Wild donkeys can be a nuisance to both humans and wildlife. The scruffy, vegetation-destroying equines, large and full of character, steal resources from native sheep and turtles, defecate in precious spring water and cost many a park manager a good night’s sleep.
However, they are unstoppable. In Death Valley National Park, researchers have captured the first photographic evidence of donkeys preying on the claws of a native predator: the cougar. The relationship is shaping the area’s wetlands, the team says, and has raised questions about wild equine management in the future.
“This is cool stuff,” says wildlife biologist Kate Schoenecker of the Fort Collins Science Center, who studies puma predation on wild horses but was not involved in the study. “It helps us understand the effect of these interactions with [wild equids] have on the North American landscape.”
Donkeys, often called burros in the Americas, first arrived with Spanish colonizers in the 1500s. They became invaluable as beasts of burden and miners to Western pioneers in the 1800s. However, at the end of the mining explosion, most of the donkeys escaped or were released.
Able to endure harsh desert conditions, the abandoned animals quickly established wild populations in Arizona and Southern California. Although the exact numbers are disputed, the Bureau of Land Management estimates the wild donkey population at about 17,000 individuals — much smaller and more localized than the wild horse population of more than 60,000. As a result, the animals attract less attention from the mainstream, says ecologist Erick Lundgren of Aarhus University, who led the new study. “Donkeys are the forgotten little desert man,” he says. “They are easily overlooked.”
The animals also don’t get much respect from the US National Park Service. Because donkeys are considered a nuisance for eating and trampling on delicate vegetation, polluting water and preventing other creatures from using resources, the agency strictly manages the population in Death Valley, where many of them live, by trapping and to animal rescue organizations for adoption. It eventually plans to remove all donkeys from the park. Part of the reason for this management was a general consensus that the donkeys, like other invasive species, have no natural enemies to control them.
Some ecologists and park workers in Death Valley have found evidence to suggest otherwise: sightings of cougars eating donkey carcasses, or carcasses stowed near cougar trails or feces. However, Lundgren says many other scientists have rejected the idea without concrete evidence.
Lundgren, who received his master’s degree from Arizona State University, Tempe, and had long been fascinated by wild equines, thought the cougars did indeed prey on donkeys and wanted to prove it. He and his colleagues set up camera traps in cougar areas in Arizona and California to monitor the activity of cougars and donkeys. Then he crossed his fingers and waited.
One day in 2019, he found more than he expected. One camera had slipped so that it was pointed at the ground, collecting thousands of useless images of dirt. But as Lundgren prepared to delete the images, a strange shape caught his eye in the corner of a photo.
Lundgren realized he saw a cougar and a donkey trapped in the middle of a fight as the cougar wrestled the donkey to the ground. With one eye closed, the bloodied cat seemed to be staring straight into the camera as its powerful front paws twisted around the donkey’s forehead. The following year, Lundgren caught another cougar killing a donkey on camera, this time at night. The camera had captured the entire sequence, from the cougar clinging to the panicked donkey’s haunches to the triumphant cat standing over its dead prey.
“Capturing that murder on camera was a great moment of validation,” Lundgren says. Immediately his mind went to trophic waterfalls, phenomena where a single type of predator hunting a particular prey can affect an entire ecosystem, down to the tiniest blade of grass. Wolves famously have this effect in Yellowstone; by hunting moose, they prevent overgrazing of delicate saplings and give other species such as beavers access to more resources. This change is only partially achieved by predation itself: once predators enter a landscape, prey learn to fear the places where their brethren have been killed and spend less time grazing in those areas. Lundgren had long suspected that cougars could have a similar impact by hunting burros.
To investigate his theory, he went to Death Valley, one of the few places where wild donkeys live and cougars are protected. Given the park’s name, it’s easy to imagine it as a dry, lifeless desert, Lundgren says, but deep down, the land is dotted with springs and wetlands. These wetlands are a magnet for thirsty donkeys, making them an ideal place for Lundgren to conduct his research. In total, he and his team monitored 14 wetlands.
At the five sites where cougars were absent, probably because they were close to human encampments that would deter the big cats, the donkeys hung out for an average of 5.5 hours at a time on hot days. However, in the places where cougars were present, donkeys only stayed for 40 minutes on average at a time and didn’t stay around much at night, when cougars hunt more often.
The vegetation in places less frequented by donkeys had more plants growing and less signs of trampling. By making donkeys wary of wetlands, the cougars promoted a greener and healthier landscape, Lundgren and his colleagues reported last week in the Journal of Animal Ecology.
For Lundgren, this relationship between cougars, donkeys, and vegetation suggests that the donkeys are not a pest to the Death Valley landscape and should not be completely removed. With cougars preventing donkeys from destroying wetlands, he says donkeys’ favorable behavior has a better chance of breaking through.
For example, Lundgren’s ongoing research has shown that by clearing the vegetation around the valley’s springs, the equines kept the pools from drying out and endangered fish could survive there. In addition, he thinks that, without donkeys to prey on, cougars would disperse elsewhere or turn to bighorn sheep, the park’s native medium-sized herbivore, which is already vulnerable to climate change and disease.
The evidence that cougars prey on donkeys and influence their behavior is strong, says Mark Boyce, an ecologist at the University of Alberta, Edmonton, who has worked with cougars and trophic waterfalls. But he doesn’t think this knowledge should affect burros management. “This is an invasive exotic species,” he says. “It would be a serious mistake to conclude that because cougars kill donkeys, we don’t need to remove donkeys.”
Schoenecker agrees. Donkeys, like horses, are a domesticated species, she notes, so they’ve been artificially selected over thousands of years to reproduce more often than untamed herbivores like bighorn sheep. Predation by cougars, she argues, is not enough to control them.
Abby Wines, a management analyst at Death Valley National Park, says the new study won’t change the park’s goal of removing donkeys. Any ecological benefits the donkeys provide, such as clearing unwanted vegetation, can also be done by the park staff, she says.
Despite her concerns, Schoenecker believes the new study is an exciting starting point for future research and continues the often difficult conversation about controlling feral domesticated animals. “It’s really interesting and helpful to see their data,” she says. “It definitely makes you want to test more of these things, and that’s a good thing.”