My friend John had been an enthusiastic snake collector for years when he and I went off at the age of 25 to seek adventure in the Peruvian Amazon. As two Isconahua Indians paddled us in their dugout canoe up a jungle-covered stream, we talked about bird-watching. But John was really watching for anacondas. When he spotted a very large snake's head protruding above the water's surface, he grabbed the snake by the neck and dragged it ashore as our Indian guides fled.
What John hadn't stopped to calculate was the size and strength of the body to which that large head was attached. Only when the whole snake was on the bank of the stream were we able to determine that it was about eight feet long, weighed about 40 pounds, and was only slightly less strong than us. Each of us in turn brandished the anaconda while the other took some snapshots. When my turn came, I looked into the snake's array of long, sharp teeth, felt its muscles resisting all my efforts to hold them in check, and wondered where it would bite me if I lost my grip. Had the snake been a foot or two longer, I would have found out. Our Indian guides knew anacondas well and had been smart to run away.
That incident satisfied all my desires for close contact with snakes, until three years ago when one of my then three-year-old twin sons, Max, fell in love with snakes and begged for one as a pet. Like any normal twin who sees his brother with a new toy, Max's brother Joshua then insisted on getting his own personal, private pet snake. As our boys' demands escalated and our reptile collection grew, we eventually had to set aside one room of our house as the Snake Room.
At the same time, my research laboratory at the UCLA Medical School began to fill up with the rattlesnakes and pythons being studied by my gifted colleague, physiologist Stephen Secor. Max, Joshua, and Steve have taught me to appreciate giant snakes once again and to understand that while they are terrifying and potentially very dangerous, they are often gentle and always fascinating.
Snakes have also proved to be ideal subjects for my own research into digestive physiology. I'm interested in how our intestine adapts to changes in the amount of food it's given to digest--for instance, what happens when we change our diet, or starve, or nurse babies, or embark on a vigorous exercise program. However, we humans consume small meals at regular intervals several times a day. Our intestine is at work most of the time and doesn't have to adapt to a very wide range of food intakes. That makes our own intestine's responses modest in scope and difficult to analyze. Many snake species, on the other hand, are notorious for consuming huge meals at long and unpredictable intervals. Hence their intestinal responses are also huge in scope, making them good, easy-to-study models of our own digestive physiology, carried to an extreme.
The thought of big snakes makes most of us shiver in disgust, perhaps because our image of these creatures was influenced by reading books like The Swiss Family Robinson when we were children. You remember the typical scene: a monstrous 40-foot-long serpent waits coiled in a tree, motionless, until an unsuspecting donkey, or person, passes underneath. Anchoring itself to the tree with its tail, the monster wraps the donkey in its slimy coils and squeezes until its victim's bones are crushed. The poor donkey is then kneaded into a shapeless mass and swallowed without even being chewed.
Parts of this horror tale are actually true--but only parts. For one thing, no snake on record has reached 40 feet in length, though a few come close. The world's largest snakes are the giant constrictors: the anaconda and boa constrictor of the tropical Americas, and four python species of the Old World and Australia--the reticulated python, the African rock python, the amethystine python, and the Indian python. Only the anaconda and the reticulated python occasionally top 30 feet, however; the record is currently held by a 37-foot anaconda. The longest snakes can be as thick as a man's waist and weigh several hundred pounds, most of it muscle.
Big snakes do indeed lie in wait for their victim, but anchor themselves with their tail only if the prey is difficult to immobilize or strong enough to drag the snake around. And their coils are emphatically not slimy: every snake I have ever held was dry and pleasant to touch. While those muscle-bound coils do allow the snakes to squeeze their prey to death, the victim usually doesn't suffer broken bones and tends to retain its shape. In fact, the body of a 14-year-old Indonesian boy swallowed by a reticulated python was still intact and recognizable in the snake's stomach when the snake was killed and cut open two days later.
Snakes do swallow their prey without chewing. A donkey, however, is too big. The biggest prey on record is a 130-pound antelope that was swallowed by an African rock python. There's also a case in which an Asian reticulated python downed a 28-pound goat and a 39-pound goat at one sitting, then proceeded to consume a 71-pound ibex a few days later.
