Have you ever had a roommate you just hated? You're a reasonable, mild-mannered enough person, but that one guy's always hogging the couch, watching movies you hate, he's late paying his share of utilities, and some mornings you open the refrigerator and all your yogurt is gone, and you think to yourself, I'm going to KILL Chad.
Welcome to the jungle. This happens all the time in the natural world between individuals of different species. It's so common, ecologists call it "interspecific competition," and it can be observed between two trees vying for sunlight in a forest, or a crocodile and a hippopotamus with dibs on the same watering hole. This kind of competition can result in fisticuffs between individuals of different species, killing, and even extinction of an entire species.
A new study of white-tailed prairie dogs published in the Proceedings of the Royal Society shows that interspecific killing doesn't just happen between big, fierce animals — and beyond that, the killings actually benefit the individual killers.
In a meadow in Colorado's Arapaho National Wildlife Refuge, two cute, chubby-cheeked herbivorous mammals spend their days nibbling grass and scurrying into their burrows when a hawk looms menacingly overhead. The diets of white-tailed prairie dogs (Cynomys leucurus) and Wyoming ground squirrels (Urocitellus elegans) are practically identical, and they live side-by-side, feeding next to each other and sometimes sharing a burrow.
From an ecological standpoint, these similarities are what make the competition between the two rodents so extreme. Here's where things get less cute: as a result of this tight competition for the same resources, some white-tailed prairie dogs in this meadow in Colorado also keep busy brutally assassinating baby ground squirrels and leaving their corpses on the prairie to rot.
Dr. John Hoogland of the University of Maryland's Center for Environmental Science has been studying prairie dogs for 43 years, but had never seen a prairie dog actively kill another herbivorous mammal until 2007.
Killing is no rare thing in the animal world — that's the basis for the whole predator-prey relationship. But one herbivore killing another simply to insure less competition for veggies?
"I had never dreamed this could be happening — nobody else had either," says Hoogland, the lead author on the new paper. "But after we saw it the first time, we noticed it happening all over the place!"
Over the course of six years, Hoogland and his students witnessed 101 killings of Wyoming ground squirrels by 53 female white-tailed prairie dogs, and inferred 62 more cases from evidence available.
"We have a situation here where one herbivorous mammal is killing another herbivorous mammal," says Hoogland. "That's been seen anecdotally before — elephants have been seen killing cattle before, for instance. But this goes way beyond the anecdotal, and it's a big part of the white-tailed prairie dog's story."
Perhaps the more stunning piece of Hoogland's research is that his team found the killers experienced higher reproductive success than the non-killers.
"We saw these killings so often over six years, we started tracking the individual animals and were able to record both their reproductive success and whether or not they killed," says Hoogland. "I was totally astonished to find that killing pays significant dividends. The killers not only have higher annual fitness, they also have higher lifetime fitness. They wean more babies and their babies survive better. It doesn't get any better than that."
According to Hoogland, these killer resort to violence for a simple reason — they're just ensuring that they and their babies have access to plenty of food.
"Prairie dogs and ground squirrels eat the same plants. When a female prairie dog kills a ground squirrel, that means more vegetation for herself and her babies," says Hoogland.
As noted behavioral biologist Arnold Schwarzenegger once said of his competitive philosophy: "My instinct was to win, eliminate anyone who is in competition, destroy my enemy, and move on without any kind of hesitation at all."