This article appeared in Science 80, the consumer sister of the journal Science, in October 1981. It helped us win that year's National Magazine Award for General Excellence, with a special citation for writing.
By Terry Dunkle
The snake is a pile of black-and-olive loops in the autumn sun, its yellowish rattle drooping over the pile like a ripe head of wheat. It is breathing, as snakes usually breathe, in waltz time: first squeezing, then inflating, and finally relaxing its slender sides. On the squeeze cycles, the skin over its backbone sags like a loose tent. The snake hasn't eaten for weeks.
A rattlesnake can get by on one meal a year, if the meal is big. This snake, a yard long and as thick as a vacuum-cleaner hose, can capture and swallow a rabbit.
Nobody has gotten a really complete look at a rattlesnake's feeding in the wild. Parts of it happen too swiftly for human vision. But recent laboratory studies have made it possible to put together details. A rattlesnake, they reveal, is a beautifully designed killing and eating machine—living proof of the inventive powers of hunger.
The rattlesnake, still dewy from the night, is waiting for its blood to reach 85 degress, the preferred operating temperature. Without moving its head, it looks around. Tiny, lidless eyes tilt right and left behind their glassy cover plates, scanning both ends of the ledge. Yellow-tinted lenses slide in and out, focusing far and near like binoculars. No food in sight.
Snake eyes differ radically from those of all other vertebrates. Amphibians, ordinary reptiles, mammals, and birds have inherited the eyes that evolved 300 million years ago in the fish. But somewhere along the line, the snake lost parts of its eyes and later rebuilt them.
In theory, the eye alterations were part of a grand transformation that created the snake 100 million years ago. A sudden change—an ice age, perhaps, or a spurt of competition from the mammals—drove the snake's ancestors underground. Tunneling in the dark, they gradually lost their eyesight, eardrums, and legs. Ribs multiplied, stretching the chest until the lungs were crowded. The left lung shrank as the right one lengthened. Dozens of changes and thousands of millennia later, up came a tubular lizard.
Among the cleverest at overcoming these handicaps were the rattlesnakes. In only a few million years, they spread from Mexico into most of Central and South America and all of the continential United States except Alaska and Maine, invading deserts and swamps, mountains and prairies, jungles and vacant lots. Today there are dozens of species, ranging in length from two to eight feet and patterned in black, white, yellow, brown, orange, pink, green, and sometimes blue.
This rattlesnake is Crotalus horridus, the timber rattler. Anyone who has hiked in the Berkshires, the Alleghenies, the Smokies, the Ozarks, or just about any other mountain range between Massachusetts and Texas, has very likely cast a shadow on a live specimen without ever knowing it. At the first sign of danger, horridus usually freezes, becoming a pile of leaves, a confusion of shadows, a cow flop. It can detect footsteps many paces away, through a fine bone linkage between its inner ear and the pan of its jaw, which rests on the ground like a stethoscope.
A lot of other snakes have the stethoscope option, but a rattlesnake also has facial pits and foldup fangs, four different crawling gears, and a rattle.
The rattle, a loose string of hollow, interlocking beads made of the same stuff as fingernails, has nothing to do with getting food. It is a stop signal. When a large animal is about to tread on the snake, the rattle vibrates at about 70 cycles per second, sounding like grease sizzling in a pan.
The facial pits are a pair of finely bored and polished little holes under the rattesnake's eyes. They look like twin automobile hood scoops. For centuries, nobody bothered much with them, but in 1936 Gladwyn K. Nobel and Arthur Schmidt, of the Museum of Natural History in New York, discovered that blindfolded rattlesnakes could still strike at hot ligthbulbs. The pits, the two men reasoned, must sense heat.
Humans have a similar faculty. We can feel where the fireplace is from the warmth on our cheeks. But rattlesnakes have carried it further. Stretched across the interior of each pit is a membrane, a thousandth of an inch thick, crammed with sensors that pick up infrared radiation from warm-blooded animals. The signals feed into the brain, where, according to recent work by Peter Hartline and Eric Newman at the Eye Resarch Institute in Boston, they are spread out like dots on a picture tube. At night a rattlesnake can see its victim glowing in the dark.
