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Rattlesnakes get the message from tail-waving squirrels

Zachary Cavas via Royal Society

This ground squirrel didn't make it. The tail of the prey hangs out of a rattlesnake's mouth.

When ground squirrels wave their tails back and forth, it's apparently meant to send a message to rattlesnakes not to mess around with them — and a newly published study suggests that the snakes are getting the message.

The "tail-flagging" study, appearing in the Proceedings of the Royal Society B, was aimed at understanding how predators and prey communicate with each other. Matthew Barbour, a biologist at the University of British Columbia, said previous studies have shown that the squirrels wave their tails when they're in the presence of snakes, their natural predators, but also when there are no snakes around. Does the tail-waving really make a difference?

"We wanted to see whether this behavior influences how the rattlesnakes hunt," Barbour told me.

So, Barbour and a colleague at San Diego State University, biology professor Rulon Clark, went out to a couple of field sites in California's Alameda County and collected 22 rattlesnakes. The researchers hooked up the rattlers with radio transmitters, then tracked them as they made their way through areas frequented by ground squirrels.

Barbour and Clark observed dozens of squirrel-snake encounters, involving 14 different snakes. One side of the experiment focused on the snakes. "Snakes only strike at squirrels that are tail-flagging if they get really close," Barbour said. If a tail-flagging squirrel is more than 20 centimeters (8 inches) away, a rattlesnake will hardly ever attack. Even when snakes decided to lunge at a tail-flagging squirrel at close range, all but one of the squirrels got away.

In contrast, Barbour said that "if the squirrel's not tail-flagging, snakes strike more than 90 percent of the time," even if that prey is at the very edge of the snake's strike range (31 centimeters, or 1 foot). Squirrels who didn't wave their tails survived a strike only about half the time (seven out of 13 strikes).

The other side of the experiment had to do with whether the squirrels tried dodging the snakes during an attack. In all five cases where squirrels waved their tails, they dodged. In comparison, the squirrels who didn't wave their tails dodged in only five of the 12 cases observed.

This video shows a non-tail-waving ground squirrel pup being bitten by a rattlesnake. The snake drags the squirrel into the burrow, releases it, and later emerges to look for the envenomated prey using chemosensory searching. Meanwhile, a tail-waving adult squirrel investigates.

"We essentially provided the first field evidence supporting the hypothesis that tail-flagging honestly advertises a squirrel's vigilance toward snakes," Barbour said.

Barbour said tail-flagging tells the snake that this particular squirrel is on guard and ready to dodge the blow. It also alerts other squirrels in the area that danger is near, further reducing the snake's chances of making a kill. That apparently persuades the snake to look elsewhere for its next meal — for instance, amid a gaggle of squirrel pups that haven't yet perfected their tail-wagging, snake-dodging tricks.

"Our study serves as a demonstration that a single anti-predator signal, even when displayed frequently in the absence of a predator, can still influence predator behavior through multiple mechanisms," Barbour and Clark write.

Barbour said the study could lead researchers to take a closer look at the evolutionary dynamics behind predator-prey signaling — not only squirrels vs. snakes, but also lizards vs. snakes or gazelles vs. cheetahs. Hey, maybe animals are smarter than we thought. But you knew that already, right?

More about squirrels and snakes:

The researchers say that "all methods adhered to were approved by the Institutional Animal Care and Use Committee at San Diego State University." The research was funded by an Animal Behavior Society Student Research grant, American Museum of Natural History Theodore Roosevelt Grant and SDSU's Mabel Myers Memorial Scholarship, provided to Barbour; and grants from the National Geographic Society and the National Science Foundation, provided to Clark.

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.