Feared pit vipers yield their biological secrets of hunting, mating, killing

December 08, 1991|By New York Times News Service

The long-held theory that the two characteristic pits between the snout and mouth of diamondback rattlesnakes are thermal sensory organs evolved to help the snakes hunt their warm-blooded prey has been challenged by new research.

In a study comparing every aspect of the habits of pit vipers such as the rattlesnake with those of related viper species lacking pits, Dr. Harry W. Greene of the University of California at Berkeley, one of the world's authorities on rattlesnakes, contends that the snakes use their pits to help from becoming prey themselves.

Dr. Greene believes that a pit viper uses infrared information from an approaching animal to determine whether the would-be predator is small enough to scare away with a threat, or whether the viper would do best to slink away meekly.

Researchers have known for years that the pits -- present in such snakes as cottonmouths, copperheads and sidewinders -- are infrared detecting organs, capable of picking up heat signals from an approaching animal to form an infrared image that augments the snake's visual picture of the creature.

The vast majority of studies of viper pits have been performed with the assumption that the pits are there to help a snake hunt, but Dr. Greene now says that neurobiologists might do better to analyze brain signals from the pits by presenting a snake not with a tempting rodent, but with, say, a badger, a crafty and vicious foe of the pit viper.

To support his thesis that the pits evolved to defend the snake against its predators, Dr. Greene compared pit vipers with their pitless relations, which are considered the more primitive variety of snake.

He could find no difference in their hunting or eating habits. Both types of vipers favor rodents and other small mammals, which they hunt largely by staying perfectly still and ambushing a passing animal.

But the snakes do differ in their tactics for protecting themselves. Pit vipers have rattles or nubby tails that can thump the ground noisily, he said, a clear indication that they will stand up against predators.

By contrast, pitless vipers have no warning gear, and they also tend to have a striped coloration, which sets up an optical illusion once they start fleeing that makes them extremely difficult to see and catch.

Pit vipers usually are covered with splotches, which is of no aid in retreat.

Importantly, many pit viper mothers will lay eggs and then hang around to guard the clutch, while pitless vipers will drop their eggs and leave them to hatch or be eaten.

Hence, pit vipers seem to have an incentive to attempt to scare away predators searching for snake eggs, said Dr. Greene, but that attempt only makes sense if the mother herself can detect an oncoming threat and appraise its dimensions. The pits allow them to make just such an assessment.

"When I first thought of this idea, I jumped out of my seat with excitement," he said. "All the pieces started fitting together."

Dr. Greene said the snakes also use the pits for hunting, but he regards this as a secondary adaptation of a pre-existing talent.

Dr. Greene's theory is against orthodoxy, "and that's something positive right there," said Dr. Alan H. Savitzky of Old Dominion University in Norfolk, Va. "Harry has been very good at questioning things that others took as so obvious as to not be worth questioning."

Mating behavior

Other researchers have used recent advances in radio telemetry to outfit pit vipers with little internal radio tags, which allow the slippery and cryptic creatures to be tracked by radio receivers wherever they may be hiding.

In the past, the same snake could almost never be found twice, but with telemetry, herpetologists have gotten to know individual snakes as intimately as Jane Goodall knows her chimpanzees.

Following prairie rattlesnakes in Wyoming, scientists have found that during mating season, males embark each morning from their den on a grueling, five-mile round-trip search for willing females.

And as they travel, they crawl in a line so straight it could have been penned by a draftsman. Should they have to deviate from their path to scoot around a pond or boulder, the vipers return to the straight and narrow as soon as possible.

In computer simulations, researchers have found a rationale for that compulsive linear behavior. They have determined that females are likely to be distributed around the environment in random clusters, as they seek a dinner of rodents, which themselves are clumped about.

Male snakes obsessively covet females, and it turns out that by following a straight path, rather than zigging to one side or zagging to the next, the males optimize their chances of encountering mates.

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