From terrorist attacks to muggings, people are clearly among the most violent of species, but new research suggests that unprovoked killing not associated with hunting could be far more common in the animal kingdom than previously thought.

It appears that killing to reduce competition between species happens frequently, even when the killer is an herbivore that could care less about eating its victim.

A new paper published in the journal Proceedings of the Royal Society B documents how herbivorous white-tailed prairie dogs regularly kill Wyoming ground squirrels, leaving behind bloody bodies that the killers have no desire to eat.

Photos: Prairie Dogs Revealed as Serial Killers

“I describe the behavior in eight words: catch them, shake them, kill them, leave them,” lead author John Hoogland of the University of Maryland Center for Environmental Science’s Appalachian Laboratory told Discovery News.

Hoogland and co-author Charles Brown had no idea what they were in for when they took on a project to study the basic behavior of veggie-loving white-tailed prairie dogs. Sitting in an observation tower surrounded by the animals, Hoogland started to notice the unprovoked killings.

The researchers identified 47 prairie dogs that made kills: 36 were female, 11 were male. Thirty percent of all females killed at least once in their lives. Nineteen females were serial killers in the same or consecutive years.

The Other Side of Otters

The scientists investigated the phenomenon further and found that the female serial killers had significantly higher annual and lifetime fitness than the other females did. The scientists believe that the killings reduce competition for space and food with the ground squirrels.

They also suspect that natural selection favors females that kill, but this selection process does not necessarily make them any more aggressive with their own kind.

“It’s not that certain nasty females only do this,” Hoogland said. “In fact, the female serial killers will often dispatch their victims and then go on to graze peacefully or play with their babies, as though nothing had just happened.”

He added, “I don’t even think the prairie dogs are physically well equipped to eat ground squirrels.”

A white-tailed prairie dog killing a Wyoming ground squirrel. John Hoogland

As for why the ground squirrels would even want to live near prairie dogs, the researchers explained that both groups feed on the same vegetation. Prairie dogs also create burrows that the ground squirrels later use. Ground squirrels also understand prairie dog alarm calls, and benefit at times from extra vigilance.

For years there have been anecdotal reports of animals killing without prior threat and not for consumption. Hoogland, for example, said that lions kill hyenas and vice versa. Bobcats are also known to kill coyotes and vice versa. Humans, of course, kill all kinds of animals just for sport.

Animals in the Yard. Pesky or Pretty? Photos

The new paper, according to professor Dirk Van Vuren of the University of California at Davis Wildlife, Fish & Conservation Biology Department, “is noteworthy because it is the first comprehensive report of interspecific killing for a mammalian herbivore, reporting frequency, context, and consequences for the killer, which goes a long way toward elucidating the cause.”

Jonathan Losos, a professor and curator of herpetology at Harvard University’s Museum of Comparative Zoology, told Discovery News, “Although carnivores are known to kill competing species, who would have thought that herbivores do so as well? In the case of carnivores, one possibility is that the species are eliminating potential predators on their young, but that doesn’t seem likely in this case.”

Yoel Stuart of the University of Texas at Austin was equally surprised by the findings. He wonders if such killings might occur among various animals even more in the future, in response “to the ever increasing challenge posed by human-aided species introductions.”