Most people accept that animals kill other animals for food, to protect their young or to defend a carcass or other food stash—it’s the circle of life and all that. But new research shows that at least one mammal is a killer that exterminates another species unprovoked: the lowly white-tailed prairie dog.

A paper recently published in Proceedings of the Royal Society B details how the white-tailed prairie dog, Cynomys leucurus, which lives in colonies in Utah, Colorado, Wyoming and a bit of Montana, often attacks the smaller Wyoming ground squirrel, Urocitellus elegans, biting and shaking them to death. It is the first time an herbivore has been recorded killing competitors with no real provocation and without turning them into a snack, according to the study.

“I describe the behavior in eight words: catch them, shake them, kill them, leave them,” study co-author John Hoogland of the University of Maryland Center of Environmental Sciences tells Jennifer Viegas at Discovery News.

“In my 43 years of research, this is perhaps the most provocative, puzzling, and far-reaching discovery I’ve ever made,” he tells Michael Greshko at National Geographic. “The results are just staggering.”

In 2007, Hoogland, who studies prairie dogs at the Arapaho National Wildlife Refuge, spotted one of his subjects attacking a small rodent. When he investigated, he realized it was a ground squirrel. He instructed his fellow prairie-dog watchers to keep an eye out for any more squirrel murders. Over the next five years, what Hoogland and his team found was that the prairie dogs killed 101 ground squirrels, with 62 other suspected killings. And it wasn’t just one or two depraved individuals. The researchers recorded 43 adult prairie dogs of both sexes attacking the squirrels.

It turns out that the bloodiest month in prairie dog towns is may, when baby ground squirrels emerge from their burrows. One prairie dog dug 7 ground squirrel babies out of their den and killed them, reports Aisling Irwin for New Scientist.

Though the prairie dogs chew on the squirrels’ chests and brains some, the act is not for a snack but rather to make sure the creatures are truly dead. “They’re killing for the sake of killing—not killing to achieve some nutritional benefit,” John Orrock a zoologist at the University of Wisconsin-Madison, tells Greshko.

Hoogland believes taking out the ground squirrels gives the offspring of killer prairie dogs an advantage, since both species munch the same prickly pears and grasses. In fact, Hoogland’s study indicates that offspring of the murderous prairie dogs may do better than children of their more mellow neighbors, probably because their parents cleared their feeding territory of food competitors.

Researchers aren’t sure if squirrel murder is common outside this particular band of prairie dogs or if it’s just an oddity. But Hoogland tells Irwin that if the killing does help prairie dog offspring survive, it may change the way we understand ecosystems.

“It begs the question of whether it’s going on in other species,” he says.