Even Ground Squirrels Got In on the Vole Feast Last Summer
By last summer, University of Wisconsin–Eau Claire professor Jennifer Elaine Smith had been studying California ground squirrels at Briones Regional Park for twelve years. There wasn’t much these rodents could do that could surprise her. Then her team saw a ground squirrel stalk, hunt, and eat a California vole. It wasn’t a fluke, like some weirdly motivated or superintelligent squirrel. Because, as the researchers found, the squirrels kept doing it. Again and again. They weren’t sit-and-wait-type predators, but instead chased down the voles over short stretches of dirt. The research team documented 27 individual squirrels hunting voles that summer. “I could barely believe my eyes,” says Sonja Wild, a postdoctoral research fellow in the UC Davis Environmental Science and Policy department who co-authored a paper in the Journal of Ethology on the unusual phenomenon. “From then, we saw that behavior almost every day. Once we started looking, we saw it everywhere.” A California ground squirrel on the move with its unusual prey: a California vole. Normally, ground squirrels eat a mostly plant-forward diet. (Sonja Wild/UC Davis)It was easy to see what was triggering it: there were just so many voles around. “This was shocking,” says Smith, a University of Wisconsin-Eau Claire professor who studies social mammals and lead-authored the paper, which was published in December last year. “We had never seen this behavior before.” California ground squirrels (Otospermophilus beecheyi), on most days, have a plant-forward diet. They have also been known to eat meat such as bird eggs, hatchlings, insects, or each other on occasion—but this is the first time in nature that they had ever been documented hunting and eating California voles. “The widespread nature of vole hunting in our population fundamentally changes our understanding of this primarily granivorous species, suggesting that they are considerably more flexible in their diet than previously assumed,” the researchers wrote. California voles (Microtus californicus) are a burrowing rodent species that range from southern Oregon down to Baja California—sometimes living (dangerously?) in ground squirrel burrows. They are ubiquitous, but since they live underground, I usually only see a handful of these rodents every year. That all changed last year. Female voles can have back-to-back litters—every 21 days—if conditions are right. Just imagine. (Vishal Subramanyan)A heck of a lot of voles In May, I was hiking in Sycamore Grove Park, a regional preserve in Livermore that I’ve been visiting for over seven years. As a wildlife photographer, I spend a lot of time in nature: being still and quiet, watching for animals. This time, from the start, I saw dozens of these tiny rodents running all over the trails. I’d only seen a couple of voles in this park over the years. I saw more of them in a few minutes than I had over the past several years. Throughout the course of my hike, I counted over 100 voles. It was a photographer’s dream. I hunched down and took dozens of photos as the voles scurried through fields, climbed on stalks, and ran in and out of their burrows. It appeared Northern California was in the midst of a vole population boom. Reports emerged of huge surges in their numbers, from San Francisco to Pleasanton to the El Dorado Hills, east of Sacramento. Smith’s team, crunching numbers from the community science platform iNaturalist, reported people logged seven times as many vole sightings in California as the average over the past decade. Livermore, like Briones Regional Park, was crawling with California voles last summer. (Vishal Subramanyan)Booms like this have occurred in the past. Just like their more famous cousins the lemmings, vole populations sometimes just go through the roof—reaching densities of up to 5,000 animals per acre. To humans, these booms may seem random. Vole populations typically cycle up and down over periods of three or four years, Smith says, but this was the biggest boom she saw over twelve years of study. One thing that’s clear: Peak Vole is achieved by female voles reproducing at much higher rates than usual, according to Phoebe Edwards. She studied meadow vole population cycles for her Ph.D. thesis and is now an assistant professor of ecology, evolution, and organismal biology at Iowa State University. “As they’re increasing from a low population density, the females that are sexually mature are having lots of litters rapidly, back to back,” Edwards says. “They can even become pregnant once they’ve just given birth, and not all mammals can do that.” Voles can give birth to new litters every 21 days, she says. At the boom’s peak, birth rates slow. What sets off such industrious behavior? Generally, Edwards says, it’s because an opportunity has arisen: there’s more food around (possibly because of the climate changing), or fewer predators, or “changes to landscape use where voles are colonizing new kinds of habitats that weren’t really suited to them before,” said Dr. Edwards. Everybody likes eating voles The ground squirrels, like many, took advantage of the situation. Over the summer of 2024, researchers observed them hunting voles on 74 occasions over just 18 days of fieldwork. Of these, 31 involved active hunting, with squirrels stalking through tall grass or chasing voles across open dirt. And the hunters were quite successful—17 of the 31 documented attempts (55 percent) resulted in a kill. Sometimes, squirrels tolerated other squirrels grabbing their killed voles. But occasionally the researchers saw squirrels fighting over their prizes. That made sense, they wrote, because “the energy contained in a single vole far outweighs that of more common