Scientists discovered that some California ground squirrels are carnivorous, hunting and eating rodents instead of their usual diet of nuts and seeds, according to new research published Oct. 18.

The study, led by the University of Wisconsin-Eau Claire and the University of California at Davis, is the first to chronicle widespread carnivorous behavior among squirrels.

“This was shocking,” lead author Jennifer Smith, an associate professor of biology at UW-Eau Claire, said in a statement Wednesday. “We had never seen this behavior before.”

Squirrels are one of the most familiar animals to people, and we have usually associated them with gathering nuts and seeds—not hunting for meat. Their usual vegetarian diet is typically made up mainly of acorns, seeds, nuts, and fruits.

On rare occasions, the squirrels have supplemented their diet primarily by eating insects, or by eating eggs or young hatchlings.

Co-author Sonja Wild, a postdoctoral research fellow at UC Davis, almost couldn’t believe what the researchers discovered.

“I could barely believe my eyes,” Wild said. “Once we started looking [at the squirrels], we saw it everywhere.”

The study, which was published in the Journal of Ethology, fundamentally changes the understanding of ground squirrels, according to researchers.

The scientists researched the squirrels between June 10 and July 30 at Briones Regional Park in Contra Costa County, located about 25 miles northeast of San Francisco. The park is made up of open grassland, walnut, and oak trees.

This is the 12th year of a long-term behavioral study of California ground squirrels. Since 2013, researchers have live-trapped, marked, and released known squirrels in June and July, when most animals are above ground.

Out of 74 interactions with voles, which are small rodents similar to hamsters, 42 percent involved squirrels hunting the small rodents, according to researchers.

The scientists were made up of three groups of observers who watched parts of the population at the same time.

The squirrels involved in hunting were juvenile and adult ground squirrels of both sexes that would chase, punch, pounce, or bite other squirrels while competing for the voles.

The carnivorous summer behavior peaked during the first two weeks of July, coinciding with an increase in the vole population at the park, according to the study.

The timing suggests the squirrels’ hunting emerged with the increase in voles, the study revealed.

Scientists didn’t see the squirrels hunting any other mammals, according to the research.

“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,” Wild said.

Many species, including the California ground squirrel, are “incredible opportunists,” Wild said.

Raccoons, coyotes, and hyenas are known for their flexibility in hunting strategies to help them adapt to the landscape, according to researchers.

Many questions remain unanswered about the squirrels’ behavior, including how widespread the carnivorous behavior was and whether it is passed down from parent to pup.

The researchers are planning to return to the field in the summer of 2025 to see what impact, if any, this year’s vole hunting will have on squirrel reproduction, compared to studies in the past decade.

About 30 years ago, a study by J.R. Callahan in 1993 indicated that about 30 species of squirrels were capable of killing and consuming fish, amphibians, reptiles, birds, and mammals.

Other researchers in the study included Joey Ingbretson, Mackenzie Miner, Ella Oestreicher, Mari Podas, Tia Ravara, Lupin Teles, and Jada Wahl of UW-Eau Claire; and Lucy Todd of the University of California–Davis.

The study was partly funded by the Ronald E. McNair Postbaccalaureate Achievement Program, Diversity Mentoring Program, and Summer Research Experience for Undergraduates. Other funding came from the Swiss National Science Foundation and the Vicki Lord Larson and James Larson Tenure-track Time Reassignment Collaborative Research Program.