On the hardscrabble lands of the American West, blood is spilled by the most innocent-looking of outlaws—the white-tailed prairie dog.

These social rodents, native to Colorado, Wyoming, Utah, and Montana, ruthlessly bite and thrash Wyoming ground squirrels to death, leaving their bloody bodies to rot, a new study says.

The killers' offspring then live longer, healthier lives—probably because their parents bumped off their competition for food.

It’s the first time that a herbivorous mammal has been seen killing competitors without eating them, suggesting that a plant-based diet doesn’t preclude mammals from having a taste for bloodsport.

“In my 43 years of research, this is perhaps the most provocative, puzzling, and far-reaching discovery I’ve ever made,” says study co-author John Hoogland of the University of Maryland Center of Environmental Sciences.

“The results are just staggering.”

Cracking the Case

Like many murder mysteries, Hoogland’s study began with a body dumped in a quiet landscape: the scrublands of western Colorado’s Arapaho National Wildlife Refuge, where Hoogland and his students studied prairie dog behavior from 2003 to 2012 with a devotion bordering on the fanatical.

“For four months every year, we live like prairie dogs,” says Hoogland, a National Geographic grantee. “We get to the colony early in the morning before the prairie dogs wake up, we sit in towers all day watching what they do, and we stay until the last prairie dog submerges, just around sunset.”

During his 2007 field season, Hoogland spotted a white-tailed prairie dog tossing around a young rodent. Initially, he suspected that it was killing another prairie dog’s pup, which is not surprising: Infanticide is common in other prairie dog species, though it has never been observed in white-tailed prairie dogs.

Prairie Dog Alarms When danger lurks, the prairie dog males fend for themselves, but the females look out for the community.

But once Hoogland examined the prairie dog’s victim up close, he realized that it was in fact a Wyoming ground squirrel, a species that eats the same grass and prickly pears as prairie dogs.

“I got out a walkie-talkie right away, and I said to my students, ‘Hey, we gotta watch to see if there are any more [killings]!’” he says.

Killer Supreme

Over the next five years, Hoogland and his students built dossiers on 101 ground squirrel murders, plus 62 suspected cases. Most murders occurred in May, when young ground squirrels emerged from their nests and began foraging.

The researchers identified 47 killer prairie dogs—both male and female, and always adults.

But comparing survival rates of non-killers' and killers' offspring revealed a method to the madness: Killers' offspring had better survival odds than those of non-killers, presumably because they had more to eat.

And shockingly, a prairie dog’s penchant for murder was the only factor that predicted its offspring’s success. (See "Friends For Dinner: Why Some Animals Become Cannibals.")

“The condition of the female, her longevity—the factors that normally influence [success]—none of them apply to this case,” says study co-author Charles Brown of the University of Tulsa.

“It seems to me is that there are major, major benefits to killing these ground squirrels.”

Unappetizing Demise

The study, published Tuesday in Proceedings of the Royal Society B, adds a demented twist to scientists’ understanding of interspecific competition—the struggle among different species for shared resources such as food.

It’s not unheard of for this competition to provoke aggression, or even death: Some animals dine on competitors on the same food chain level. Even herbivorous mammals such as rats will take on other prey opportunistically, sometimes eating eggs, birds, and one another, according to John Orrock of the University of Madison-Wisconsin. (See "Why Do Animals Sometimes Kill Their Babies?")

But tellingly, white-tailed prairie dogs don’t eat their kills.

Hoogland notes that they occasionally gnaw at the squirrels’ chests, or nibble at their brains, but just to ensure that the squirrels are definitively dead. The prairie dogs then abandon the squirrels’ broken carcasses, which nearby birds then scavenge.

“They’re killing for the sake of killing—not killing to achieve some nutritional benefit," says Orrock, who wasn’t involved with the study.

Murky Motives?

Despite the fact that scientists have caught killer prairie dogs red-handed, however, the animals’ motives aren't completely understood.

The weight of the evidence shows that murderous prairie dogs have longer-living children when they kill ground squirrels.

But more research will need to eliminate a more remote possibility: that killer prairie dogs may just happen to live in areas with richer vegetation, which would attract more ground squirrels and trigger more conflicts over food, say the study's authors.

Regardless, the results suggest that for herbivores of all stripes—including white-tailed prairie dogs and ground squirrels—competition won't just bring hardship. For many, there will be blood.