food items, such as seeds or grasses.” Population booms of small mammals like voles impact whole ecosystems, affecting predators and other animals. A slew of animals prey on voles, as Smith and team noted in their paper—“hawks, owls, egrets, long-tailed weasels, coyotes, skunks, mountain lions, and garter snakes”—all of which likely had more to eat. Burrowing rodents like voles are often ecosystem engineers, too, creating tunnels that other animals use. So more voles could also mean more habitat for those species. But these booms don’t last forever—so as vole populations crash, predators may be once again forced to turn to other prey, and small animals will have fewer places to live. While the vole boom was a boon for animals with a taste for rodents, it touched human lives a bit differently. Grape grower Dane Stark, who runs Page Mill Winery in Livermore, noticed one summer day that some unknown vandal had nibbled a ring out of the bark on many of his youngest vines. He waited and watched, and quickly learned that the culprits were voles. They got to nearly all his vines. “I’ve been growing grapes for twenty years, and this is the first time I’ve ever noticed something like this,” Stark says. He hoped that the surge in vole numbers would bring in more predators to help control their exploding populations. Researchers documented last summer’s sharp spike in iNaturalist observations of California voles in their paper in the Journal of Ethology. (Courtesy of the authors)Have we passed Peak Vole? It’s hard to know when or if the vole population boom is over. It would likely require an intensive field survey to get an accurate idea of their numbers. However, on my recent hikes this winter, I’ve observed far fewer voles compared to last summer. Community science reports on platforms like iNaturalist, which were essential in recording the vole boom last year, may also help understand the timing of the boom’s end. Bobcats were among those that likely cashed in on a surfeit of voles last summer, along with “hawks, owls, egrets, long-tailed weasels, coyotes, skunks, mountain lions, and garter snakes,” according to researchers. (Vishal Subramanyan)The boom also raises other ecological questions, such as whether California ground squirrels learn hunting strategies socially or if it is a genetic predisposition. Wild and Smith are also interested in disease implications of the novel squirrel–vole interaction. “Parasites might be shared between voles and squirrels,” says Smith. “Future research will reveal the extent to which these interactions have positive or potentially negative consequences for ground squirrel populations.” I’ll remember it fondly, as a wildlife photographer, given the abundance of photo opportunities the voles gave me. One evening, at a local park in Fremont. Down on the ground, voles were scampering across the fields. I watched as a bobcat quietly stalked prey alongside the trail. After patiently waiting for a few minutes, the bobcat pounced, grabbing one of the many voles that scattered these fields. It immediately took the vole and started trotting towards the cover, disappearing over the ridge as the sun set. In a prey boom, the mandate is the same for photographers as for bobcats: strike while it’s hot. VIDEO A video compilation of ground squirrels hunting. Note: this contains some graphic imagery. (Sonja Wild, UC Davis)
For the first time, scientists documented concerted carnivory by California ground squirrels. But why were there so many voles? The post Even Ground Squirrels Got In on the Vole Feast Last Summer appeared first on Bay Nature.
By last summer, University of Wisconsin–Eau Claire professor Jennifer Elaine Smith had been studying California ground squirrels at Briones Regional Park for twelve years. There wasn’t much these rodents could do that could surprise her.
Then her team saw a ground squirrel stalk, hunt, and eat a California vole.
It wasn’t a fluke, like some weirdly motivated or superintelligent squirrel. Because, as the researchers found, the squirrels kept doing it. Again and again. They weren’t sit-and-wait-type predators, but instead chased down the voles over short stretches of dirt. The research team documented 27 individual squirrels hunting voles that summer. “I could barely believe my eyes,” says Sonja Wild, a postdoctoral research fellow in the UC Davis Environmental Science and Policy department who co-authored a paper in the Journal of Ethology on the unusual phenomenon. “From then, we saw that behavior almost every day. Once we started looking, we saw it everywhere.”
It was easy to see what was triggering it: there were just so many voles around. “This was shocking,” says Smith, a University of Wisconsin-Eau Claire professor who studies social mammals and lead-authored the paper, which was published in December last year. “We had never seen this behavior before.”
California ground squirrels (Otospermophilus beecheyi), on most days, have a plant-forward diet. They have also been known to eat meat such as bird eggs, hatchlings, insects, or each other on occasion—but this is the first time in nature that they had ever been documented hunting and eating California voles. “The widespread nature of vole hunting in our population fundamentally changes our understanding of this primarily granivorous species, suggesting that they are considerably more flexible in their diet than previously assumed,” the researchers wrote.
California voles (Microtus californicus) are a burrowing rodent species that range from southern Oregon down to Baja California—sometimes living (dangerously?) in ground squirrel burrows. They are ubiquitous, but since they live underground, I usually only see a handful of these rodents every year.
That all changed last year.
