In June 2024, Jennifer E. Smith of the University of Wisconsin-Eau Claire received a disturbing report from her research team conducting fieldwork at Briones Regional Park in Contra Costa County. It's a sprawling expanse of grasslands and oak woodlands known for scenic trails, views of Mount Diablo and a thriving bird-watching scene. However, Smith's students had documented something far more unsettling than a rare bird or damage on a trail: an unsettling description of a California ground squirrel actively hunting, killing and ripping apart the bodies of tiny California voles.

"At first, we questioned what was going on," Smith says. "But seeing the videos was astounding and shifted my perspective on a species that I have spent the last 12 years of my life studying."

For Smith's team, it was extraordinary behavior to see in a species most people think of as docile and harmless -- and one not ever known to display "widespread carnivorous behavior," particularly across an entire population. It's something never before observed in the species and could have widespread implications for what changing environmental conditions could mean for California's woodland species.

California ground squirrels are traditionally classified as granivorous, relying primarily on seeds, grains and vegetation. While there are isolated accounts of individuals scavenging carcasses, preying on bird eggs or occasionally killing small animals such as lizards under unusual circumstances, such behavior has been considered rare and opportunistic. However, a new study published in the Journal of Ethology on Dec 18. suggests a significant shift, stating that these observations "fundamentally change our understanding of the dietary flexibility of this ground squirrel."

Smith, the lead author of the report, and Sonja Wild, a co-author and postdoctoral research fellow at UC Davis, lead the Long-term Behavioral Ecology of California Ground Squirrels Project at the park, which is supported by Save Mount Diablo. The multiyear project aims to study mammalian behavior based on changing ecological conditions, though researchers didn't expect to find this type of behavior specifically.

When Smith, who is also an associate professor of biology at the University of Wisconsin-Eau Claire, first received reports of students observing the carnivorous behavior, she was sure they must be in error.

"When I first heard about the behavior, I suspected that it was a female squirrel killing the young of a different squirrel -- infanticide," she says. "Wow, was I sorely mistaken."

For Wild, she thought the behavior may have been some version of squirrels having fun. "We sometimes observe juvenile squirrels chasing after birds on the ground, which appears to be a playful interaction," she says, "at least from the squirrel's perspective." However, as soon as Smith and Wild confirmed the behavior, they made it their mission to collect as much video and photo evidence as possible.

The researchers observed interactions between voles and ground squirrels in June and July and found that 42% involved squirrels killing voles. What makes this discovery so unusual is that the percentage should have been zero. "Vole-squirrel interactions are normally very rare," Smith says, "and never before had we seen any interactions between the two species at all in our 12 years of study." Over the study period, researchers observed squirrels of all sexes and ages killing and consuming adult prey, indicating that carnivorous adaptation is rampant among all sections of the population.

The study provides detailed observations of the squirrels' unexpected and opportunistic hunting methods. While a few squirrels employed stealthier tactics, reportedly staying "low to the ground and minimizing noise (stalking) before launching an attack," most opted for high-speed chases through open spaces before pouncing on their prey. However, it was their post-kill behavior that proved especially unsettling, defying what may be expected of a species just beginning to hunt.

For squirrels observed consuming their kills immediately rather than carrying the carcass back to a den, the behavior followed a grimly methodical pattern. In every case, the squirrels "first removed the head of the vole" before pulling meat from the torso. They then "stripped fur from each of the body parts" before devouring the exposed flesh, organs and even cartilage, behavior that was reminiscent of a more seasoned predator.

Both Smith and Wild think the squirrels' behavior is an environmental adaptation, most likely triggered by an increase in voles. Vole populations in California are thought to fluctuate based on the availability of certain minerals in the environment, which may explain the dramatic increase, Smith notes. This spike in vole numbers was first documented through the citizen science platform iNaturalist, where reports indicated populations were five to six times higher than the 10-year average. Smith suggests that the squirrels "simply took advantage of superabundant prey" rather than being driven by resource scarcity, as she says it's unlikely the voles were depleting the squirrels' typical food sources.

For now, given the unexpected nature of the discovery, it's hard to say what conclusions can be drawn for the future of the carnivorous, vole-eating wild squirrels. It's possible, Smith says, that the hunting adaptation could be seen in other squirrels around the state. "The carnivorous behaviors could be transmitted socially from one individual to another," she says, "or developed independently through trial-and-error learning."

Another question for the researchers is whether the behavior will be passed down to offspring after vole populations reset. While it's unlikely that an evolutionary effect would take hold that quickly, Smith says an exciting aspect is "the possibility that mothers will pass on these behaviors to their offspring through processes like social learning."

Even if they don't, it's likely there will still be effects from the squirrels' foray into hunting and killing, both for squirrels and the animals that feed on them. Rodents are income breeders, meaning that the more they eat, the more offspring they'll have. So squirrels that eat their fill of protein could ultimately impact the food web by having additional babies during the next breeding cycle.

Fortunately, park visitors likely have little reason to fear a near future where protein-fueled super squirrels become oversized or expand their hunting options. Smith says the unprecedented speed with which squirrels switched to a carnivorous diet shows that they're "flexible and opportunistic in what they eat." That means the squirrels will likely revert back to consuming primarily seeds and whatever else in their usual diet is available at Briones Regional Park once the vole population stabilizes.

When it comes to squirrels switching to other small invertebrate prey, Smith says that "anything is possible" but describes it as "pretty unlikely."

Wild and Smith acknowledge that significantly more research is needed to answer questions about what the long-term effects could be, whether other squirrel populations may do something similar and how the change may impact other small mammals in the area. For Smith, who studied the hunting behavior of hyenas in Kenya, the findings are still hard to believe.

"I had no idea that I would ever bear witness to carnivory in ground squirrels," she says. "I was truly shocked by this behavior, and there are so many unanswered questions going forward."