SACRAMENTO BEE (California) 14 August 07 Squirrels put the heat on snakes, study finds (Carrie Peyton Dahlberg)
When it comes to looking like the biggest, meanest squirrel mom around, California ground squirrels have a special weapon in their arsenal: hot tails.
Squirrels heat their tails an extra 3 degrees when trying to chase off rattlesnakes, which perceive infrared, UC Davis researchers have discovered. Indicating they can tell one snake species from another, the squirrels don't apply the same heat for gopher snakes, which don't have thermal sensors.
Making the case for infrared communication even stronger, rattlers appear to get the message. The snakes are more cautious around specially built "robo squirrels" with warm tails than around the same furry robots with cooler tails.
"I was definitely surprised that we got a difference between gopher snakes and rattlesnakes," said Aaron Rundus, who did his doctoral research on squirrels' use of infrared.
His findings, published online this week by the Proceedings of the National Academy of Sciences, earned Rundus an award for best student paper from the Animal Behavior Society.
"It's a very exciting finding," said John Hoogland, a University of Maryland behavioral ecologist who was not connected with the study. "The infrared really turns your head."
The study highlights the novel and creative ways animals try to outwit each other, often in realms beyond human senses.
"When we look hard enough and are able to put away our biases, we just find the most incredible things about animals," said Hoogland, whose own research focuses on prairie dogs, another type of ground squirrel.
In the long evolutionary battle between squirrels and snakes, California ground squirrels have toughened up plenty.
Adult squirrels have developed an immunity to rattlesnake venom, so a snake cannot poison them for easy dining. Since vulnerable squirrel pups remain a potential snake dinner, adult squirrels, particularly mothers, have emerged as defenders of the burrow.
In that role, squirrels will fluff their hair, wave their tails rapidly, kick dirt or small stones at a snake and bite if given a chance. They'll make little forays to provoke telltale rattling, giving them clues to size and warmth, which indicate the relative ferocity of their foe.
"You have almost equally matched adversaries in snakes and squirrels," said Rundus.
He began to wonder if this standoff might include a weapon squirrels could aim at rattlers' distinctive sensory organ, a pit organ lined with tens of thousands of thermoreceptors, probably used to locate warm prey.
Using a dozen female ground squirrels captured near Winters, he filmed them with an infrared imaging video camera as they were exposed to rattlesnakes, gopher snakes and other squirrels, as well as when they were left alone.
"It was quite an initiative on his part," said Donald Owings, a UC Davis psychology and animal behavior professor who is among five co-authors of the new paper. "I thought it was a very interesting idea, but I thought there was a limited chance he would find something."
Owings figured that even if his graduate student found no thermal effect, the films would still be a rich source of data to be mined for other aspects of squirrel-snake encounters.
Rundus didn't need that fallback strategy. As the films were analyzed, he saw similar behaviors -- tail waving, dirt throwing -- when squirrels encountered either species of snake. Torso temperature stayed about the same, too. Only the tails differed, with average surface temperatures of 78.4 degrees for rattlers, compared with 75.1 degrees for gopher snakes. In the most dramatic test, a squirrel's tail measured 87 degrees at its base during a rattlesnake confrontation.
No one is sure exactly how the squirrels do it, but the most likely explanation is that they shunt more blood to their tails to warm them up, said Rundus, who is now doing postdoctoral research at the University of Nebraska.
The hotter tails were interesting, but communication is two-way. It's not enough to just say "keep away" with a warm tail. Whatever is watching has to understand the message.
Researchers did a second batch of testing to show that rattlesnakes, apparently, understand warm tail talk just fine.
Rundus and UC Davis engineering professor Sanjay Joshi, a robotics specialist, devised the robo squirrels, which could wave their tails menacingly with or without extra heat.
Higher temperatures produced more coiling, more rattling and other defensive behavior.
"Everyone I've talked to is kind of blown away by the discovery," said Owings.
Among those intrigued is Daniel Blumstein, a UCLA professor who studies the evolution of communication and anti-predator behavior. He wrote a commentary that will accompany the printed version of the squirrel paper, saying the work should encourage researchers to look more broadly for ways that prey animals aim messages to specific predators.
"It's one of these papers that makes us look elsewhere for similar sorts of things, and take a step back and appreciate the diversity of life," Blumstein said in a phone interview. "There's a lot of interesting stuff going on out there that we're blind to."
Squirrels put the heat on snakes, study finds

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