Squirrels Outwit Rattlesnakes in Infrared

Aug. 13, 2007 — Like Wile E. Coyote's hapless quest to catch the Roadrunner, rattlesnakes have attempted to catch California ground squirrels for well over a million years, and now scientists know one reason why the squirrels usually win.

Their tails freak out the snakes, researchers report in this week's Proceedings of the National Academy of Sciences.

The secret is the squirrel's ability to heat up its tail before flicking it in admonishment at a threatening rattlesnake, the researchers believe. For the first time, the scientists captured the phenomenon using a special infrared camera.

Lead author Aaron Rundus, who conducted the research while he was a member of the Animal Behavior Graduate Group at the University of California, Davis, explained to Discovery News that rattlesnakes have pit organs that are ultra-sensitive to infrared. This form of light consists of electromagnetic waves, just below the range of visible light, that correspond to radiated heat.

"When combined with other squirrel harassment activities, the squirrel tail flagging puts rattlesnakes on the defensive," Rundus said.

Squirrels will chatter with their big teeth and even "throw sand and loose pebbles at the snakes," he said. At the same time, the squirrels wave hot, glowing tails puffed out to make themselves look enormous to the snakes.

Since squirrels can neutralize rattlesnake venom, such battles usually occur between mother squirrels and rattlesnakes targeting pups, which have not yet developed the immunity.

At first, Rundus and his team were not sure if the hot squirrel tail was simply due to squirrels becoming physiologically aroused in the face of a threat.

To test this possibility, the scientists filmed 12 mother squirrels defending their pups against non-venomous gopher snakes. During these conflicts, the mother squirrels rarely emitted infrared.

"While adult female squirrels facing both rattlesnakes and gopher snakes bushed out their tails and flagged them in a similar manner, we now know that the tail's infrared serves as a targeted defense against rattlesnakes," Rundus said.

He and his team also created a lifelike, odiferous squirrel robot, which they presented to 14 adult northern Pacific rattlesnakes. The snakes were placed in a testing chamber containing a simulated squirrel burrow that contained a warm (but dead) rat pup.

When the rattlesnakes slithered toward the dead rat, out would come the squirrel robot, wagging its tail. Wags combined with infrared were much more effective at warding off the snakes.

Owen Maercks, a snake expert and co-owner of the East Bay Vivarium in California, told Discovery News that "while we tend to think of rattlesnakes as perfect predators, squirrels must have evolved infrared as a counter-offensive against the snake's infrared-sensing tactics."

"Snakes also are not bright animals," Maercks said. "By contrast, squirrels are strategists. I watch them chatter, flick their tails and show their butts to my three little dogs in our backyard."

"The squirrels taunt the heck out of potential predators," he added. "It even seems like they enjoy outwitting them."

Rundus and his colleagues don't yet know the mechanism that allows the squirrels to emit the infrared. He and his colleagues think warm body core blood moves to the tail region, but further testing is needed to confirm that speculation.

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