Another way to appreciate the size of a snake's meal is to look at it in terms of relative body weights. Even the most gluttonous of us humans would have difficulty eating 10 or even 5 pounds at a sitting--and that would represent well under 10 percent of our body weight. Many snake species, though, regularly consume 25 percent of their unfed body weight. Secor's pythons can easily eat 65 percent of their weight and sometimes 96 percent. The record, however, is held by a viper that swallowed a lizard 1.6 times its own weight. In human terms that would be like a 140-pound person who normally scarfs down a 35-pound chunk of meat occasionally swallowing a 224-pound chunk.
Big snakes can indeed kill and swallow human beings, usually small women or children. But there are only a few well-authenticated cases. That's not so surprising when you consider that most people are unlikely to stand idly by for the hour or so it would take for a serpent to swallow one of their friends. (Such an act has never even been seen in the lab, before the eyes of a dispassionate scientist.) Nevertheless, there are a number of accounts of snakes interrupted in the act of swallowing a person, or found with a big bulge that, once slit open, yielded a human being.
To avoid being killed by a snake, a quick rule of thumb is to stay out of the way of any snake over 11 feet long. When Secor brought his friendly 11-foot pet python Linus to our lab for a visit, I found that Linus could gently but irresistibly pull me in whatever direction she wanted. Pythons a few feet longer have killed unarmed humans--an American man was recently killed by his hungry 15-foot pet python when he imprudently tried to handle it unassisted. No unarmed person would have a chance against a 30-foot snake.
Here's what I see when I watch one of Secor's small pythons approach a rat. In one lightning move taking less than a second, the snake bites into the rat and simultaneously throws several coils around it. The bite merely serves as an anchor: the snake's teeth curve backward toward the gullet, so the rat cannot pull itself out of the snake's mouth. (Remember: The next time you find a python starting to swallow you by the arm--as happened recently to a 21-year-old woman cleaning her hungry 12- foot-long, 60-pound pet python's cage--don't pull back. You'll just get badly cut, and you won't get free. Instead remain calm, ask someone to pry the snake's jaws open, push your arm in further until the teeth disengage, and then pull your arm out.) What kills the rat is the snake's grip, a grip so tight that the rat is unable to move--except, perhaps, to wiggle a protruding foot or tail. Each time the rat breathes out, the python tightens the coils even more so the rat's lungs and chest can't expand when it's time to inhale. The rat quickly dies of lack of oxygen.
After the rat is unconscious or dead comes the seemingly impossible part: the fat rat has to pass through the python's thin head and neck. You or I simply couldn't do it, but the python's skeleton has been designed by natural selection for just this purpose. The snake's jaws can open to an angle of 130 degrees, while ours can only open a mere 30. When the jaws are wide open, a snake's upper jaw will point nearly vertically upward while its lower jaw points nearly vertically downward. Yet that alone isn't enough to get the rat down the snake's throat. It also requires a hinged jaw--instead of a single bone like ours, the snake's lower jaw is split in two, with the halves connected by an elastic ligament. Parts of its skull and palate are similarly flexible, not rigid like ours. This allows the snake's head to literally stretch around its victim.
A snake has no hands with which to push its prey down its throat, so it uses its muscular throat and cheeks, which are flexible like a hand. The prey is taken into the snake's mouth headfirst, so that the hair, legs, spines, or feathers will be pressed back, the prey won't get stuck, and the snake won't get punctured. Snakes will swallow porcupines with their quills, deer with their antlers, and goats with their horns. Despite the precautions, there are still cases of snakes getting pierced from inside by an antelope's horn or by the spiny fin of a fish.
Once the prey is in the snake's mouth, the snake will alternately extend the opposite sides of its muscular face around it, much as you would pull a long, tight sock over your foot. Eventually the prey will be totally engulfed, with the possible exception of a leg or tail. At that point the snake's powerful body muscles take over, milking the prey rapidly down the esophagus into the stomach much as you would squeeze water out of that same long sock.
