The California ground squirrel seems innocent. A familiar sight to hikers, the plump rodent can often be seen contentedly chewing on grasses, seeds and berries.
But according to new research published on Wednesday in the Journal of Ethology, wildlife biologists have documented a fierce feature of the creature: It is a committed carnivore, hunting and feasting on unlucky little voles.
Like a rodent version of the notoriously violent 1969 film “The Wild Bunch,” extensive video footage recorded at Contra Costa County’s Briones Regional Park offers repeated evidence of the squirrels’ savagery.
Squirrels chase the voles, then catch and pummel them until they’re a bloody pulp. Like cats, the squirrels sometimes let the mortally wounded creature limp off. Then they pounce again. Even as the vole squirms, clinging to life, they take a bite.
It’s been known that California ground squirrels won’t pass up an easy meal of meat. Prior research found clues of ingested voles, as well as quail eggs, insects and shellfish.
But the new study, led by the University of Wisconsin-Eau Claire and UC Davis scientists, is the first to chronicle widespread predatory behavior.
“This was shocking,” said lead author Jennifer E. Smith, an associate professor of biology at UW-Eau Claire who with Sonja Wild of UC Davis leads the Long-term Behavioral Ecology of California Ground Squirrels Project.
“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,” she said.
With its rolling, grassy hills and secluded, shady canyons, Briones is heavenly habitat for ground squirrels.
Prolific and plentiful, the squirrels are intriguing because they have a larger and much more complex social structure than other squirrel species, according to Smith.
The behavior of two different ground squirrel populations is the focus of research by faculty and students from UC Davis, Wisconsin and other institutions.
Dubbed Team Squirrel, the researchers follow specific animals year after year, gathering extensive data on the social and biological histories of many individual creatures across their lives.
They analyze recordings of alarm calls. They study squirrel poop to learn about gut health. They witness tense rattlesnake vs. squirrel standoffs.
In 2024, something unusual happened. Parts of California experienced an unusual explosion in the abundance of voles.
Defenseless bundles of fur, voles are small and pudgy, with short rounded ears, a rounded head and restless energy. They are engineers, digging burrows which turn soil and move seeds. Active night or day, they are driven by an enormous appetite and the ever-present need to feed their offspring. Voles may produce as many as six or seven broods a year.
Scurrying through a meadow in broad daylight is risky for voles, the researchers learned.
Through videos, photos and direct observations at the regional park, they documented ground squirrels 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 of vole numbers at the park reported by citizen scientists on iNaturalist, a free app where users have recorded millions of observations of wild animals and plants.
The squirrels didn’t seem interested in hunting other small mammals.
Watching video, “I could barely believe my eyes,” said Wild. “From then, we saw that behavior almost every day. Once we started looking, we saw it everywhere.”
The study changes our understanding of ground squirrels — suggesting that what was considered a grain-eating species actually is an opportunistic omnivore, say researchers.
It’s known that many other species, including raccoons, coyotes and humans, are flexible in their diet and behavior.
The new study shows that the California ground squirrel can also respond to a surge in the availability of prey, according to the study.
“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,” said Wild.
Other questions remain unanswered. For instance, it’s not known whether the hunting behavior is widespread, if it is passed down from parent to pup, and how it affects the larger ecology of the natural landscape.
The researchers plan to return to Briones next summer to see what impact, if any, this year’s vole diet had on the numbers of squirrels.
By then, however, the squirrels may have become someone else’s dinner. Hungry snakes, weasels, hawks and eagles rely on them for sustenance.
Ground squirrels are predators. But, like voles, they’re also prey.