Researchers Find Vole-Munching Squirrels in East Bay Park

Its fluffy body stays low to the ground, silently stalking its prey. While the small rodent remains unaware, the predator goes in for the kill, pouncing quickly, pinning the animal to the ground before brutally decapitating it and starting to feast.

It’s all light work for this unlikely predator: the California ground squirrel.

“I could barely believe my eyes,” said Sonja Wild, a postdoctoral research fellow at UC Davis. “Once we started looking, we saw it everywhere.”

For the first time, researchers at UC Davis and the University of Wisconsin-Eau Claire captured squirrels in Contra Costa County hunting and killing voles, small rodents related to hamsters that typically weigh less than two ounces.

Wild worked on the study with lead author Jennifer Smith, an associate professor of biology at UW-Eau Claire.

But it was undergraduates on their team that first discovered the behavior, Smith said.

“I was very excited to hear about it and also a little bit taken aback about how these cute squirrels that we’ve been studying for over a decade were engaging in this carnivorous behavior,” Smith said.

Researchers — affectionately dubbed “Team Squirrel” — have been observing the creatures in Briones Regional Park for 12 years as part of the Long-term Behavioral Ecology of California Ground Squirrels Project, which Wild and Smith co-lead.

This summer was the first time they observed widespread carnivorous behavior among the ground squirrel population.

“It really helps to have the study subjects habituated to us,” Smith said. “There are humans in the park all of the time, but the squirrels respond differently to members of Team Squirrel. We walk differently. We are very, very cautious. We observe at a distance.”

Their findings turn the common understanding of squirrel foraging behavior on its head. Previously, the consensus was that ground squirrels primarily ate seeds and other vegetation — only occasionally scavenging for lizards, birds, eggs and bugs.

“We’ve known for a long time that they are omnivores, and they can eat meat, and they can scavenge all of these things,” Smith said. “But the missing link was: do these squirrels actively target and pursue prey in their natural habitat?”

A peak in the carnivorous behavior coincided with an explosion of the vole population in the park at the beginning of July, suggesting that it may have been in response to the prey being more available than usual, the authors wrote in the study published in the Journal of Ethology.

Besides hunting and killing voles, researchers also observed the squirrels competing over their prey.

Squirrels are also social mammals, so researchers are interested in further understanding how the hunting behavior became so widespread throughout the population.

Smith said another potential area of future research is the impact of hunting voles on the survival of ground squirrels.

Researchers expect the vole population to crash next summer due to population cycles, so Smith said it will be interesting to see how the squirrels respond and pivot their eating habits.

“They’re incredibly behaviorally flexible,” Smith said, “and my guess is they’ll just take advantage of whatever foods are around.”