Viper heat sensors locate cool spots. (Snake Pits).

Researchers who glued minuscule plastic balls onto the faces of live rattlesnakes say the project has revealed the first experimental evidence of an overlooked role for the viper's heat-sensing organs. The newly tested function: finding places for the desert snake to hide from the scorching sun.

Rattlesnakes can sense heat via special receptors sunk in two tiny pits on their faces, explains Aaron Krochmal of Indiana State University in Terre Haute. Decades of experiments have focused on how the pits enable the animal to turn into a heat-seeking missile for warm-blooded prey.

Now, Krochmal and George Bakken, also of Indiana State, report on the first tests of whether the facial pits also help western diamondback rattlesnakes protect themselves from overheating. After researchers blocked rattlesnakes' facial pits, the animals had trouble finding cool refuges, Krochmal and Bakken report in the Aug. 1 Journal of Experimental Biology.

Bakken says that experiments on seeking refuge may help ecologists learn how animals deal with a patchy environment.

Krochmal adds that the need for thermoregulation may have driven the evolution of early pits and the snakes' spectacular prey targeting came later.

When Krochmal decided to test rattlesnakes, he hadn't ever handled poisonous snakes. He practiced for months with a harmless but cranky black rat snake. Still, he says, "the first time [working with a rattlesnake] was the most unnerving experience of my natural-born life"

First, he tested 12 wild-caught snakes in a simple Y-shaped tube with one branch kept comfortably cool at 30[degrees]C and the other heated to a stressful 40[degrees]C. Snakes put into the Y's stem, also at 40[degrees]C, slipped into the cool end about 75 percent of the time.

Then Krochmal anesthetized all the snakes and blocked their 2-millimeter-wide facial pits with tiny plastic balls and aluminum foil temporarily glued on top. Snakes with blocked pits ended up in the cool spot only half the time, as if by chance. When Krochmal unblocked the pits, though, the snakes' performance bounced back.

The researchers repeated this test with a choice of four boxes, one of them cool, and then with four natural-looking burrows. Each time, the snakes' performance dipped when their pits were blocked but bounced back when the paraphernalia came off.

Snake specialist Harry Greene at Cornell University asks how well the test setups, where there's less than 1 meter from a snake's decision point to the cool nook, represent choices in the real world. He suggests yet another important use for the pits: assessing danger from predators. However, he says that the idea that facial pits evolved for body-temperature control "deserves serious consideration."