The shift from herbivorous to carnivorous diet is a response to a growing number of other rodents, study finds.

California ground squirrels, long known for their plant-based diet, are now exhibiting unexpected carnivorous behavior towards other rodents. A new study, published in the Journal of Ethology, presents the first documented evidence of these mammals hunting, killing, and eating voles, signaling a striking shift in their feeding habits.

Traditionally, squirrels are associated with hoarding nuts and seeds, but while some species have been known to consume meat occasionally, instances of them actively preying on live vertebrates have been rare.

The research, part of the Long-Term Behavioral Ecology of California Ground Squirrels Project, based in Briones Regional Park explores how these native grassland rodents adjust to environmental changes—specifically, a surge in the local vole population.

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Northern California has seen an increase in vole infestations, and researchers observed a significant rise in vole numbers at the study site over the past decade. Historically considered herbivores or granivores, California ground squirrels are now engaging in predatory behavior, demonstrating their ability to adapt to ecological shifts.

This study dramatically alters the perception of squirrels, who have proved to be remarkably resilient I response to environmental pressures such as climate change or drought, says lead author Jennifer Smith, an associate professor of biology at the University of Wisconsin-Eau Claire.

California ground squirrels thrive across various elevations and consume a wide range of plants, but the quick adaptation of their behavior to an abundance of voles is something unseen before.

Squirrels play a key role in California’s ecosystems, and understanding how they acquire new food sources may provide insight into how other species adapt to environmental changes, she explained.

During ten days of June 2024, researchers captured, marked, and released California ground squirrels every two weeks to track their behavior. Each squirrel was tagged with identification markers and dyed for easy tracking.

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On non-trapping days, scientists observed the squirrels in three study groups, recording 74 interactions between them and voles. Of these encounters, 42% involved active hunting and consumption of voles, with both juvenile and adult squirrels—male and female—participating.

Beyond hunting, the study uncovered complex interactions between squirrels and voles, ranging from social behaviors like foraging and greeting to competitive encounters involving chasing, pouncing, and biting, with voles already recognizing squirrels as predators rather than just competitors for resources.

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The findings suggest that California ground squirrels may be better classified as opportunistic omnivores. When food sources fluctuate, they demonstrate dietary plasticity—the ability to shift their diet based on availability. With protein being a vital resource, voles may offer a more efficient nutritional boost than seeds – which is a smart wat to take advantage of abundancy.

While some might find this behavioral shift surprising, researchers emphasize that it’s a natural reaction to food availability. Voles, known for their rapid population growth, are considered pests, and having a new predator could help regulate their numbers.

The study authors, who represent the University of Wisconsin and the University of California Davis, published a short video with several episodes of California ground squirrels hunting and eating voles.

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