The typical victim—a mouse, a rat, a chipmunk, a squirrel, a prairie dog—is nearsighted in a way that even glasses wouldn't help. Its retinas lack a fovea, the fine-grained region that primates use for looking directly at things. A rodent sees head-on what a man sees out of the corners of his eyes.
But rodents have a compensating talent: fast reflexes. Signals from their eyes must race only a few inches to the brain and into the legs. Donald Owings, of the University of California at Davis, has videoed encounters between rattlesnakes and ground squirrels. The squirrels sometimes attack the snakes, lunging, biting, and leaping away as the fangs brush by.
Rodents are in fact one of the rattlesnake's greatest enemies. They nibble on snakes that are sluggish with cold, exposing them to fatal infections. This, and not the squeamishness of visitors, is the main reason zookeepers avoid live feedings. Mice have been known to chew their way out of snakes.
A rattlesnake is a delicate creature. Pick one up wrong, and its neck breaks. Trap it under 20 minutes of noonday sun, and it goes into convulsions. Lay it in a line pointing too steeply uphill, and its heart, meant for pumping horizontally, may fail. The trauma of being picked up may lead it to starve itself.
Rattlesnakes, although dangerous, aren't the killers that many people assume. In an average year in the United States, fewer than a dozen persons die of rattlesnake bite, some of them showmen or hunters who are deliberately handling snakes. More people die from bee stings.
It is not just that rattlesnakes are shy. Joseph Gennaro, of New York University, has discovered they adjust their venom dosage to the size of their prey. The bigger the victim, the more they inject—up to a point. Human victims, perhaps a fifth of the time, get no venom at all. They are too big to swallow.
The venom, a modified saliva, oozes into a bunch of little bulbs arranged like grapes behind the rattlensake's jaw. Clear and flavorless, it does no harm to humans when taken by mouth, although minutes later the tip of the nose may tingle. As venoms go, it is weak. The Australian tiger snake can kill a man with one drop; a rattlesnake might need a teaspoonful. But a rattlesnake can inject that much in a fraction of a second, where most other reptiles can't.
Poisonous lizards, such as the Gila monster, must work their venom into the wound like a vaccination, for it merely empties into the channel between the reptile's lips and teeth. In primitive snakes like the cobra, two of the front teeth are connected to venom ducts, but the fangs are short and inefficient—a cobra usually hangs on and chews. Rattlesnake fangs, mounted on hinges, fold up along the roof of the mouth when not in use. They can grow to two-thirds of an inch. Each is a hollow needle, slightly curved, with an orifiice on the side instead of at the tip, to prevent clogging with flesh. After a couple of uses, it drops out and a fresh one, growing in a sheath behind it, ratchets forward and installs itself in the socket. No human ever wielded a deadlier hypodermic.
The rattlesnake feels an urge to move. Its heart-shaped head slowly rises, the long neck cantilevers outward and lightly touches down, the pile of coils unravels. The snake moves along the ledge by bunching itself like an accordion, straightening the pleats to advance the head, and drawing up the rear again—traveling in what herpetologists call concertina. At the far end, it turns right and shifts into lateral undulation, flowing up the slope in a wavy line.
Still in lateral undulation, the rattlesnake arrives at a huckleberry patch a few yards up the hill and threads itself in, disappearing under the canopy of red leaves. Rattlesnakes have two other gears. In sidewinding, a snake buggies along on two or three points of its body, the rest flowing down and through and up and around them almost like a corkscrew. In rectilinear, a snake lying in a straight line can creep foward by the millimeter, its head imperceptibly growing in the victim's field of view.
In the warm, pinkish twilight under the huckleberry canopy, a chipmunk is sitting on its haunches, its nose and whiskers jiggling as it nibbles on a dried huckleberry between its paws. It pops the berry into a cheek pouch, darts forward, scrambles over a fallen branch, and halts at an acorn lying on the other side. It stands up with the acorn and begins gnawing off the point, its long, self-sharpening teeth making faint scratching sounds.
