First evidence of carnivorous squirrels documented in California

Ground squirrels are commonly associated with their diet of nuts, seeds, and grains, but a groundbreaking new study reveals an unexpected twist in their behavior. 

Researchers from the University of Wisconsin-Eau Claire and the University of California, Davis, have discovered that California ground squirrels also hunt, kill, and eat voles. 

This surprising finding, published in the Journal of Ethology, marks the first documented evidence of widespread carnivorous behavior among squirrels.

The findings fundamentally challenge long-held assumptions about California ground squirrels as primarily granivorous. Instead, the study suggests that these animals are opportunistic omnivores, capable of adjusting their diet to include meat when the opportunity arises.

A shocking discovery of carnivorous squirrels

The surprising observations occurred in 2024 during the 12th year of the Long-term Behavioral Ecology of California Ground Squirrels Project at Briones Regional Park in Contra Costa County. 

Study lead author Jennifer E. Smith is an associate professor of biology at UW-Eau Claire who co-leads the project with Sonja Wild of UC Davis. 

Over the course of June and July, the researchers recorded 74 interactions between squirrels and voles, 42% of which involved active hunting by the squirrels.

“This was shocking,” said Professor Smith. “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 Wild, a seasoned observer of ground squirrels, the behavior was initially hard to believe. 

When undergraduate students first reported seeing squirrels hunting voles, Wild admitted skepticism. “No, I’m not sure what you’re referring to,” she said, until she watched the video evidence.

“I could barely believe my eyes,” Wild recounted. “From then, we saw that behavior almost every day. Once we started looking, we saw it everywhere.”

Opportunists in a changing landscape

The study documented squirrels of all ages and genders hunting and consuming voles, often competing over the prey. This behavior peaked in early July, coinciding with a sudden increase in vole populations reported by citizen scientists on iNaturalist. 

Interestingly, the researchers observed squirrels hunting only voles and no other mammals, suggesting that the behavior arose in response to the temporary abundance of this particular prey.

Wild emphasized the adaptability of California ground squirrels in the face of environmental changes. 

“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.”

Smith echoed this sentiment, pointing to the broader trend of mammals adapting to human-altered landscapes. 

“Many species, including the California ground squirrel, are incredible opportunists,” she said. “From raccoons and coyotes to spotted hyenas and humans, the flexibility these mammals apply to their hunting strategies helps them change and adapt with the human landscape.”

Questions for future research

While the discovery sheds new light on ground squirrel behavior, it raises numerous questions about the ecological and evolutionary implications of their hunting habits. The researchers are particularly curious about how widespread this behavior is among other squirrel populations and whether it is learned or instinctual.

“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 team also plans to investigate whether vole hunting has any impact on squirrel reproduction, comparing data from this year to the previous decade.

Significance of carnivorous squirrels

This discovery of carnivorous behavior in California ground squirrels adds a fascinating layer to our understanding of a species we thought we knew well. It demonstrates not only the behavioral flexibility of these animals but also highlights the value of direct observation and collaboration in scientific research.

The findings suggest that even the most familiar animals can surprise us with behaviors that challenge established assumptions. As researchers prepare for next year’s field season, they remain eager to uncover what other secrets these seemingly ordinary squirrels might reveal.

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