Being a herbivore might sound like a peaceful way to live, but not all vegetarian animals appear to subscribe to the non-violent ethos. A new study in the Proceedings of the Royal Society B documents the murderous behavior of female prairie dogs in Colorado, which have been observed slaying Wyoming ground squirrels despite being herbivores. Given that these chubby-cheeked rodents do not kill for food, researchers set out to establish the reason for their violent tendencies.

Female prairie dogs rear their offspring in burrows, and obtain the majority of their food from the region immediately surrounding this burrow while lactating and raising infants. Because of this, they have been known to defend the territory around their home from both predators and other animals that may compete for food.

Ground squirrels tend to live in the same habitats as prairie dogs, and share a very similar diet, with the six most commonly consumed plant species being identical for both animals. As such, they represent major competition for food.

Over a six-year period, researchers observed the interactions between prairie dogs and ground squirrels in the Arapaho National Wildlife Refuge in Colorado, documenting a total of 101 incidents in which prairie dogs killed ground squirrels. Around 80 percent of kills were perpetrated by lactating females, with 96 percent occurring less than 20 meters (65 feet) from the killer’s burrow.

Though around 13 percent of killers did have a nibble on their victims, they never ate more than 5 percent of the carcass, which was typically later scavenged by birds, therefore ruling out the possibility that prairie dogs engage in this behavior for nutrition. Such an observation would appear to back up the theory that this inter-species violence serves to eliminate competition for food, particularly in the restricted area to which new mothers have access.

During the six-year period, a total of 47 different prairie dogs were observed killing at least one ground squirrel, with 19 identified as serial killers, meaning they killed on multiple occasions. Of these, the most prolific included a female that killed nine squirrels over four years, and another that slaughtered seven juvenile squirrels from the same litter on a single day.

^{Researchers discovered that the size of a female prairie dog's annual litter increased with the number of squirrels she killed. John L. Hoogland and Charles R. Brown}

Interestingly, prairie dogs that killed squirrels were found to have superior reproductive fitness than those that did not kill, and the more victims a female claimed, the more offspring she was able to raise. Serial killers tended to produce litters that were three times larger than those of non-killers.

Amazingly, the number of squirrels killed in a given year was the single biggest predictor of overall fitness for female prairie dogs, underlining the importance of eliminating competition for food while raising young.

This in turn raises the question as to why ground squirrels continue to occupy the same habitat as prairie dogs, as the laws governing natural selection would suggest that populations living elsewhere should thrive while those being killed in this way should die out. However, the study authors propose that proximity to prairie dogs may actually bring a net benefit to the squirrels, since they are able to react to the alarm calls of prairie dogs and avoid predation by up to 15 types of birds.

Therefore, while populations may lose a certain number of members each year to competitive female prairie dogs, this figure is outweighed by those that are able to avoid being killed by predators.

Summing up, the researchers claim their study provides the first evidence of interspecies killing for reasons other than predation, shattering the illusion that herbivores are peaceful and non-violent.