It’s an herbivore-kill-herbivore world

White-tailed prairie dogs — those stand-up, nose-wiggling nibblers of grass — turn out to be routine killers of baby ground squirrels. And the strongest sign of successful white-tailed motherhood could be repeat ground squirrel kills, researchers say.

At a Colorado prairie dog colony, females that kill at least two ground squirrels raise three times as many offspring during their lives as nonkiller females, says John Hoogland of the University of Maryland Center for Environmental Science in Frostburg. The “serial killers,” as he calls repeat-attack females, rarely even nibble at the carcasses and aren’t getting much, if any, meat bonus. Instead, the supermom assassins may improve grazing in their territories by reducing competition from grass-snitching ground squirrels, Hoogland and Charles Brown of the University of Tulsa propose March 23 in Proceedings of the Royal Society B.

“This really caught me by surprise,” Hoogland says. Carnivorous mammals killing other carnivore species wouldn’t be surprising, but prairie dogs and ground squirrels eat plants. He knows of no other systematic study documenting routine fatal attacks by one herbivore species on another.

“It’s also striking because it’s so subtle,” he says. He had been watching prairie dogs in general for decades and the white-tailed prairie dogs in the Arapaho National Wildlife Refuge for a year before he noticed an attack. A female “jumped on something, shook it, shook it hard, kept attacking — and then walked away,” he says. The encounter lasted just minutes. Hoogland rushed from his observation tower to the scene of the fight and, to his surprise, retrieved a dead baby ground squirrel.

Once he and his colleagues knew what to look for, they saw 101 such lethal attacks (mostly from females, but also from some males) over six years and inferred 62 more from carcasses. A propensity for killing ground squirrels turned out to be the only factor (among such possibilities as body mass, age and number of neighbors) that predicted a tendency toward lifetime success in raising lots of young. That factor, which biologists describe as fitness, is a big deal in analyzing how populations change and species evolve.

Hoogland and Brown propose that prairie dogs and ground squirrels compete for grazing. An analysis of the animals’ diets finds at least six plant species in common, the researchers say. Hoogland didn’t directly test to see if the serial killer prairie dogs just had great territories that attracted lots of ground squirrels and thus provided more opportunities for killing. But if that were true, he says, he would predict that the holders of this prime territory would have robust body sizes, and therefore there would be some link between maternal body size and high offspring number. No such link shows up, he says. The best hypothesis explaining the benefit of killing squirrels that Hoogland can think of, he says, is that prairie dogs slay the competition for food resources.  

Still, the idea that prairie dogs and ground squirrels compete for plants needs more information, says ecologist Liesbeth Bakker of the Netherlands Institute of Ecology in Wageningen. The total of ground squirrel kills was an impressive number, she says, but it’s unclear what percentage it represents. If the deaths remove only a small proportion of ground squirrels, competition isn’t likely to ease much. Also, any effect would be weakened by the relative sizes of the species. “The ground squirrels are about half the size of the prairie dogs and thus eat less food,” she says. 

Behavioral ecologist James Hare wonders why ground squirrels venture into prairie dog territory if it’s so dangerous. One of the ideas Hoogland suggests is that prairie dog vigilance in raising alarms about predators might make the risks of hanging out in a colony worthwhile. Hare, at the University of Manitoba in Canada, also wonders whether ground squirrels have trouble finding good habitat free from prairie dogs.

Hoogland too is left with questions, including one about the big-family bonus of interspecific killing. “Is this really unique to prairie dogs or is this more common?”