A ground squirrel with cheeks stuffed with nuts, seeds or grains, is a common sight. But a new study provides the first evidence that California ground squirrels also hunt, kill and eat voles. The study, led by the University of Wisconsin-Eau Claire and the University of California, Davis, is the first to chronicle widespread carnivorous behavior among squirrels.

Published in the Journal of Ethology, the study fundamentally changes our understanding of ground squirrels. It suggests that what was considered a granivorous species actually is an opportunistic omnivore and more flexible in its diet than previously assumed.

The observations occurred in 2024 — the 12th year of the Long-term Behavioral Ecology of California Ground Squirrels Project conducted at Briones Regional Park in Contra Costa County. Out of 74 observed interactions with voles between June and July, 42% involved active hunting of these small rodents by ground squirrels.

“This was shocking,” said lead author Jennifer E. Smith, an associate professor of biology at UW-Eau Claire who leads the long-term ground squirrels project with Sonja Wild of UC Davis. “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.”

Wild has observed hundreds of squirrels in nature and yet, even for her, when the undergraduate students came in from field work and told her what they had witnessed, she said, “No, I’m not sure what you’re referring to.” Then she watched the video.

“I could barely believe my eyes,” said Wild, a postdoctoral research fellow in the UC Davis Environmental Science and Policy department. “From then, we saw that behavior almost every day. Once we started looking, we saw it everywhere.”

Opportunists amid rapid change

Through videos, photos and direct observations at the regional park, the authors documented California ground squirrels of all ages and genders hunting, eating and competing over vole prey between June 10 and July 30. The squirrels’ carnivorous summer behavior peaked during the first two weeks of July, coinciding with an explosion in vole numbers at the park reported by citizen scientists on iNaturalist. This suggests that the squirrels’ hunting behavior emerged alongside a temporary increase in the availability of prey, the study said. The scientists didn’t 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,” Wild said.

Smith added that many species, including the California ground squirrel, are “incredible opportunists.” From raccoons and coyotes to spotted hyenas and humans, the flexibility these mammals apply to their hunting strategies help them change and adapt with the human landscape.

“Through this collaboration and the data coming in, we’re able to document this widespread behavior that we had no idea was going on,” Smith said. “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.”

The researchers said many questions remain unanswered, including how widespread the hunting behavior is among squirrels, whether and how it is passed down from parent to pup, and how it effects ecological processes. 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 on squirrel reproduction compared to the past decade.

Co-authors include Joey Ingbretson, Mackenzie Miner, Ella Oestreicher, Mari Podas, Tia Ravara, Lupin Teles and Jada Wahl of UW-Eau Claire and Lucy Todd of UC Davis.

Several co-authors conducted field work during their undergraduate studies. Their work was partly funded by the Ronald E. McNair PostbaccalaureateAchievement Program, Diversity Mentoring Program and Summer Research Experience for Undergraduates. Additional funding sources include the Swiss National Science Foundation and the Vicki Lord Larson and James Larson Tenure-track Time Reassignment Collaborative Research Program.