Like humans, snakes make the job of swallowing their food easier by lubricating it with saliva. The head of an African boy, pulled from the mouth of a rock python interrupted in the act of swallowing him, was covered with saliva. When the python that swallowed the Indonesian boy was cut open, the boy's body was found to be coated all over with a "dirty- looking slime"--probably a mixture of saliva and gastric juices. In addition, his legs were crossed, his left hand was wedged between his legs, and his right arm was bent behind his head. If the current crop of horror movies is too tame for you, then picture how it might feel to be slowly forced headfirst down a python's throat, drowning in its saliva, with your arms pinioned at your sides by the grip of its cheek muscles.
The process of swallowing can take up to several hours, especially if the prey is large in relation to the snake. The next step, digestion, also proceeds at a leisurely rate. There's an enormous amount of food to be processed. Yet there's a problem: the snakes find themselves in a race with the foreign microbes that live in their prey's intestines to see who can digest the prey first. Everyone knows that dead animals begin to rot and stink as bacteria break down their meat and give off toxic chemicals. A whole rabbit in your stomach is of no use if you let bacteria rot it away before you can get to work digesting it.
The risk of prey rotting in a snake's stomach is a real one, especially if the prey is large. When a small python swallows a big rat, the python's body becomes immediately distended by the rat inside it, then distends half as much again within a day as the bacteria in the rat's corpse begin rotting it away from the inside, filling it with gas. A greedy little python that swallows too big a rat may end up vomiting it, yielding a very swollen, very smelly, very rotten dead rat. Occasionally snakes are unable to vomit and are killed by the putrefying prey inside them.
That's one reason snakes will bask in the sun to warm themselves, or coil up to conserve heat--it speeds their digestive processes. An Indian python, fed a rabbit, completed digestion in four or five days when kept at 82 degrees, in a week when kept at 71 degrees, but still had rabbit in its stomach after two weeks when kept at 64 degrees. Indeed, if the temperature gets too cool, a snake will refuse to eat at all. In my mostly unheated house in Los Angeles, my boys' snakes go all fall and winter and much of the spring without eating.
How long does digestion normally take? In humans the residues of a meal typically appear in the feces within less than a day. When my colleagues and I studied hummingbirds, we found that it took less than an hour for nectar to travel from the mouth through the intestines. For big snakes, on the other hand, transit times have to be measured in days or weeks. It takes 12 to 14 days for one of Secor's rattlesnakes to process a rat; at the San Diego Zoo, a snake fed 15 times during the year defecated only eight of those meals before it was fed the next meal several weeks later.
The interval between swallowing and defecation is long, but the time between meals can be even longer. In the wild, feeding intervals for rattlesnakes and big constrictors range from a few weeks to a few months. Pregnant female rattlesnakes will normally go one and a half years without eating; snakes in zoos have refused food for over two years. In other words, snakes swallow huge meals, digest them at leisure, and then wait a long time before eating again. Thus the snake's intestine has to be designed so that it can be called upon unpredictably, at short notice, to have a big load of digestive work dumped on it, and then do nothing for a long time until the dinner gong sounds again.
To launch our own studies into snake digestion, Secor took advantage of our laboratory's location in a medical center. When patients were not being X-rayed, Secor took some of his snakes down to the radiology department and X-rayed them at various times after they had swallowed a rat. The rat's skeleton, hidden in a python's stomach, showed up clearly on the films. By examining them we learned that the rat is dissolved headfirst; two days after it is swallowed, its skull is gone, though the rest of its body is intact. By four days, the chest and front legs are dissolved and the hair has become detached from the skin. By six days, only an occasional vertebra or hind leg remains within the snake's stomach.
At the same time, a mush composed of partly digested rat flesh passes from the snake's stomach to its intestine. The mush is gradually absorbed until all that remains of the rat is a few mats of its hair in the large intestine, awaiting expulsion as feces. The rest of the rat, including its bones--and, if it was a different prey animal, its horns or antlers--is thoroughly digested.
