California ground squirrels (Otospermophilus beecheyi) might look like your run of the mill bushy-tailed rodent. But these squirrels are anything but ordinary. For the first time, they have been observed hunting, killing, and eating other animals-tiny rodents called California voles (Microtus californicus). The predation is described in a study published December 18 in the journal Journal of Ethology and provides the first evidence of carnivorous behavior in this species of squirrels.
Most squirrels are omnivores, but are usually seen eating nuts, seeds, and grains. However, some species are known to eat some animal protein. The eastern gray squirrel will supplement its diet with insects, frogs, and bird eggs. But these California ground squirrels appear to have developed a taste for mammal meat.
The new observations occurred over the summer and were part of the Long-term Behavioral Ecology of California Ground Squirrels Project conducted at Briones Regional Park in Northern California. Out of the 74 observed interactions with voles between June and July 2024, 42 percent involved active hunting by the ground squirrels.
"Our findings of vole hunting build on reports of infanticide in California ground squirrels, the killing of newly emerged young animals, and isolated events of killing of small prey such as lizards in captivity," study co-author and University of California, Davis biologist Sonja Wild tells Popular Science. "It is therefore plausible that hunting is innate in these squirrels to an extent. Yet, in our twelve years of conducting observations on hundreds of squirrels we had never observed any incidents of vole hunting before this summer."
"This was shocking," study co-author and University of Wisconsin-Eau Claire biologist Jennifer E. Smith said in a statement. "We had never seen this behavior before. Squirrels are one of the most familiar animals to people. We see them right outside our windows; we interact with them regularly. Yet here's this never-before-encountered-in-science behavior that sheds light on the fact that there's so much more to learn about the natural history of the world around us."
Even for seasoned biologists, this observation was quite a surprise. When several undergraduate students came in from the field, they showed the video to study co-author and University of California, Davis biologist Sonja Wild.
"I could barely believe my eyes," Wild said in a statement. "From then, we saw that behavior almost every day. Once we started looking, we saw it everywhere."
Using a combination of direct observations, photos, and videos the team documented California ground squirrels of all ages and sexes hunting, eating, and competing over vole prey between June 10 and July 30, 2024. They noticed that their carnivorous summer behavior peaked during the first two weeks of July. This peak also coincided with a large number of vole sightings in the park reported to the citizen scientist website iNaturalist. The team believes that squirrels' hunting behavior emerged alongside a temporary increase in the availability of prey. Over the course of the summer, the scientists did not observe the squirrels hunting other mammals.
"The fact that California ground squirrels are behaviorally flexible and can respond to changes in food availability might help them persist in environments rapidly changing due to the presence of humans," said Wild.
According to Wild, it also suggests that vole hunting behavior likely emerged in response to the unusual peak in California voles at the team's field site. This uptick provided easily accessible prey for the squirrels.
Many species are "incredible opportunists," according to Smith. Raccoons, coyotes, spotted hyenas, and humans are all known to adapt their hunting strategies. For non-human mammals, this flexibility helps them in a landscape that continues to be dominated by humans.
"Through this collaboration and the data coming in, we're able to document this widespread behavior that we had no idea was going on," said Smith. "Digital technology can inform the science, but there's no replacement for going out there and witnessing the behavior because what animals are doing always surprises us."
Numerous questions remain unanswered about this pattern. We still do not know quite how widespread hunting behavior is among squirrels, whether and how it is passed down between generations, and how it affects ecological processes and the food web in the area. The team plans to return to the field in summer 2025. The authors are also excited to return to the field next summer to see what impact, if any, this year's vole hunting may have had on squirrel reproduction compared to the last 10 years.