Are Yellowstone Elk Afraid of the Big Bad Wolf?

Fear of death or injury keeps humans well away from dangerous places or risky situations, so it seems reasonable to assume that wild prey species will do the same when it comes to avoiding their deadly predators. But do they really? Are animals so fearful of predators that it dictates their daily movements?

Answering this question is not easy. Indeed, while it is possible to actually observe prey gathering into groups or being more vigilant in response to the presence of predators, detecting active avoidance of risk across space and time is challenging. It requires continuous monitoring of the movements of both predators and prey in their natural habitat.

As a PhD student, Jeremy Cusack became slightly obsessed with the idea of tracking predator avoidance behavior using remotely triggered camera traps. But after struggling to find strong avoidance patterns in communities of large mammals in east Africa —and unsure whether this was because these responses didn't exist, or because the method to detect them wasn't right—he decided to turn to a predator-prey system that had been extensively studied: elk and wolves in Yellowstone National Park (YNP).

A unique dataset

This is how he came to team up with researchers Michel Kohl and Dan MacNulty from Utah State University, Matt Metz from the University of Montana, and Doug Smith and Dan Stahler from the U.S. National Park Service. These newfound collaborators comprised scientists that had been working on wolf and elk dynamics in YNP for over 20 years. They suggested Jeremy first try to confirm the existence of avoidance behavior by elk in response to wolves using tens of thousands of Global Positioning Satellite (GPS) locations they had collected over the years from collars placed on both species.

The GPS-collars had recorded the successive positions of a sample of adult female elk during the winters of 2012 to 2016 every one to three hours. In the case of wolves, a member of each of the dominant packs active in northern Yellowstone had been collared since 2004. Together, these data provided comprehensive past and current information on how members of both species used the landscape.

Unfortunately, knowing where elk and wolves roam in northern Yellowstone—despite representing considerable research effort in itself—is not enough to truly understand if and how the former respond to the risk of predation posed by the latter. It is also important to consider other cues that elk might be responding to aside from wolves themselves, such as where elk had previously been killed, or open grasslands were wolves often hunt successfully.

Perhaps more importantly, however, it is essential to know how elk would have moved in the landscape had they completely ignored predation risk. The team solved this problem by simulating alternative elk movements across the landscape that represented their expectations of how they would behave if they simply didn't care about wolves.

The role of fear under question

The analysis, published in the Journal of Animal Ecology, revealed no difference in the extent to which real and simulated elk trajectories overlapped with the different measures of predation risk.

This suggests that elk do not proactively avoid risky areas where they were more likely to fall prey to wolves. Not only this, but elk didn't even avoid being in close proximity (i.e. less than 500m away) to wolves. Such instances occurred once every seven to 11 days, a rate that was similar to that measured from simulated trajectories. In fact, the only evidence for avoidance behavior we found was a small proportion of elk that stayed clear of open grassland habitats during daylight hours when wolves were most active.

To really understand the implications of these findings, a bit of background to the elk and wolf story in Yellowstone is needed. Wolves were reintroduced to Yellowstone in 1995 to 1997 as part of the U.S. Fish & Wildlife Service's legal mandate under the Endangered Species Act to recover populations of this iconic predator in the lower 48 States.

The subsequent growth of the wolf population within YNP was concurrent with a steady decline in elk numbers, which was in part ( but not solely ) due to increased predation. Other factors, such as recovering cougar and grizzly bear populations, human hunting outside the park, and periods of severe drought and winter weather also pose risks to elk, and contributed to declining numbers.

In addition, research undertaken on the growth of key tree species within YNP, which had been negatively impacted by an abundant elk population since the early 20 th century, reported signs of vegetation recovery in some areas of the Park. One of the proposed explanations for this recovery was the newly restored fear of wolves, which kept elk vigilant and on the move, thus reducing browsing pressure on young shoots.

In this context, the findings of our study are important as they add to a growing body of literature that questions the role of fear in driving the movement behavior of elk, and thus the recovery of plants such as aspen and willow—the so-called behaviorally mediated trophic cascade.

Do elk really not care about wolves?

Although elk probably do care about the risk posed by wolves, they likely care more about finding good food during the winter. In fact, they have an inherent tendency to return to the same summering and wintering grounds year after year—a behavior known as philopatry. Familiarity with an area helps them find the high quality forage they need, even if that means putting up with a small chance of encountering and falling prey to wolves.

Another reason for the absence of avoidance behavior is that female elk frequently survive their encounters with wolves due to their large body size and tendency to be in groups. Previous research has shown that only 10 percent of wolf hunts targeting adult elk are successful. Added to this is the fact that wolves are not around-the-clock hunters, meaning that elk may exploit potential risky areas, such as open grasslands, during the night when wolves are less active. This suggests that for some elk the response to the threat posed by wolves is more temporal rather than spatial.

As humans we are more likely to think of a threat as being scarier if it is unpredictable in time and space. Wolves, in contrast, are quite predictable in terms of where and when they roam. It is this predictability that has allowed researchers, as well as many other 'wolf-watchers', to become such effective observers of these predators on the Yellowstone landscape. We generally know where and when to look for wolves, and we often find them, even without telemetry. Elk no doubt do the same.

Jeremy J. Cusack is a postdoctoral researcher at the University of Stirling.​

Dan MacNulty is an associate professor of wildlife ecology in the S.J. and Jessie E. Quinney College of Natural Resources at Utah State University. He has studied wolves in Yellowstone National Park since they were reintroduced there in 1995.​

Views expressed in this article are the authors' own.

The headline of this article originally appeared as: Wolves Were Reintroduced to Yellowstone 24 Years Ago. But The Elks Weren't Afraid