What's dissolving those bones? It seems obvious that some very potent chemicals must be doing a lot of work. In fact, one of them is the same hydrochloric acid that goes to work on the food in your stomach. Meat dissolves in acid, as you can easily convince yourself by dropping a bit of meat into a bottle of acid in a child's chemistry kit, and so does bone. Acid can even eat away at iron nails. Your stomach turns acidic whenever you eat and neutralizes when it empties: hence it goes through several cycles of acidity and nonacidity each day.
To test whether the same holds true for snakes, Secor had some of his pythons swallow a rat with a pH probe attached to it to measure acidity. (If this doesn't convince you that snakes can be gentle and cooperative, nothing will.) He found that the stomach turned acidic just a few hours after the rat was swallowed. But unlike acid in the human stomach, the acid in the snake's stomach remained for the six days it took to digest the rat.
Digesting a rat--not to mention an antelope or a goat--requires huge amounts of acid. And that acid costs energy to make. Where does the energy come from? From metabolic fuels and oxygen, of course. When you run or exercise, you breathe faster or deeper, or both, so you can take in more oxygen. Just as a car engine mixes gasoline with oxygen and burns it to produce energy in the form of heat, your body mixes its fuels-- carbohydrates, fats, and proteins--with oxygen to produce heat or chemical energy. This bodily combustion is called metabolism. When you exercise, you need more oxygen so you can burn more fuel, and thus you breathe more; people who run can increase their oxygen consumption by up to ten times the amount consumed when they're resting.
We also need energy to produce acid and digest our food, though not nearly as much. Instead of the tenfold increase in oxygen consumption found in a human runner, a human eater experiences an increase of just 20 to 25 percent. However, a snake that's swallowed an animal weighing a large fraction of its own weight faces an enormous amount of digestive labor. Soon after it swallows its prey, the snake begins breathing faster and deeper. We found that pythons consuming prey equal to 65 percent of their body weight increase their oxygen consumption by a factor of 36. The only comparable increase in animals is found in a greyhound, racehorse, or wolf running at full speed. But keep in mind that a meal that's 65 percent of a snake's body weight isn't even close to the limit--some snakes consume meals that are 160 percent of their body weight. That would demand even more oxygen, possibly 100 times the amount consumed at resting level. And greyhounds, racehorses, and wolves keep up their sprinting and high oxygen consumption for only a short while, then collapse in exhaustion; snakes go on like this for days.
Part of this fantastic metabolic spurt goes toward producing the necessary digestive acids. Another use for the energy became clear when we looked at the snake's intestine. When you embark on an exercise program, your muscles start to grow; if you're a weight lifter, you can watch with pride as your biceps visibly develop almost daily. Other organs, such as your heart, kidneys, and liver, are also growing with use. Although that growth is invisible, it too requires metabolic energy. Thus the impressive amount of food consumed by many athletes is used not only to fuel the immediate cost of the exercise itself but also to invest in building up the muscles and other organs.
By analogy, digestion may be thought of as exercise for the intestine. Anything you do that requires an increase in food intake--not just physical exercise but also things like nursing a baby or staying warm in a cold climate--will result in intestinal growth. Athletes' biceps do indeed begin to bulge, but so do the linings and propulsive muscles of their intestine. The intestine similarly bulges inside nursing mothers. When a runner hangs up her sneakers, or when a nursing mother weans her baby, the intestine atrophies.
When a python swallows a rat, its intestine doubles or triples in weight overnight. Each cell of the intestine's lining grows taller and wider and develops longer projections. All that extra surface area means that there's more intestine to make digestive enzymes--up to 60 times more enzymes--and to break down and absorb food.
Synthesizing extra intestine accounted for a lot of the extra oxygen consumption that we saw in our fed snakes. When a rattlesnake swallows a mouse, its intestine grows quickly: to keep up, a 150-pound man would have to add 6 pounds to the weight of his own intestine overnight. In fact, the cost of making extra intestine and digestive acids is so high that the snake uses about a third of the energy it gets from a mouse's body just to digest the mouse's body! And a larger python, eating an even larger meal, can use up to half the energy derived from the meal.
