Squirrels are famous for their love of nuts and seeds—and for their cuteness. Yet their fluffy tails and playful scampering belie the fact that these rodents have the ability to become lethal predators, in the right conditions.
While squirrels are generally recognized as herbivores, they have been known to eat insects, bird’s eggs, and even scraps of meat left out by humans. Active hunting is very rare, though it has occasionally been observed in certain species.
That’s why the 2024 discovery that the California ground squirrels in Briones Regional Park (nearly Berkeley and Oakland in Contra Costa County) had become carnivorous (or, more accurately, omnivorous), was so surprising. The squirrels had never before been observed hunting live mammals, yet they now seem to actively prey on voles, a smaller rodent sometimes considered a pest due to its booming population in parts of Northern California. The carnivorous behavior appears to be directly related to the recent massive increase in voles.
For a species with a mostly vegetarian diet, the squirrels in Briones Regional Park adapted to the abundance of voles in the area remarkably quickly. Last summer, researchers from UC Davis and the University of Wisconsin-Eau Claire captured, marked, and released California ground squirrels in Briones Park, then tracked their interactions with voles. During the period between June 10th and July 30th, 2004, 74 interactions were observed. In 42% of those interactions, the squirrel killed and ate the vole. This was observed in both male and female squirrels and in both juveniles and adults.
Although their sudden interest in eating meat seems to have been precipitated by an overabundance of voles rather than a lack of vegetarian options, the squirrels' dietary plasticity suggests that they should be considered opportunistic omnivores. And though the behavior may seem worrying to the average person, the researchers consider it evidence that squirrels have the resilience to adapt to a changing environment.
A new item on the menu:
- Rather than being shocked that the squirrels had suddenly started hunting and killing voles (in a fairly gruesome way), biologists were pleased that they were capitalizing on the sudden abundance of protein. The squirrels are also helping their ecosystem by becoming predators and keeping vole numbers in check.
- The researchers have yet to learn whether this carnivorous behavior has spread beyond Briones Regional Park, where the vole population is particularly high. Another questions is whether it is likely to be passed on to future generations of squirrels.
- According to citizen scientist data recorded on the app iNaturalist, California’s vole population was around seven times greater in 2024 than the average over the past decade.