California ground squirrels are as commonplace as the seeds, acorns, and grains they’re often seen nibbling. Yet, scientists in Northern California recently uncovered an astonishing fact about these bushy-tailed rodents: they are not just herbivores but also active hunters.

Over the summer of 2024, researchers at Briones Regional Park in Contra Costa County documented ground squirrels hunting, killing, and eating voles. The study reveals a behavior previously unrecorded in these squirrels and fundamentally shifts our understanding of their diet.

“This was shocking,” Jennifer E. Smith, a biologist at the University of Wisconsin-Eau Claire and the study’s lead author, said in a press release. “Squirrels are one of the most familiar animals to people. Yet here’s this never-before-encountered behavior that sheds light on how much we still have to learn.”

Carnivorous Squirrels

The discovery came during the 12th year of a long-term study on the ecology of California ground squirrels. From June to July, researchers documented 74 interactions between squirrels and California voles. Almost half (42%) involved active hunting.

“We had never seen this behavior before,” said Sonja Wild, a co-author and postdoctoral researcher at UC Davis. The surprise began when undergraduate field researchers showed her a video of the squirrels chasing voles. “I could barely believe my eyes,” Wild recalled. “From then, we saw that behavior almost every day.”

The hunting coincided with a population explosion of voles in the park, reported by citizen scientists on iNaturalist. This abundance of prey likely triggered the squirrels’ predatory behavior. So, the study suggests that these squirrels, once thought to be primarily granivores (seed-eaters), are instead highly flexible omnivores.

Opportunists in a Changing World

Such dietary flexibility may help California ground squirrels adapt to rapidly changing environments. “The fact that these squirrels are behaviorally flexible and respond to changes in food availability might help them persist in human-dominated landscapes,” Wild explained.

Opportunism in foraging is certainly not unique to squirrels. Raccoons, coyotes, and even humans exemplify this trait, adapting their diets to suit their environments. Still, the researchers were unprepared for the extent of the squirrels’ carnivory. “It’s incredible to document a behavior we had no idea was happening,” said Smith.

Ground squirrels’ predation on voles raises new questions. Could this behavior influence the region’s food web? Is it learned socially or instinctual? And will it persist beyond this summer’s vole boom? Scientists aim to return to the site and explore how this hunting behavior might impact reproduction and ecological balance.

The Bigger Picture

California ground squirrels are a keystone species, serving as prey for hawks, snakes, and other predators. Understanding their dietary shifts is crucial for grasping their role in the ecosystem. As Smith noted, their success can ripple outward, offering opportunities for other species to thrive.

This discovery also highlights the importance of direct observation in science. “Digital technology can inform the research,” Smith said, “but there’s no replacement for going out there and witnessing the behavior because what animals are doing always surprises us.”

For now, the image of a ground squirrel, cheeks stuffed with nuts, may need a dramatic update — one that includes sharp teeth and a vole caught in its paws.

The findings appeared in the Journal of Ethology.

This article originally appeared in December 2024 and was updated with new information.