"Eat or be eaten,” some say, is the golden rule of the animal world. But some prairie dogs kill—not for a meal—but to take out the competition. White-tailed prairie dogs (Cynomys leucurus) and Wyoming ground squirrels (Urocitellus elegans) eat the same grasses, live in the same prairies, and sometimes even use the same burrows. These close quarters and shared tastes pit the two species directly against each other in competition for food. Over 6 years of studying the prairie dogs in Colorado, researchers saw them chase and kill ground squirrels more than 100 times, according to a study published online today in the Proceedings of the Royal Society B. Other than a bit of chewing, in most of these instances the prairie dog simply walked away afterward, leaving the whole carcass as food for scavengers. The more squirrel murders these mostly female prairie dogs committed, the more offspring they were able to raise. And serial killers—those that killed at least two squirrels per year—successfully raised three times the offspring of nonkillers over the course of their lives. This is the first time scientists have shown that these sorts of killings actually benefit the killer in the wild. And given that it took researchers 4 years of studying prairie dogs to even notice the murders, they say it may be widespread—but sneaky—in other animals, too.