When people think of squirrels, most of the time they perceive them as amiable happy-go-lucky creatures of the Earth–or a dog’s worst enemy.
However, researchers at UC Davis and the University of Wisconsin-Eau Claire have discovered recent changes in the diet of the innocuous California Ground Squirrel. Instead of just acorns, they have developed a taste for something else: Voles.
In the summer of 2024, during a longer-term study on squirrel population at Briones Regional Park in Contra Costa County, researchers from UC Davis observed 74 interactions between squirrels and voles, with 42 percent ending in the act of squirrels hunting the small rodents. This was especially surprising given documentation occurred in only a two-month interval between June and July of 2024.
For as long as they have been studied, squirrels’ diets have primarily comprised nuts, seeds, and vegetables, with the occasional insect. This introduces the question as to what caused this dramatic change in the squirrels’ diet.
Coauthor Sonja Wild said in an article from the UC Davis College of Agricultural and Environmental Sciences, “I could barely believe my eyes,” but that “Once we started looking, we saw it everywhere.”
While squirrels have been noted to rarely engage in infanticide and hunt smaller animals in prior studies, the frequency in the 2024 paper far exceeds any previous work. Whereas the authors note only 16 unambiguous records of squirrels predating small birds and mammals between 1940 and 2020, the 2024 study found 31 instances of predation on voles, suggesting this behavior may be more common than previously thought.
Another compelling facet of this study is that the research method the team at UC Davis supplemented their data utilizing the app iNaturalist, which is a citizen app used to document biodiversity around the world.
Science Teacher Jack Reardon described iNaturalist as a “very useful tool because it lets us see species that we interact with on a daily basis,” adding that it’s “a great tool for learning about ecology as well as naturalistic research and the scientific method.”
The study proposes that this dietary adjustment is a consequence of a relative increase in the local vole population, as documented through iNaturalist, and that the squirrels were opportunistically exploiting an increase in a novel food source.
Diego Gomez ’25 further speculated, “In my opinion, one of the main causes of that may be the pesticides gardeners have been putting in the plants. Not only to stop weeds, but they also have been killing off plants.”
Additionally, predation was most common in areas of high squirrel density rather than areas of high vole density, and squirrels were observed competing over vole carcasses, which the study attributes to the relatively much higher nutrition voles provide relative to their conventional food sources.
The study further concludes given how prolific vole predation was in the sampled population, coupled with the aforementioned handful of earlier reports killing small animals, that squirrels may be genetically predisposed to circumstantially engage in carnivory, rather than this being a learned behavior.
Furthermore, this suggests that ground squirrels are far more dietarily flexible than previously believed, with the authors suggesting they would be better characterized as opportunistic omnivores, rather than the traditional label of strict granivores.
This demonstrates an often unappreciated degree of adaptability and nuance among even the most ostensibly mundane wildlife, and that even the ordinary can defy expectation.
