An Eye for Heat

Rattlesnakes, and such venomous relatives as copperheads and water moccasins, have in their heads two small organs called "pits." Scientists have long known that the pits are sense organs which respond to heat, but they did not understand clearly how they work. In last week's Science, Drs. Theodore H. Bullock and Raymond B. Cowles of the University of California, Los Angeles, told how they hooked up a rattlesnake's pits and studied their actions as if they were microphones.

Having prudently given the rattlesnake a paralyzing injection of curare, the researchers uncovered one of the nerves leading out of a pit organ and connected it through an electrode to an apparatus that amplified and recorded its electrical impulses. When they blindfolded the snake but did not excite it otherwise, the sound that came from the amplifier sounded "like grease cooking slowly in a pan." But when Dr. Bullock moved his warm hand near the snake's pit, the sizzling sound increased "as if you had turned the heat up." A lighted match or cigarette produced the same effect. On the other hand, a cold object, such as an ice cube, cut the sizzling down.

The temperature of the nearby air had no effect. Warm objects could be detected by the pit organ even through the cold air of a refrigerated room. But when a sheet of glass, opaque to long infra-red rays, was placed between the snake and a warm object, it "blinded" the pit. Drs. Bullock and Cowles conclude that the pit is a sort of "heat eye," sensitive to the infra-red rays that come from warm objects. It detects cold objects by giving less response than it does to the snake's room-temperature surroundings. A glass of water only one degree above or below room temperature is clearly "visible" to the sensitive pit.

This organ must be very useful to the snake, say Bullock and Cowles. Rattlesnakes have good eyesight, but they do most of their hunting at night or underground, and so must be grateful for an organ that points out warm prey. A snake crawling down a dark burrow after a warm mouse quivering at the end of it can "see" its prey through its pits.

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