Hot Squirrels

The squirrel on top is in the presence of a rattlesnake. The one at the bottom is still "tail flagging," but in the presence of a gopher snake. image: PNAS

Graduate student Aaron Rundus and his colleagues at UC Davis exposed California ground squirrels to rattlesnakes in the lab. When they viewed the interaction through an infrared video camera, they saw that the squirrels' tails heated up like a red-hot poker. But if they put a gopher snake, which cannot sense infrared radiation, in with the squirrels, the squirrels' tails remained dark and cold while they waved them. But questions still remained.

"Once we found out they produced this infrared signal, the next logical question was to ask how this functions," says Rundus, who is now doing a postdoc at University of Nebraska at Lincoln.

In other words, does this added component really have a significant effect on rattlesnakes beyond the tail flagging?

Rundus says he considered several ways to test that, like by covering the tail with something that would block the radiation from reaching the snake, or by administering a drug that could prevent the squirrel from thermo-regulating its tail. In the end he decided that the best way to control for other variables was to use a robotic squirrel with a tail that could be heated, or not.

As Rundus and his colleagues wrote in Proceedings of the National Academy of Sciences, the rattlesnakes were much more afraid of the robotic squirrel when its tail was heated.

Rundus considers this a marvelous story of evolution, where a special ability in rattlesnakes (sensing infrared radiation) triggered evolutionary changes in squirrels that they employ against a specific predator.

Squirrels probably had some ability to heat up their tails long ago, but it might not have been fully employable. But over time, it became the kind of ability that would better maintain the species through generations.

"Squirrels probably tail flagged without this component first," Rundus says. "But the groundwork was there for them to increase the temperature, and over time squirrels' ability to do it was probably selected for, and probably increased in frequency and power throughout the population."

The rattlesnake is more afraid of the robot squirrel (lower left) when its tail is heated. image: Aaron Rundus

"One of the most interesting things is the reminder that when we look at animals and understand how they work, we need to take into account the perceptual working domains of the animals, and not be constrained by our own human perceptual biases," he says. "When you do that, you can find some really interesting phenomena."