A California ground squirrel carries a vole in its mouth after hunting the rodent. Sonja Wild, UC DavisIn the rolling green hills of Californias Briones Regional Park, not far from the urban bustle of Oakland and Berkeley, the squirrels are not what they seem.Instead of simply stuffing their cheeks with fruits, nuts or seeds, the California ground squirrels there are now known to hunt, kill, decapitate and consume voles, a fellow rodent species, according to research published Wednesday in the Journal of Ethology.This was shocking, study lead author Jennifer E. Smith, a professor of biology at the University of Wisconsin-Eau Claire, says in a statement. We had never seen this behavior before.Most people are very familiar with squirrels, accustomed to seeing the animals on their streets or in their yards. And Smith is even more sosince 2013, she has led the Long-term Behavioral Ecology of California Ground Squirrels Project, which monitors squirrel genetics, social behavior and physiological stress responses in the San Francisco Bay area. But even with her intimate knowledge of the rodents, it was only this year that her research team observed ground squirrels actively pursuing and eating meat.The initial discovery was made by undergraduate researchers, who returned from the field site in Briones Regional Park earlier this year with videos of the squirrels behavior to show their incredulous supervisors.At first, we questioned what was going on, Smith tells Suzie Dundas of SFGATE. But seeing the videos was astounding and shifted my perspective on a species that I have spent the last 12 years of my life studying.Vole hunting: Novel predatory and carnivorous behavior by California ground squirrelsWatch on Smith and her team documented 74 interactions between ground squirrels and voles between June and July. In 42 percent of the interactions, the squirrels were hunting the smaller rodents.From then, we saw that behavior almost every day, Sonja Wild, a postdoctoral researcher at the University of California, Davis, who co-leads the project with Smith, says in the statement. Once we started looking, we saw it everywhere.Though theyre largely vegetarians, squirrels have long been known to occasionally consume insects, small bird eggs or meat left out by humans, but direct study of hunting behavior by squirrels remains rare, according to the study. This widespread carnivorous behavior, Smith tells SFGATE, was therefore groundbreaking.In most recorded cases included in the paper, a single squirrel would pursue a single vole across the open landscape, rather than lying in wait or hunting in a group. Sometimes with a stalking motion and a pounce, it would tackle the vole, restrain it with its forepaws, bite the voles neck and remove its head. Then, the squirrel would either strip the fur from the vole to expose cartilage, meat and organs, or tear the meat directly out from the torso.The authors of the study suggest this change in diet is a response to a booming vole population. Using reports from citizen scientists on iNaturalist, an app that allows users to submit photos of plants and animal sightings, the researchers found vole abundance in the state was seven times greater this year than over a ten-year average, and sightings were particularly high in Briones Regional Park.What is most striking and incredible is the speed at which they shifted their behavior to this local surge in vole abundance, Smith tells CNNs Julianna Bragg.Its a wonderful way for them to capitalize on a very abundant resource to provide enough sustenance for many [squirrels] to use, John Koprowski, a wildlife biologist at the University of Wyoming who was not involved in the study, tells CNN. Tia Ravara from UW-Eau Claire, left, and Ryann Su of UC Davis, both members of the research group called "Team Squirrel," watch a California ground squirrel during the 2023 field season. Sonja Wild, UC DavisSquirrels of all ages and sexes took part in the vole hunt, an indication that this dietary flexibility is widespread across the species and may serve as a crucial survival mechanism in response to fluctuating environmental conditions, especially in areas with high human populations.In the face of human insults such as climate change and drought, these animals are resilient and have the potential to adapt to live in a changing world, Smith adds.If acorn or seeds fall into low supply, for instance, squirrels appear primed to find other sources of protein and nutrients. The less stubborn an animal is about its diet, the better suited it is for survival. Although in this case, the squirrels dont seem to be motivated by a scarcity of any other food source, per SFGATE, rather, the high population of voles appears to be the reason for their hunting.But because the study was focused on one regional park, it is unclear if the squirrels behavior willor already hasspread across the state. It is also unknown if the taste for fellow rodents (and the hunting techniques used to capture them) will be passed onto future generations of squirrels once this vole surge abates. What the opportunistic squirrels door eatnext remains to be seen.Digital technology can inform the science, Smith says in the statement, but theres no replacement for going out there and witnessing the behavior, because what animals are doing always surprises us.Get the latest stories in your inbox every weekday.Filed Under: Animals, California, Hunting, Mammals, New Research, Rodents and Shrews, Scientists, Squirrels