Serial-killer prairie dogs in the US have been observed targeting baby ground squirrels to reduce competition for food. While this behaviour is often observed among carnivores competing for resources, it is the first time it has been seen in herbivorous mammals.
John Hoogland, from the University of Maryland Centre for Environmental Science, and colleagues spent six years watching prairie dogs in the state of Colorado. During this period, the team witnessed many cases of white-tailed prairie dogs killing, but not consuming, Wyoming ground squirrels. They published their findings in the Proceedings of the Royal Society B.
Over the course of the study, they saw 101 cases of prairie dogs killing squirrels. "In a typical [kill], the prairie dog repeatedly bit the ground squirrel in the head, neck or thorax over a period of one to three minutes until death, and then abandoned the carcass and resumed foraging on nearby vegetation," the authors wrote.
The kills were carried out by 47 individuals. Nineteen were categorised as 'serial killers' in the same or consecutive years. They also found that 30% of female prairie dogs killed at least one squirrel over the course of their lifetimes.
How many squirrels the female prairie dogs was important, as this was found to be the only significant predictor of prairie dog fitness – mean annual fitness was twice as high for serial killer than non-killers. How many squirrels a female killed was also linked with her litter size. This is likely due to decreased competition for vegetation, the researchers said.
But why would squirrels continue to live alongside these killers? The researchers said there are several reasons. First, the vegetation in the meadows inhabited by the prairie dogs is highly nutritious. The prairie dogs also often a certain level of protection – the squirrels listen to their warning calls and use them to escape other predators. Similarly, they use unused prairie dog burrows as a means of quick escape from other hunters.
"Living with prairie dogs thus involves a compromise for ground squirrels: they incur higher mortality [from the interspecific attacks], but they probably incur lower mortality from predation," they wrote. Consequently, ground squirrels probably receive a net benefit from living with prairie dogs and perhaps for this reason natural selection has not led to clear character displacement that would reduce competition with prairie dogs."
The team says the findings could lead to other scientists discovering this behaviour in other herbivorous species. "It might be happening frequently but covertly right before their eyes, with animals they know well, and with significant consequences for fitness."