For California ground squirrels, survival against rattlesnakes often comes down to one basic question: Are you willing to "shake it?" By holding their tails upright and thrashing them from side to side, the animals notify predators that any attempt to turn them into a meal is likely to end in failure. Now researchers have discovered that this tail-waving behavior has a dual purpose. Not only does it ward off predators, but it also warns other squirrels of potential danger, forcing rattlesnakes to find new hunting grounds.
Pacific rattlesnakes (Crotalus oreganus oreganus) are patient hunters. They wait for hours in or around the burrows of California ground squirrels (Otospermophilus beecheyi). When an unsuspecting squirrel gets close enough, the snake delivers a venomous bite, releases the animal, and hunts for the dead body later. Scientists knew that some squirrels avoid attack by shaking their tails after seeing a snake, but sometimes squirrels did this even when they didn't detect a predator. Were they clued into some sort of alarm call or just waving at random?
Matthew Barbour and Rulon Clark decided to investigate things from a snake's perspective. Armed with snake tongs and bags, the San Diego State University ecologists trekked into the California wilderness and captured and anesthetized 22 rattlesnakes, surgically implanting them with small tracking devices. As soon as the snakes recovered, the duo released them back into the wild, keeping tabs on them with the tracking devices and security cameras set up around several squirrel burrows.
Barbour and Clark sifted through hours of footage, noting which squirrels got attacked, how far away they were from the snake, and whether they had been waving their tails. In the end the researchers found that that tail-wavers were able to get very close to rattlesnakes and still avoid being bitten. In fact, when squirrels that unknowingly stumbled into areas where there was a hidden snake became aware of the danger, they would stop, stare directly at the snake, and wave their tails fearlessly—even at distances of only 13 cm (see video). These squirrels didn't get attacked. Those that did not wave their tails, however, did. The researchers say the tail waving likely signals that the squirrels are ready to dodge out of the way, should the snake strike. In other words, snakes may think twice about attacking tail shaking squirrels because they will probably miss.
The team also noticed something else. Whenever a squirrel that encountered a snake began shaking its tail, nearby squirrels often followed suit, even if they didn't see the snake. And snakes would avoid them, too. The finding suggests that tail shaking in squirrels is an "honest" signal, says Barbour. Squirrels that begin thrashing their tails after seeing another squirrel do it have picked up on a true danger signal, he says. They're not just shaking their tails at random. Some researchers thought that the squirrels might be pretending to be vigilant, but now the animals "have been vindicated," says Barbour. "They weren't lying about being ready at all."
Rulon and Barbour have added "a really insightful study to an area of the field that needs a lot of attention," says Troy Murphy, a behavioral ecologist at Trinity University in San Antonio, Texas. Studies that follow natural predators are notoriously hard to do, adds William Cooper, a behavioral ecologist at Indiana University-Purdue University, Fort Wayne. "Rulon and Barbour's study is impressive for having overcome these problems."
*This item has been corrected on 11 July. The description of why tail shaking deters snakes has been changed to reflect that the shaking does not scare snakes but instead makes them think they are going to miss striking their prey.