A heck of a lot of voles
In May, I was hiking in Sycamore Grove Park, a regional preserve in Livermore that I’ve been visiting for over seven years. As a wildlife photographer, I spend a lot of time in nature: being still and quiet, watching for animals. This time, from the start, I saw dozens of these tiny rodents running all over the trails. I’d only seen a couple of voles in this park over the years. I saw more of them in a few minutes than I had over the past several years. Throughout the course of my hike, I counted over 100 voles.
It was a photographer’s dream. I hunched down and took dozens of photos as the voles scurried through fields, climbed on stalks, and ran in and out of their burrows. It appeared Northern California was in the midst of a vole population boom. Reports emerged of huge surges in their numbers, from San Francisco to Pleasanton to the El Dorado Hills, east of Sacramento. Smith’s team, crunching numbers from the community science platform iNaturalist, reported people logged seven times as many vole sightings in California as the average over the past decade.
Booms like this have occurred in the past. Just like their more famous cousins the lemmings, vole populations sometimes just go through the roof—reaching densities of up to 5,000 animals per acre. To humans, these booms may seem random. Vole populations typically cycle up and down over periods of three or four years, Smith says, but this was the biggest boom she saw over twelve years of study.
One thing that’s clear: Peak Vole is achieved by female voles reproducing at much higher rates than usual, according to Phoebe Edwards. She studied meadow vole population cycles for her Ph.D. thesis and is now an assistant professor of ecology, evolution, and organismal biology at Iowa State University. “As they’re increasing from a low population density, the females that are sexually mature are having lots of litters rapidly, back to back,” Edwards says. “They can even become pregnant once they’ve just given birth, and not all mammals can do that.” Voles can give birth to new litters every 21 days, she says. At the boom’s peak, birth rates slow.
What sets off such industrious behavior? Generally, Edwards says, it’s because an opportunity has arisen: there’s more food around (possibly because of the climate changing), or fewer predators, or “changes to landscape use where voles are colonizing new kinds of habitats that weren’t really suited to them before,” said Dr. Edwards.
Everybody likes eating voles
The ground squirrels, like many, took advantage of the situation. Over the summer of 2024, researchers observed them hunting voles on 74 occasions over just 18 days of fieldwork. Of these, 31 involved active hunting, with squirrels stalking through tall grass or chasing voles across open dirt. And the hunters were quite successful—17 of the 31 documented attempts (55 percent) resulted in a kill.
Sometimes, squirrels tolerated other squirrels grabbing their killed voles. But occasionally the researchers saw squirrels fighting over their prizes. That made sense, they wrote, because “the energy contained in a single vole far outweighs that of more common food items, such as seeds or grasses.”
Population booms of small mammals like voles impact whole ecosystems, affecting predators and other animals. A slew of animals prey on voles, as Smith and team noted in their paper—“hawks, owls, egrets, long-tailed weasels, coyotes, skunks, mountain lions, and garter snakes”—all of which likely had more to eat. Burrowing rodents like voles are often ecosystem engineers, too, creating tunnels that other animals use. So more voles could also mean more habitat for those species. But these booms don’t last forever—so as vole populations crash, predators may be once again forced to turn to other prey, and small animals will have fewer places to live.
While the vole boom was a boon for animals with a taste for rodents, it touched human lives a bit differently. Grape grower Dane Stark, who runs Page Mill Winery in Livermore, noticed one summer day that some unknown vandal had nibbled a ring out of the bark on many of his youngest vines. He waited and watched, and quickly learned that the culprits were voles. They got to nearly all his vines. “I’ve been growing grapes for twenty years, and this is the first time I’ve ever noticed something like this,” Stark says. He hoped that the surge in vole numbers would bring in more predators to help control their exploding populations.
Have we passed Peak Vole?
It’s hard to know when or if the vole population boom is over. It would likely require an intensive field survey to get an accurate idea of their numbers. However, on my recent hikes this winter, I’ve observed far fewer voles compared to last summer. Community science reports on platforms like iNaturalist, which were essential in recording the vole boom last year, may also help understand the timing of the boom’s end.
The boom also raises other ecological questions, such as whether California ground squirrels learn hunting strategies socially or if it is a genetic predisposition. Wild and Smith are also interested in disease implications of the novel squirrel–vole interaction. “Parasites might be shared between voles and squirrels,” says Smith. “Future research will reveal the extent to which these interactions have positive or potentially negative consequences for ground squirrel populations.”
I’ll remember it fondly, as a wildlife photographer, given the abundance of photo opportunities the voles gave me. One evening, at a local park in Fremont. Down on the ground, voles were scampering across the fields. I watched as a bobcat quietly stalked prey alongside the trail. After patiently waiting for a few minutes, the bobcat pounced, grabbing one of the many voles that scattered these fields. It immediately took the vole and started trotting towards the cover, disappearing over the ridge as the sun set. In a prey boom, the mandate is the same for photographers as for bobcats: strike while it’s hot.
A video compilation of ground squirrels hunting. Note: this contains some graphic imagery. (Sonja Wild, UC Davis)