Squirrels Ward Off Snakes With Hot Tails

This is a robotic squirrel. Courtesy of Aaron Rundus/PNAS

Thursday, August 23rd, 2007--

Coyotes, raptors, and snakes all want to eat California ground squirrels (Spermophilus beecheyi). When attacked by avian and mammalian predators, ground squirrels typically sound a vocal alarm. In the presence of snakes, however, squirrels exhibit a behavior called tail flagging, which includes "bushing up their tail fur, picking their tail off the ground and waving it from side to side," explains Aaron Rundus, a post-doctoral researcher at the University of Nebraska-Lincoln and an author on a study on tail flagging published in the Proceedings of the National Academy of Sciences.

Rundus and his colleagues examined tail flagging in response to rattlesnakes (Crotalus oreganus) and gopher snakes (Pituophis melanoleucus). Previous studies had shown that ground squirrels sometimes tail flag more vigorously in the dark--a curious finding given that a visual cue like tail flagging would ostensibly be less effective in the dark.

With this in mind, the researchers watched the squirrels interact with the snakes through an infrared camera, which senses heat. The researchers discovered that when confronted by a rattlesnake, California ground squirrels not only wave their tales, but heat them as well. Rundus says, "They are actually increasing their tail temperature, or increasing the amount of infrared radiation being emitted from their tail when they encounter rattlesnakes." This is the first time that researchers have seen any animal use infrared radiation as a communicative signal, Rundus says. LISTEN TO RUNDUS>>(mp3)

But ground squirrels don't heat their tails for all snakes. While the squirrels heat up for rattlesnakes, they keep their tails cool when flagging gopher snakes. The difference in signaling is likely related to the fact that rattlesnakes can sense infrared radiation, while gopher snakes cannot. Rattlesnakes are equipped with pit organs--small cavities filled with heat-sensitive nerves that allow them to sense infrared radiation.LISTEN TO RUNDUS>>(mp3) "It would appear that these squirrels are producing a signal that capitalizes on this sensory bias that the rattlesnakes possess," Rundus says.

But what does a hot tail mean to a rattlesnake? To find out, the researchers built a biorobotic California ground squirrel. The robotic ground squirrel could be programmed to heat its tail or not, giving the researchers a chance to see how the rattlesnakes changed their response to hot versus cool tails.

The experiment was contingent on fooling the rattlesnakes into believing the biorobotic squirrel was real. "We used an actual squirrel taxidermy model ...so the exterior is a real squirrel. Then we stuffed the squirrel with components in order to get the squirrel to move the way we wanted it to move," says one of the robo-squirrel's designers, Sanjay Joshi, a professor in the department of mechanical and aeronautical engineering at the University of California, Davis. The biorobotic squirrel was also stored in real squirrel-bedding so that it would smell like a real squirrel. Key to making this robot, and any robot that is designed to interact with another (living) species, is figuring out "what makes a robotic animal believable to another animal," Joshi says. LISTEN TO JOSHI>>(mp3) In this case, the snakes were fooled by the robotic squirrel.

The biorobot was designed to react autonomously to the snake, varying its tail flagging depending on the distance of the snake from the robot. LISTEN TO JOSHI>>(mp3) When confronted with a hot-tailed robo-squirrel, the snake backed-off. Rundus explains: "We found that rattlesnakes shift from predatory mode to more defensive behavior when they see tail flagging, but when that infrared component is included in the tail flagging that shift is even greater. They become much more defensive than when it's not included."

Why would rattlesnakes be cowed by a heated tail? Rundus says that adult ground squirrels are "pretty evenly matched adversaries to these rattlesnake and gopher snake predators." The rattlesnakes are really after squirrel pups. If an adult squirrel is tail flagging, it means the rattlesnake has been sighted. And squirrels don't stop with tail flagging: they will also kick pebbles and dirt in the snake's direction, and even take a swipe at a snake, Rundus says. Adult squirrels can afford to be fearless because they have proteins in their blood that will neutralize rattlesnake venom. Rundus says: "The tail flagging probably signals to these rattlesnakes that a) they've been discovered and b) they're going to be harassed not only by that squirrel but by other squirrels in the area, so it's probably a good decision to leave or take refuge." LISTEN TO RUNDUS>>(mp3)

-Flora Lichtman

What did you think of the story? Send us some feedback.

--Flora Lichtman