The branch is breathing.
The chipmunk keeps on gnawing, flecks of acorn dust falling from its mouth.
The breathing deepens. Slowly the snake's head turns and levitates an inch off the ground. The head recedes, pushing the neck into an S. Muscles tighten.
The chipmunk stuffs the acorn into its other pouch, then drops down on all fours and darts back in the direction it came from.
The neck muscles trip, shooting the snake's head forward so fast that a car with the same acceleration would go from zero to 60 in half a second. The jaws fly open. The fangs flip out like fingers on a reaching hand. The syringes fire.
A flash of belly, the chipmunk dashing away.
The snake is back in position in less than a second. Some observers would say it has never opened its mouth. No one would be certain whether the strike has succeeded. Rattlesnakes occasionally miss.
This time, only a tiny drop of venom has gotten into the chipmunk's blood. The animal's pulse, over 1,000, disperses it in seconds. Some of the alien proteins unlock a few enzymes from the chipmunk's cells. Others snip open a few bonds here and there. Suddenly, capillaries burst. Blood squirts out into the tissues. The chipmunk staggers, collapses, kicks its hind legs for a minute, gasps, and lies still.
For some minutes, the rattlesnake does nothing. Its pea-sized brain is programmed to wait until the victim is safely dead. Finally, it extends its neck and, swinging it back and forth like a boom, touches the ground here and there with its flickering, pink tongue. Finding the right spot, it lowers itself and streams into the chipmunk's trail.
Each time a snake pulls its forked tongue inside its mouth, the tines brush an organ that analyzes the chemistry of whatever they have touched. Humans have the same organ in the roof of the mouth, but we lose it before birth. The snake's can detect loose molecules from animals that have passed days before.
But the chipmunk has left two trails: one arriving, one leaving. The wrong trail may draw the snake to where the chipmunk slept last night, perhaps hundreds of feet off course. A rattlesnake can't afford that kind of energy. This one can't afford the time. Above the canopy, the sun is sinking, the temperature about to plummet.
David Duvall and Jeanne Trupiano, of the University of Wyoming; David Chiszar, of the University of Colorado; and Charles Radcliffe, of the Denver Zoo, have recently shown that when a rattlesnake is offered two mice, one killed by snake bite, the other by neck wringing, the snake usually takes the bitten one. Duvall believes rattlesnake venom may cause the right side of the victim's heart to pump harder than the left, forcing telltale body fluids into the lungs and the tip of the nose. He thinks the victim's rapid breathing may spray an unmistakable line of droplets on the ground.
That theory is too elaborate for some of Duvall's colleagues, who think the trail is urine.
In a few minutes, the rattlesnake arrives. The chipmunk, lying on a bed of freshly fallen leaves, is twice the rattesnake's neck diameter.
Swallowing will be difficult, but not digestion. Special compounds in the venom are already tenderizing the chipmunk from the inside. In the rattlesnake's stomach, everything will dissolve but the hair and the teeth.
The rattlesnake crawls leisurely around the chipmunk, approaching it from different angles and touching it in different places with its tongue—the chin, a shoulder, the belly, a half-shut eye, the nose. Its jaws clamp the nose. The left half of the bottom jaw advances independently of the right, sinks a row of sharp teeth into the underside of the chipmunk, and pulls it a fraction of an inch in. The right half follows, and again the left, tractoring the chipmunk deep into the snake's throat.
So regular and mindless is the action that if two snakes happen to seize the same prey, one occasionally ends up swallowing the other.
The rattlesnake pauses for a minute, a short length of tail protruding from its mouth. Its head and jaws are sprung so badly out of shape, they might have been run over by a car. Its neck is a chipmunk-shaped lump, encased in pinkish skin with rows of dark polka dots on it: the disconnected scales.
Soon, the neck muscles begin contracting in a wave, squeezing the chipmunk toward the middle of the snake. Finished, the rattlesnake lifts its misshapen head and yawns, snapping everything back into place.Top
