A squirrel running with a vole in its mouth in Briones Regional Park, California. (Image credit: Sonja Wild, UC Davis)
Ground squirrels usually eat seeds and nuts, but in a park in California, these cute, furry creatures have turned into carnivorous killers, mercilessly hunting down voles to peel the flesh from their bones.
The unusual new behavior, which researchers caught on camera in Briones Regional Park in Contra Costa County, could be driven by an explosion in vole numbers, according to a new study published Wednesday (Dec. 18) in the Journal of Ethology.
Students have been observing squirrels in Briones Regional Park for 12 years as part of a long-term research project at the University of Wisconsin-Eau Claire and the University of California, Davis. In all that time, researchers had never witnessed anything like what they saw in the summer of 2024.
Study lead author Jennifer Smith, an associate professor of biology at the University of Wisconsin-Eau Claire, co-leads the park research project -- called "Team Squirrel." When two students first approached her with a video of the killer squirrels, she didn't know what to make of it.
"I was shocked, in disbelief, and, frankly, very excited to learn more about what was going on," Smith told Live Science in an email. "At first, I questioned whether the squirrels were actively hunting the voles in the area. Team Squirrel made it our mission to document full hunting, killing and consumption events from start to finish using digital photography of these carnivorous squirrels."
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The squirrel family is known to be fairly flexible when it comes to their dinner. California ground squirrels (Otospermophilus beecheyi) are mostly vegetarian, but they will also eat insects, eggs, hatchling birds and even each other's young, according to the study. However, actively hunting adult mammals still marks a significant shift in the squirrels' behavior.
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The team observed the squirrels hunting California voles (Microtus californicus californicus) almost every day between June 10 and July 30, which suggests the behavior became fairly common this year. A typical hunt involved a single ground squirrel chasing a vole down and biting it, often at the neck or head, Smith said.
"Not all hunts were successful, but once captured, the squirrels typically started to eat the head - bones and all - and then move to pull meat from the bones of the voles," Smith added.
In some cases, one squirrel would get a vole, and then another squirrel would come along to try to steal the kill. The study authors noted that this competition over meat led to aggressive interactions between squirrels.
Why are squirrels killing voles?
The squirrel rampage coincided with a boom in the local vole population. Some vole populations are stable, but most increase and decrease over a multi-year cycle, peaking every three to five years, according to the study. Vole numbers in California appear to have peaked this year, with many more voles reported compared with previous years -- roughly seven times more than the 10-year average.
The study authors linked the squirrels' new hunting behavior to this sudden increase in vole availability. Smith said that the voles aren't likely to be as abundant next year, so the squirrels are likely to switch back to eating seeds.
Team Squirrel will continue to monitor the squirrels next year to better understand the impact of this hunting behavior.
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"Perhaps one of the most fascinating next steps will be to understand the fitness consequences -- in terms of survival and reproduction -- in response to this carnivorous behavior," Smith said. "Squirrels, like most rodents, are what we call "income breeders." More food, especially a high-quality meat diet, should mean more babies coming out."
When asked about the possibility of squirrels taking on rats, Smith noted that it was possible the squirrels might start hunting other rodents, but it's unlikely to be rats.
"Ground squirrels are roughly 10 times heavier than voles, so this size advantage likely was an important factor contributing to their successful hunting strategy," Smith said. "Rats, on the other hand, are of more comparable size to ground squirrels and equally smart, presenting an even bigger challenge to a ground squirrel."