The growth of a snake's intestine is faster and more dramatic than the growth of a human's intestine and seems to be triggered differently as well. In our body, the intestine begins growing only when food arrives downstream, near the rectum, prompting the release of intestinal growth hormones. It's as if the far end of the intestine were telling the rest, "Well, you didn't succeed in absorbing all that food; some of it still reached me down here. I guess this owner of ours is going on an eating binge, so we'd better grow and get ready to work harder."
In a snake, intestinal growth is nearly complete in less than a day, when the prey is still sitting in the stomach. The intestine knows it's going to get a big load--that's the way snake digestion works. So in the snake's case the prey sitting in its stomach prompts the stomach to either release hormones or fire nerves to alert the intestine. The snake's trigger, in other words, is upstream, not downstream. It's as if the snake's stomach were calling to the intestine, "Hello down there! A flood of mouse has just arrived and is on its way to you. Get ready for some hard work, starting tomorrow."
A python dining on a rabbit is thus more than just a disgusting spectacle--it's a window into our own physiology. Despite our differences, we can learn a lot by looking at snake digestion. Like the snake's, our gut has to adapt to fluctuations in its work load: we eat in three pulses each day, and we eat more while we're pregnant, nursing a baby, training for a marathon, or working outdoors in the dead of winter. We may go on a high- carbohydrate diet, then get bored with pasta and switch to a high-meat diet. Our food consumption changes if we develop diabetes or cancer or break a leg or get burned. In fact, throughout most of human history our eating habits were even more like those of snakes; our hunter-gatherer ancestors binged when they killed an elephant and then made do with less food until they bagged another.
Our intestine adapts to all those fluctuations in its work load by growing and atrophying, synthesizing more or fewer enzymes, releasing hormones, and firing nerves. And the digestive processes don't differ fundamentally between snakes and us. The hydrochloric acid we use is the same, and the protein molecules used to secrete the acid or absorb nutrients are quite similar. The snake's responses, however, are comparatively gigantic: a 3,600 percent increase in oxygen consumption instead of our pitiful 20 percent, and a 60-fold rather than a 2-fold increase in the amount of enzymes. To understand the human intestine in health and disease, the snake intestine may well be the best place to start.
That strategy is typical of science in several ways. We have no alternative to using animal models to understand human physiology and health, because we refuse to sacrifice the lives of our own children in medical experiments to save the lives of children to come. Also, a particular biological process may be more highly developed and thus easier to study in one species than another. In other words, it's good policy to study running in greyhounds, reproduction in rabbits, and digestion in snakes.
Finally, snake digestive physiology illustrates the often misunderstood role of what's called basic scientific research. Though the federal government foots about half the bill for scientific research in the United States, most elected officials are not scientists and don't understand how science really works. They think scientific experiments should be designed to address a practical question directly. If you want to cure cancer, they say, you should test different chemicals and see whether they can dissolve a tumor.
That's called applied research. While it's certainly important, it rarely results in huge increases in knowledge, because it involves scientists' tinkering with variations on what we already know. Major advances in solving problems tend instead to come from way out in left field, from a scientist studying something that has nothing to do with the problem at hand and making a discovery that proves unexpectedly relevant. Look at the first wonder drug, penicillin. Its development stemmed from--of all seemingly useless things--the observation of a speck of mold that fell on a bacterial colony. It turned out that the mold killed the bacteria by secreting a chemical that proved to be penicillin.
Almost two decades ago, Senator William Proxmire established the notorious Golden Fleece Awards. Proxmire scanned the titles of government- funded research grants until he found one that seemed of no applied relevance, then publicized and ridiculed it as a supposed example of wasted government money. I can only imagine Proxmire's reaction to the study that led to the discovery of penicillin: "A grant to study molds? Give me a break. Molds! Can you beat that for waste of taxpayer dollars?"
It's in the spirit of the penicillin discovery that I defend research on snake digestive physiology as a wise use of taxpayer dollars. Many of us will experience serious intestinal problems at some point during our lives, and some of us will die of them. The snakes' digestive achievements make them worthy of serious study. While I like snakes, I'd still rather use them than human beings to figure out solutions to our intestinal problems. So the next time you're out hiking and come across a snake, don't kill it or shrink in horror. Instead reflect on what an amazing creature it is, and how much it has to teach us.