After more than a decade studying California ground squirrels, Jennifer Smith felt she had a solid understanding of their behavior. Then, in the summer of 2024, her students spotted something she never expected: one of the squirrels chased, killed and ate a vole, a small rodent common across the western coast of North America.

Until now, the California ground squirrel (Otospermophilus beecheyi) had been described as an animal that ate mostly acorns and grass seeds. They’ve been observed eating bark, flowers, leaves and the very occasional insect or bird egg.

At first, Smith, an evolutionary ecologist at the University of Wisconsin, U.S., couldn’t believe it. “I was shocked and very skeptical,” she told Mongabay by email. “In twelve years of studying these ground squirrels, we have never seen anything like this before!”

Over the next seven weeks at Briones Regional Park in California’s Contra Costa county, Smith’s team documented 74 cases of squirrels hunting voles (Microtus californicus). The findings have now been published in a study. From a population of 125 squirrels, they recorded at least 27 individuals taking part: male and female, young and old.

The carnivorous behavior wasn’t passive; the squirrels stalked, chased and pounced on their prey. And for an animal that had never before been recorded hunting, they were surprisingly proficient at it. Out of the 31 documented hunting attempts, 17 resulted in a kill, a success rate of 55%.

“The sounds on the videos were quite shocking as the squirrels crunched on the skulls of the vole prey!” Smith said. It’s still not known whether the squirrels learned hunting behavior from each other or if individual squirrels learned how to hunt on their own through trial and error.

In 2024, California saw an unprecedented vole population boom, with sightings seven times higher than the decade’s average. Scientists are still trying to understand the reasons behind the explosion in vole numbers but suspect the squirrels seized the opportunity for easy prey.

The discovery fundamentally changes existing understandings of the species, which will now be reclassified as opportunistic omnivores instead of a granivore, or seed eater, the authors said in the study.

Scientists also want to know whether the protein-rich diet will give the squirrels an edge. “Next year we will assess the consequences of this novel behavior for the California ground squirrels,” Smith said. “We will see the effects of carnivory on the reproductive output and overwinter survival — two major components of fitness.”

If the species is truly more flexible in its diet than previously thought, it may also prove to be more resilient and adaptable in the face of rapid environmental changes caused by climate change and other human-induced environmental impacts, the authors said.

Banner image: A California ground squirrel, which usually eats seeds and acorns, hunts a vole. Image courtesy of Sonja Wild/UC Davis.