Hot bother: ground squirrels taunt in infrared.

Ground squirrels are the first animals reported to broadcast an infrared signal, and the message seems to be "Nyah, nyah."

When adult California ground squirrels discover a lurking rattlesnake, they often harass it, says Aaron Rundus of the University of California, Davis. They dash into its striking range, kick sand at the snake, nip at its tail, and whip their own tails back and forth in a display called flagging (SN: 10/9/99, p. 237). A snake subjected to such vexations sometimes slithers off to lurk somewhere else.

Infrared videos show that ground squirrels' tails, which are generally cooler than the bodies, heat up during a bout of rattlesnake baiting, Rundus repotted in Oaxaca, Mexico, last week at the annual meeting of the Animal Behavior Society. Rattlesnakes have infrared sensors inside little pits below their eyes, and Rundus proposes that the tail's heat enhances the harassment display.

In contrast, the ground squirrel tails didn't warm appreciably during similar taunting of a gopher snake. This predator doesn't have infrared sensors.

"I don't recall ever hearing of anything like this, and it's fascinating," says snake specialist Harry Greene of Cornell University.

"I think it might be a way to divert the snake's attention so it strikes at the tail," suggests Gordon Burghardt of the University of Tennessee in Knoxville.

The ground squirrel attacks on rattlesnakes aren't as insane as they may seem at first glance. The ground squirrels have evolved blood proteins that partially neutralize rattlesnake venom, so an adult usually doesn't die from a bite. And although a snake strikes fast, a ground squirrel can move even faster. "Many, many times we've seen a squirrel dodge out of the way" Rundus says.

Ground squirrel pups, however, don't have enough of the protective blood proteins to neutralize the venom. An earlier study estimated that when pups are available, they can account for 69 percent of a rattlesnake's diet. They also can amount to nearly half of the diet of gopher snakes, which aren't poisonous.

In his study, Rundus videotaped 12 adult ground squirrels during with a series of 10-minute sessions with a caged snake in a laboratory setting. When the cage held a rattlesnake, the ground squirrel's tail temperature began to rise within a few minutes and increased 2[degrees]C at both the tip and base to reach, on average, 26[degrees]C. In the presence of a gopher snake, the squirrel's tail temperatures rose only 0.2[degrees]C at the base and 0.1[degrees]C at the tip.

"I was rather shocked to find they were discriminating between the snakes" says Rundus.

The ground squirrels probably raise their tail temperature by shunting more blood there, says Rundus. That's the same mechanism they use for dissipating excess heat.

It's likely that the rattlesnakes take note of the heat increase because they're "exquisitely sensitive," Rundus says. Some early experiments suggest their heat receptors can react to a temperature change of as little as 0.003[degrees]C.

To learn more about the infrared-emitting tail's effect on rattlesnakes, Rundus is finishing up production of a robotic squirrel that can heat its tail or not during a flagging display.