emlen said. "I work on rhino beetles, and I study animal weapons -- beetle horns. I spend most of my time thinking about why weapons are so big. It's been a blast to work on antlers, because those are giant weapons. It's been a wild ride. That a beetle biologist who studies weapons can collaborate with elk and wolf biologists adds a fun twist to the story."

The study was funded in part by the National Science Foundation's Long Term Research in Environmental Biology program, which aims to help sustain research projects lasting 10 years or longer.

"This is a really sophisticated look at a complicated behavior," said Kendra McLauchlan, NSF program director. "They were able to make this discovery because of 13 years of careful observations. Sometimes it is really important to have that long-term record of observations."