It might sound nuts. But the squirrels are definitely listening.
For years, researchers have known that squirrels can eavesdrop on other species’ alarm calls to cue into dangerous situations that they might not notice themselves. Now, a team of scientists has uncovered another facet of these rodents’ nosy nature: To figure out when a threat has passed, squirrels may also snoop on the casual conversation of birds—a clear sign that things are, once again, all good in the wooded neighborhood.
The study, published today in the journal PLOS ONE, hints that, even for wildlife, the language of safety operates on something of a sliding scale. Rather than relying on the mere absence of an alarm, squirrels will prick their ears for the presence of another signal to confirm the coast is clear—even if that means dissecting the dialect of another species.
This rudimentary multilingualism is a testament to squirrels’ cognition, which is “much more complex than people give them credit for,” says Suzanne MacDonald, who studies animal behavior at York University in Canada, but was not involved in the study. “Safety is sometimes seen as the default: When you don’t hear this sound, it’s safe. But this takes it a step further...and that makes it much more interesting.”
Most humans may not care for it (anymore), but squirrel meat has lost none of its allure with the rest of the animal kingdom. Whether they’re traversing the trees or skittering across manicured lawns, these rodents have plenty of predators to fear—which means they need all the help they can get to avoid becoming lunch.
So perhaps it’s no surprise that they’ve learned the lingo of certain birds, which will squawk, caw, chirp, or even shriek out a warning after they sense a predator on the prowl. Though these alarm calls are probably intended for the birds’ feathered kin, keen-eared squirrels have been caught tuning in as well.
But a few years ago, Oberlin College behavioral ecologist Keith Tarvin started to wonder if alarm calls were only half of the squirrel eavesdropping equation. “There’s a lot of information in alarm calls,” he says. “It dawned on us that cues of safety might be equally informative.”
When a squirrel has reason to believe a crisis is afoot, it’ll drop whatever it’s doing and reallocate all its energy into a set of vigilance behaviors, like rearing up on its hind legs to scan its surroundings, freezing in place, or fleeing the danger zone entirely. But staying in this stressful state is exhausting, and saps time and energy from other essential tasks like foraging for food or wooing a potential mate. That means having an obvious off switch for predator surveillance could be as important as the trigger that turns it on.
To explore this side of the story, Tarvin and his students, Marie Lilly and Emma Lucore, both Oberlin undergraduates at the time, decided to scare the crap out of a scurry of eastern gray squirrels (Sciurus carolinensis)—and then try to calm them back down. On mild winter days, Lilly would hoist a set of speakers onto her bicycle and search out squirrels near the university’s campus. Once she found a potential candidate, she’d play it a brief recording of the cries of a red-tailed hawk (Buteo jamaicensis)—a known squirrel predator—and wait for the rodent to react.
Unsurprisingly, the squirrels went on high alert the moment they heard the fearsome bird’s calls, exhibiting the classic suite of vigilance behaviors. Then, as the rodents began to slowly eke their way back to baseline, Lilly played some of them another track: the chatter of several species of songbirds, gabbing away as if all was hunky dory.
These sounds are distinct from cries known as sentinel or “all clear” calls, vocalizations that evolved specifically to communicate that a known threat has passed, Tarvin says. Chatter, on the other hand, is more like blase bird babble that probably wouldn’t be happening if danger was nigh.
Hearing this soothing soundtrack seemed to speed up the squirrels’ return to business as usual, compared to a second group that had been played a recording of only background noise, sans bird chatter. Both sets of rodents were probably ultimately headed toward the same fate, Tarvin says. But in the aftermath of panic, bird chatter might be a special something that puts a nervous squirrel at ease.
“This really adds something to the [scientific] literature,” MacDonald says. “Not only does [bird chatter] say there’s no predator...it tells the squirrels, ‘Now, you can really relax.’”
Of course, the timing of these events is probably a lot more variable under natural circumstances. After all, squirrels aren’t always in the vicinity of birds shooting the breeze. But ambient chatter is “free, public information” that anything within earshot could potentially take advantage of, Tarvin says. In the wild, it’s the closest squirrels can get to wiretapping their loud-mouthed feathered friends.
“Squirrels are incredibly successful,” says Amanda Robin, a squirrel researcher at the University of California, Los Angeles who was not involved in the study. “This could be one more example of how they’re able to take advantage of things that are in their environment.”
It’s still not clear how squirrels learn to tune in to avian chitchat. But the calls played in the study spanned about a dozen different birds, hinting at the possibility that these rodents have the cognitive flexibility to decipher similar signals from a wide range of species (including, perhaps, some of their noisiest neighbors: humans).
“This highlights how interconnected ecosystems are,” Robin says. “If you removed one species [like a bird], you might be changing the entire life of another species and not know.”
Understanding these subtle ecological networks will be essential for future conservation efforts, Lilly says. With noise pollution levels on the rise, we humans may already be covering up communication in the environment that we’re not even aware of yet.
MacDonald agrees. “Animals aren’t just paying attention to other squirrels—they’re paying attention to everything in the environment,” she says. “That’s why biodiversity is so important.”