California squirrels are eating another rodent for the first time, new study finds

A California ground squirrel in Briones Regional Park in Contra Costa County feeds on a vole as an adaptive behavioral response to an increase in the local vole population, a new study found. (Sonja Wild / UC Davis via CNN Newsource)

Traditionally known for stuffing their cheeks with nuts, squirrels can be carnivorous — though recorded instances of the rodents hunting and killing other live vertebrates are rare, with few species known to have done so. Now, scientists have found unprecedented evidence of another type of squirrel exhibiting carnivorous behaviours, including hunting, killing and eating voles, according to a new study.

The research, published Wednesday in the Journal of Ethology, is part of the Long-Term Behavioral Ecology of California Ground Squirrels Project at Briones Regional Park in Contra Costa County. The project examines how California ground squirrels — native to the state’s grasslands — adapt their behaviour in response to environmental changes, in this case an increase in the local vole population.

In parts of Northern California, vole infestations have been observed. At the research site, the study’s authors noted a significantly higher number of voles than average over the past decade. California ground squirrels have typically been considered herbivores or granivores, eating mostly plants and seeds. The new findings offer the first documentation of the species actively preying on other live vertebrates — underscoring its ability to respond to changes in the ecosystem.

“This research radically changes our perception of squirrels, one of the most familiar mammals in the world,” said lead study author Jennifer Smith, associate professor of biology at the University of Wisconsin-Eau Claire, via email. “In the face of human insults such as climate change and drought, these animals are resilient and have the potential to adapt to live in a changing world.”

Squirrels are an important part of California ecosystems, and understanding whether food discovery is a socially transmitted trait in these populations could provide key insights into how other species will adjust to shifting environments, Smith said.

An unexpected shift in squirrel behaviour

Smith and her colleagues observed the squirrels’ meat-eating behaviour during a study period from June 10 to July 30. During that time, the scientists used live traps to capture, mark and release California ground squirrels on a biweekly basis.

The study team collected detailed data on each squirrel, including its sex, reproductive status and body mass, labeling every animal with two identification tags and a unique fur mark using dye to ensure all the squirrels studied could be tracked throughout the observation period.

Squirrels are particularly useful for studying animal resilience to environmental changes because they are active during the day and easy to trap, mark and observe, Smith said.

The researchers split the squirrels into three groups and observed the animals’ behaviour on non-trapping days. The team recorded 74 interactions between the squirrels and voles, and 42 per cent of the encounters involved squirrels — including both male and female juveniles and adults —actively hunting and consuming the voles.

“We already knew that (California ground squirrels) could live (in) a range of elevations and feed (on) a breadth of plants but what is most striking and incredible is the speed at which they shifted their behaviour to this local surge in vole abundance,” Smith said.

The study revealed other previously undocumented social dynamics between squirrels and voles as well. Some interactions between the animals were positive, including foraging, greeting and playing. Other behaviours were competitive, characterized by chasing, physical pushing, pouncing and biting.

“Voles (have come to) recognize the squirrels as predators,” said John Koprowski, dean of the Haub School of Environment and Natural Resources at the University of Wyoming, who was not involved in the study. “There’s likely some really neat interplay between the two species because being eaten is usually not a great way to continue passing on your genes.”

Evolving dietary patterns

The study’s findings suggest squirrels might be best classified as opportunistic omnivores based on their willingness to hunt and consume live prey, particularly when there is an abundant supply. Protein is a limited yet necessary resource for squirrels to thrive, and hunting voles likely provides them with a faster, more accessible nutrition boost than seeds, Koprowski explained.

“It’s a wonderful way for them to capitalize on a very abundant resource … to provide enough sustenance for many (squirrels) to use,” he said.

Though humans typically categorize animals by their eating habits, it’s not uncommon for even the strictest carnivore to eat fruit every once in a while, Koprowski added.

This phenomenon in which an animal capitalizes on a necessary resource when it’s made available to them is known as dietary plasticity, according to Koprowski. If an animal doesn’t adapt to utilize the resource, it’s likely another species will take advantage of it.

While this discovery may initially seem concerning to some, Koprowski said such a change in diet is quite normal for animals. Voles frequently experience surges in population growth and are considered pests, so having new predators such as California ground squirrels will help keep vole numbers under control, he said.

The new findings may also help set the stage for future research on the adaptability of different squirrel species and other mammals in the face of environmental changes, according to Smith.

“Animals can either adapt or go extinct in a human-impacted world,” Smith said. “These animals are showing … us the incredible resilience of some species and by studying these processes we can offer meaningful insights for conservation.”