The first time it happened, it was over so quickly that John Hoogland almost missed it. It was a spring day in 2007. Hoogland was sitting in a two-meter-tall observation tower near Walden, Colorado, watching a colony of white-tailed prairie dogs. During his stakeout, he saw one of the small, burrowing rodents—a  female called “Head 6”—jump on something. She grabbed it, shook it for a few minutes, and then walked away.

Hoogland assumed he had just seen a case of infanticide, that the female had killed a neighbor’s baby. But when he walked over to investigate, he realized he was wrong. The victim was actually a baby ground squirrel—a completely different species of burrowing rodent, about half the size of a prairie dog.

“We had never seen anything like this,” he says. Not in 35 years of observing wild prairie dogs, and not in four years of watching the white-tailed species specifically. “But once we knew what to look for, it seemed like it was going on everywhere. The next day, I saw another kill. And then more.”

John Hoogland

Between 2007 and 2012, Hoogland and his assistants witnessed 101 instances of prairie dogs dispatching ground squirrels. “It’s so quick and subtle,” he says. Brutal, too. The prairie dog would grab the smaller animal by the neck or chest and violently shake it for a few minutes, either breaking its neck or puncturing its heart. Most attacks happened after a ground squirrel entered the prairie dog’s territory. But in rare cases, the killer waited in ambush outside the victim’s home-burrow, or actively dug it out. In six occasions, “the female actually stalked a baby almost like a tiger coming in for a kill,” says Hoogland.

But unlike tigers or other predators, prairie dogs are grass-eaters. In rare cases, they took small nibbles of the squirrels they killed, but they most often left the body and went back to grazing. Often, a gull or crow would come by and fly off with the carcass, and Hoogland actually saw 58 such acts of scavenging in the absence of any obvious kill. “Even when we were watching non-stop all the time, there were cases we didn’t see,” he says.

The perpetrators were usually females, and the victims were almost always babies. At least a quarter of the prairie dog females under Hoogland’s surveillance took out at least one squirrel, and 19 of them were serial killers. One especially prolific female executed seven babies in a single day. (By contrast, the prairie dogs never killed their own kind.)

This kind of “interspecific killing”—where animals kill competitors, rather than prey—is surprisingly common. It happens in at least 97 species of meat-eating mammals: cougars killing bobcats, lions killing hyenas, American badgers killing skunks, and (as immortalized in this award-winning photograph) red foxes killing Arctic foxes. But in these cases, the killers’ motives are unclear. They could well be after a meal. “That’s clearly not the case [with the prairie dogs],” says Tim Caro from the University of California, Davis. “It is killing competitors pure and simple. Cuddly little white-tailed prairie dogs, no more!”

This is, in fact, the first recorded case of interspecific killing among wild, plant-eating mammals. “When you’re talking about lions killing hyenas, they’re trained killers. They’re doing what they do naturally on something that happens to be competing with them,” adds Hoogland. “But prairie dogs are eating grass all the time, but then become violent killers over the course of three minutes. That’s extraordinary.”

The consequences of the kills were even more extraordinary. Hoogland showed that females who killed ground squirrels produced larger litters every year, and successfully raised more youngsters over their lifetimes—and the more they killed, they more successful they were. No other factor correlated with their success as parents—not body weight, colony size, or even longevity! Put it this way: Killing ground squirrels is the number one way in which female prairie dogs can get an advantage over their peers.

This isn’t just about aggression. Prairie dogs will sometimes fight each other but Hoogland found that such acts of aggression don’t correlate with either the number of squirrels they kill, or their reproductive success. So it’s not that more violent dogs are both more likely to kill squirrels and foster more young.

Instead, this is probably about competition. Prairie dogs and ground squirrels eat similar foods, so the loss of one squirrel means more grass for a prairie dog and her offspring. Those extra blades of grass might be especially important in spring, when mothers need energy for making milk, and when a lot of food is still buried under snow. “It’s hard to imagine that getting a little extra grass by removing a competitor could make such a difference,” says Hoogland. “But the prairie dogs are saying: No matter what you think, this is what is happening.”

And it’s happening so much that in many years, prairie dogs killed more ground squirrels than all other predators combined. Coyotes, bobcats, American badgers, Swainson’s hawks, golden eagles—these traditional predators were a lesser threat than the cute, adorable, plant-eating prairie dogs.

“I hope it’ll force people who are studying animals with nearby competitors [from other species] to consider that this sort of killing could be just as subtle or unanticipated as it was here,” says Hoogland. For example, many species of marmot share valleys with ground squirrels that eat the same food, and many bird species feed on the same levels of the forest.

“This is top-flight natural history,” says Caro. “It shows what years of dedicated observational research can uncover.” Indeed, when I spoke to Hoogland about his discovery, he was in another observation tower at the Valles Caldera National Preserve, north of Albuquerque. He was 8,100 feet above sea level, wearing six layers of clothing, and settling in for a long day of prairie dog-watching. Who knows what mayhem he’ll see this time?