Something odd happened in a California park last summer. Ground squirrels the chunky, sunbathing herbivores you step around on hiking trails started killing things.
Researchers watching California ground squirrels (Otospermophilus beecheyi) at Briones Regional Park in Contra Costa County recorded 74 separate incidents in which the animals hunted, killed, and ate California voles (Microtus californicus). The squirrels involved weren’t just opportunistically nibbling on something that wandered too close. They were stalking prey through tall grass and running it down across open ground. Of 31 recorded active hunts, 17 ended in a kill a success rate of around 55%, which is hard to explain away as accident.
The behavior clustered in early July and tracked a population spike in voles at the park, documented by citizen scientists through iNaturalist. That timing matters. California ground squirrels are textbook omnivores seeds, roots, the occasional insect but voles offer far more energy per mouthful than any of that. When a high-calorie food source suddenly became abundant, the squirrels, apparently, took notice.
This fits reasonably well with optimal foraging theory, which predicts that animals will expand their diet when richer options appear. What’s harder to explain is how fast and broadly the behavior spread. Twenty-seven individually identified squirrels took part in hunting activity between June 10 and July 30. Juveniles, adults, males, females all of them were in on it. That’s not one eccentric individual. That’s something closer to a population-wide behavioral switch being thrown.
Two explanations have been floated, and they aren’t mutually exclusive. The first is that carnivory was always latent in these animals a capacity that sits dormant under normal conditions and surfaces when circumstances change. The second is that some squirrels figured it out and others watched and copied them. Social transmission of hunting behavior, if confirmed, would be a striking finding in its own right. But the study doesn’t yet resolve which explanation carries more weight, or whether both are operating simultaneously.
The ecological implications are worth taking seriously. Voles are herbivores, and their population sizes have knock-on effects on vegetation. Adding a novel, widespread predator to the mix even intermittently could shift local food web dynamics in ways that are hard to predict in advance. The researchers also note that behavioral flexibility like this may help the species cope with climate-driven disruptions to food supply. That’s plausible, though speculative for now.
A lot remains open. Whether this was a one-summer anomaly or a recurring pattern is unknown. Whether hunting gets passed down to offspring, whether it spreads to other populations, whether it actually improves survival or reproduction none of that has been established yet. Longer-term study across multiple sites would start to answer those questions.
For now, what’s clear is that a species biologists thought they knew well just added a new entry to its behavioral profile. The California ground squirrel, it turns out, is a predator at least when the conditions are right.
Reference:
Smith, J. E., et al. (2025). Vole hunting: novel predatory and carnivorous behavior by California ground squirrels. Journal of Ethology, 43(1), 3–12.
