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.
"In a world where your resources are distributed in random clusters, and you don't know where they are, it turns out that the straighter the path, the higher your probability of colliding with them," said Dr. David Duvall of Arizona State University in Phoenix. "And the path of a male rattlesnake searching for females is 99 percent straight. It's really wild stuff."
Other scientists are investigating the highly ritualized battles that competing males engage in for the privilege of mating with ** females, fights that resemble nothing so much as disembodied arm wrestling.
Rearing up and entwining their necks like swans in love, the males do not bite, but merely attempt to force one another to the ground.
Loser syndrome
The fights last for hours and often result in an apparent draw. But woe to the snake that cannot stand its ground. Scientists who study copperheads have lately learned that a trounced pit viper is so demoralized that for days afterward even far punier males, which normally would be loath to pick a fight, will take on the loser and defeat it.
"Losing seems to put the snake in a real psychological dilemma, and the snake may maintain that wimpy attitude for a week," said Gordon W. Schuett, who is completing his doctorate at the University of Wyoming at Laramie.
He and his co-workers are now measuring hormone levels in freshly defeated males to see whether defeat wreaks its psychological devastation by raising stress hormones such as cortisol and depressing testosterone concentrations that lend male snakes their randy aggression.
The scientists have also discovered that, on occasion, a female copperhead will take advantage of the loser syndrome.
Upon approaching a potential mate, she will mimic another male, rearing up as though ready for battle. Should the mock display terrify the suitor, she will take it as evidence that the male is a loser and reject him as unfit for paternity.
"Females mate with winner males almost exclusively," said Mr. Schuett.
And rarely as well. Most female pit vipers breed less than once every three to five years, which is why a fertile female is highly prized and fought over.
"With pit vipers, you get these explosive mating assemblages," said Dr. Duvall. "When the female emerges in spring, ready to mate, there may be hundreds of males waiting for her outside the den."
Once she appears, the frenzied male tournaments commence. After hours or days of fighting, a winner emerges, who then approaches the female and courts her with gentle chin rubs and tongue flicks.
Eventually, he may be awarded with copulation, an act that itself lasts for hours or days, as the male locks himself into the female with one of his two barbed and bifurcated genitals, called hemipenes.
Chemosensory talents
Other biologists are attempting to determine the limits of the pit viper's extraordinary chemosensory talents, which seem to be among the most powerful in the animal kingdom.
A viper's strike may take only a fraction of a second, but in this snatch of time the snake picks up with its forked tongue a distinct chemical signature of the creature.
The few odor molecules are conveyed from the tongue to a pair of sensory organs called vomeronasary glands, located on the roof of the viper's mouth, creating a chemical memory. The envenomated animal is allowed to wander off and die, but the viper can track it down wherever it may stumble.
Researchers at the University of Colorado in Boulder have attempted in exacting laboratory experiments to water down the scent of a prey rodent after the pit viper has tasted it.
But they have not yet, after hundreds of dilutions, succeeded in throwing the snake off the trail.
Fascination with venom
The venom of a pit viper also holds fascination for many researchers, who are trying to further understand it, and to devise improved anti-venoms.
About 8,000 people a year are bitten by pit vipers in the United States, the great majority of them young and often drunken men who tease or maul snakes they find by the side of the road.
Professional and recreational snake handlers in Western states consider it the essence of machismo to endure an occasional rattlesnake bite and often do not bother seeing a doctor afterward, with sometimes serious consequences.
Even a relatively minor snakebite on the finger can result in permanent paralysis of the hand, and more serious bites cause extensive destruction of tissue, internal bleeding, dangerously lowered blood pressure and death.
Dr. Greene, who has not been bitten since he endured a tiny nip by a copperhead as a teen-ager, displays photographs in his lab of people who have been far more severely envenomated, showing legs and arms that are horrid masses of blackened, blistered flesh.
"This is to instill in everybody the awareness that pit vipers are not toys," he said.
The snakes normally will not bite unless touched or seriously pro
voked, he said, but they must always be treated with respect and the utmost caution.
Scientists have learned that snake venom serves a variety of purposes, which together account for its deadly potency.
"Venom is a mixture of neurotoxins, blood toxins and other proteins," said Dr. Richard C. Dart, director of clinical toxicology at the University
of Arizona in Tucson. "There are dozens of individual components, each causing its own toxic effect."
Not only does the poison help subdue and kill a prey animal, he said, it also begins digesting the creature from the inside out. A pit viper swallows its meal whole, moving its vertebral column over a creature that may be 1 1/2 times its own body size.
The snake then allows the prey to digest internally over days or even weeks. Were it not for the degradative effects of the injected venom, the prey would begin to putrefy within the snake's belly.
Most of the 144 species of pit vipers live in North and South America, although several types are found in Asia. Pit vipers thrive in the harshest deserts, the wettest Amazonian rain forest and the gentlest meadows and woodlands.
"Pit vipers live on the ground, underground or in the trees," said Dr. Carl Gans of the University of Michigan at Ann Arbor.
"Pit vipers can be very bulky or very slender, short or long. They take on a whole variety of different architectural patterns."
"These really are magnificent, complex and sophisticated animals," said Dr. Savitzky. "Snakes generally are fascinating, but pit vipers have a suite of adaptations that are truly elegant."
On pit vipers
For the first time, the many studies of the spellbinding snake family are being pulled together into a single volume, "Biology of the Pitvipers," to be published later this year by Selva Press.
The book, edited by Jonathan A. Campbell and Edmund D. Brodie Jr. of the University of Texas at Arlington, presents a sweeping view of the pit viper's evolution, its anatomy, its alternately violent and gallant mating behavior, the lethal brew of toxins and degradative enzymes found in its venom, its singular approach to eating and other solutions the creature has devised to the problem of being a snake.