As winter approaches in the Northern Hemisphere, people retreat indoors, and the pace of life seems to slow—but not for squirrels. Across forests, parks and your backyard, these animals go into overdrive, scurrying ceaselessly through the undergrowth and stuffing nuts and seeds into the soil.
Although it might look like a mad dash to survive the winter, the frantic vibe masks some meticulous preparation. A single squirrel can bury up to 3,000 nuts in a season in a process known as caching. It can store nuts across dozens of locations and even spatially organize them by type. What’s behind this obsessive pantry planning? Do squirrels just randomly retrieve whatever they sniff out, or do they actually remember where they place this precious stash?
A growing body of research suggests that they do remember. “They’re not just burying a bunch of stuff and hoping that they’ll find it in the future. They’re strategizing quite a lot,” says Lisa Leaver, a researcher who studies the behavioral and cognitive adaptations of squirrels and other animals at the University of Exeter in England.
In fact, squirrels take two methodical approaches to storing their food: larder hoarding, in which the fluffy-tailed rodents bury their entire bounty in one or two locations, and scatter hoarding, which involves the squirrels splitting a stash among multiple locations dotted across a landscape.
“In a squirrel’s mind, there are a lot of factors at play” in which method they choose, says Pizza Ka Yee Chow, who studies the evolution of cognition at the University of Chester in England. The foods’ location, availability and type, the squirrels’ local habitat and vulnerability to predators “and how many other buddies are around when they are doing the caching” all combine to steer them toward scattering or hoarding, Chow explains.
These two strategies exist along a continuum, and some squirrels go with the “mixed” method, where they will do both, Chow says. Usually different squirrel species will practice one or the other approach, however.
For instance, American red squirrels (Tamiasciurus hudsonicus) often depend on a small number of pine trees for their food, says Lucia Jacobs, a professor emerita of psychology and neuroscience at the University of California, Berkeley. They’ll gather pine cones and create a midden—a large pile of cones and scales left over from eating—typically at the base of a home tree. The convenient setup allows the animals to oversee and defend their bounty at close range, making larder hoarding worth their while.
Meanwhile the Eurasian red squirrel (Sciurus vulgaris), the fox squirrel (Sciurus niger) and the eastern gray squirrel (Sciurus carolinensis)—the most common backyard squirrel in the eastern U.S.—tend to favor scatter hoarding. Depending on where they live, these species rely on a range of food sources, including hickory nuts, walnuts, hazelnuts and acorns. This variety pushes these species to forage over a larger area, compared with the American red squirrel, which makes it difficult to closely guard a single large stash—and may explain why they scatter hoard. Although this strategy leaves more caches for pilferers to find, each cache’s smaller size eliminates the risk that the squirrels will lose their entire stash in one go.
Jacobs and her colleagues have also observed scatter-hoarding squirrels taking extra steps to protect their most coveted stash. Fox squirrels presented with almonds and peanuts will bury the almonds, which they prefer, farther away from the source and at lower densities than the peanuts, Jacobs says. “So the squirrel carries [a nut] a species-specific distance and caches it at a species-specific density.” These burial tactics help to throw off nut-snacking competitors. But do they also make it tough for the burier to keep tabs on all of its stash?
Not according to a few studies. In 1991, Jacobs and her team provided eight hand-raised gray squirrels with 10 nuts each to bury in the same enclosed area. When the researchers released each squirrel back into the area several days later, the animals “were retrieving twice as many of their own [nuts] as [those of] another squirrel’s cache,” Jacobs says. Interestingly, the squirrels also followed a different path when retrieving their nuts, compared with the one they’d taken to bury this food. “They could plan a trajectory through their 10 caches, which they could only do if they had a memory of where those caches were,” she says.
That study took place under highly controlled conditions, Jacobs cautions. But others have gone on to document squirrels’ impressive memory span. In a 2017 experiment, Chow gave lab-reared squirrels a task that required manipulating the right set of levers to release hazelnuts from a rectangular plexiglass puzzle box. Then, 22 months later, Chow presented them with another puzzle box that was triangularly shaped and featured different colors and a different lever layout to make it appear to the squirrels like a novel task. This task still required the same lever strategy to release the nuts as the previous one, however—and that’s the approach the squirrels applied. “The solution [the squirrels] used was the same as two years before,” Chow says. “That’s how we knew that they still remembered it.”
Meanwhile Jacobs’s lab has made some striking findings on squirrel brains. This research shows that while most small mammals experience brain shrinkage during the approach to winter, squirrels’ brain expands at this time, which may indicate a seasonal increase in cognitive load.
Others have uncovered clues about how squirrels might locate their hidden nuts. The fervent nut hunters do rely partially on their sense of smell to help them pinpoint their food, yet a 1986 study suggested that it’s a last resort: they first prioritize other tools such as visual and spatial cues to guide them to their stash. In fact, a 1997 study showed that gray squirrels adjusted where they dug for their buried nuts based on the relocation of flags that were originally planted beside the caches. That indicated that the squirrels were likely also using these spatial cues. Gray squirrels in the experiment could remember up to 24 cache locations for up to two months. More recently Chow has shown that lab-reared squirrels can use the relative position of nearby landmarks such as bushes and trees to guide them to their caches in an enclosed study area.
Spatial mapping would make sense in gray squirrels, Leaver says. The animals “have relatively small home ranges that they know inside [and] out. If you spent your whole life hiding bits of food that you relied on in your house, you would know where you’d put it,” she says.
Further research from Jacobs’s lab suggests that the fox squirrel’s tendency to carefully bury nuts of the same type close together may indicate an information-streamlining strategy called “chunking,” which humans also use. In squirrels, organizing nuts by type likely “reduces memory load and hence should increase accuracy of recall,” Jacobs explains.
She adds that some gray squirrels have a quirky habit of revisiting their burial sites, where they’ll paw through the overgrowth and then carefully rearrange the leaves. Sometimes squirrels will even excavate and then rebury their nuts. This strikes Jacobs as a kind of geographic revision: “It’s not like they cache in September and then they have to remember through till February,” she says. “They are out there every day rehearsing, rehearsing, rehearsing.”
And when they’re not refreshing their own memory, these crafty creatures continue working to throw others off their trail—with some surprisingly deceptive tactics, Chow says. “[Researcher] Mike Steele, he found that some squirrels do fake digging to protect their cache, but they don’t actually put any nuts in it,” Chow adds. “They trick others into thinking, ‘Hey, I put my nuts in here!’ just to distract them.”
There’s a lot still to learn about how these sharp-brained little rodents find and protect their food. Yet we can be sure that behind their seemingly scatterbrained fall behavior, there is some impressive mental arithmetic at play, even in the ubiquitous urban gray squirrel. “Because it’s such a common urban species, everyone thinks, ‘Oh, that’s just a squirrel,’” Jacobs says. “But it’s actually a very unique animal.”