A father and his two sons sat in the jungle, trimming the wood they had just chopped. The humid heat, the jungle’s windless silence, and the work’s monotony had so sapped the men’s alertness that they never even saw the giant lizard until it was nearly upon them. It had seemingly appeared out of nowhere and was now advancing, its forked tongue flicking rapidly in and out of its mouth, its seven-foot-long body tensed in a crouch.
As the monster charged, the three men leapt up and began to run. Unfortunately, one of the sons crashed against a low-hanging vine. In an instant the monster seized him by the buttocks and tore off a large chunk of flesh. Although the others managed to drive the lizard back into the jungle, the damage had been done. Within half an hour the young man bled to death before the eyes of his helpless father and brother.
This nightmarish story is not fiction; it happened in Indonesia a few decades ago. The killer was the world’s largest lizard, the Komodo monitor, alias the Komodo dragon. (It gets its name because it belongs to the group of lizards known as monitors and because it lives on the tiny Indonesian island of Komodo.) While its usual diet consists of animals rather than people, some large individuals do become dangerous to humans. Their victims have included European tourists as well as Indonesian villagers.
When European scientists finally learned of the Komodo dragon’s existence in 1910, they were astonished that such a large animal had escaped their notice for so long. Not surprisingly, they immediately began asking some obvious questions, to which we have only recently begun to get answers. For example, how big does the lizard really grow? Why does this largest of lizards live on tiny, obscure Komodo, of all unlikely places?
What does it normally eat, when it’s not stalking unwary fathers and sons? How does it capture its normal food?
These questions may seem to refer to just one unique species, and to lack broader significance. In fact, they’re part of a deeper puzzle. The Komodo dragon is not unique: if you’re one of the many people who find the thought of it gross and frightening, wait until you hear about the really frightening dragons that used to inhabit Australia. All these dragons raise the broad question of how supposedly primitive animals (like lizards) manage to persist. After all, the advantages that warm-blooded mammals enjoy over cold-blooded reptiles are many and clear: for example, mammals can run much faster and farther, grow faster, and don’t need to wait for the sun to warm them up. Why, then, does the world still support more species of reptiles than of mammals? How did a cold-blooded lizard, the Komodo monitor, come to be the biggest carnivore in its part of the world, instead of the usual big carnivorous mammals (like tigers and bears) that one finds elsewhere? Do scientists really believe in the existence of dragons that lived by killing elephants?
But I’m racing ahead of my story. Let’s begin with the Komodo monitor itself, which we now understand much better, thanks to field studies begun two decades ago by zoologist Walter Auffenberg of the University of Florida. First let’s straighten out a misconception stemming from the beast’s name. The Komodo monitor is not confined to Komodo; it also occupies a couple of neighboring small islands, as well as the western part of the much larger nearby island of Flores, which Auffenberg estimates to be the home of more Komodo monitors than is Komodo itself. Thus, a more accurate (though unwieldy) name would be monitor of Flores and neighboring small islands. Still, there’s no doubt that the monitor has by far the smallest geographic range of any of the world’s top carnivores--a puzzle to which we’ll return.
As for size, when dragons hatch from their eggs they’re only a foot long and weigh about three ounces. The biggest adults measured have been just over ten feet long. While zoologists have understandably been too terrified to weigh such monitors in the wild, Auffenberg has weighed eight- footers that he’s captured; they tipped the scales at up to 120 pounds, and he’s estimated that some ten-footers, after a good meal, could exceed 500 pounds. That’s enough, by a wide margin, to guarantee the title of world’s largest lizard.
Since the size of the Komodo monitor varies so much with age, it’s not surprising that its habits would also. Little hatchlings live in trees, where they catch insects and small lizards. Medium-size ones live on the ground and catch rats and birds. Only the biggest ones combine the roles of scavenger and top predator, like hyenas. Carrion is apparently located by its smell, which attracts monitors to a carcass from miles away. The smellier and more rotten the carcass, the more easily they can find it- -Indonesian government rangers attract dragons for tourists by letting a dead goat rot in the tropical sun for a couple of days. Some individual dragons have been found digging into Komodo’s cemetery to feast on recently dead corpses. These scavenging monsters continue to catch and eat small live prey, like rats and chickens--and younger Komodo dragons. But they also catch and kill big mammals, ranging from pigs, goats, and deer smaller than the lizards themselves up to horses and water buffalo ten times their own weight. (That’s as if one of you 200-pound readers were to kill a one- ton bull, while armed only with your teeth and hands.)
Adult monitors spend the night curled up in burrows to reduce their heat loss. After sunrise they bask in the sun for a couple of hours to raise their body temperature, then set off in search of food. As for hunting behavior, adults can reach a speed of 11 miles per hour in a short sprint, but they’re unable to sustain that speed, so they can’t run down their prey in a long chase as can lions or wolves. Instead, the monitor captures its prey by ambush or stealth. It looks for sleeping deer, creeps up on animals (as one did to that woodcutter’s son), or waits beside a game trail and charges when its prey passes within a few feet. To disable its victim, it takes a swift big bite out of the belly or buttocks, then waits for the victim to collapse from shock or hemorrhage.
Zookeepers who have cared for Komodo monitors come to respect them as among the most intelligent of lizards. In the wild, the animals learn individualized hunting strategies that show evidence of advance planning. For example, they periodically swim several hundred yards through the ocean, across a channel with whirlpools and strong tides, to reach an island when villagers stock it with goats. They also keep tabs on the condition of pregnant goats and horses, in order to be present at a natural birth or to goad the mother into a miscarriage and then catch the newborn as it drops--and sometimes the incapacitated mother as well.
Individual lizards vary greatly in their temperament and prey preference. Certain big individuals are notably aggressive toward people or specialize in hunting big mammals. Villages that happen to fall within the territory of such a monitor find it difficult to keep livestock.
Watching a Komodo dragon eat isn’t recommended for the fainthearted, or for children being indoctrinated with that parental advice on manners: Eat slowly, and chew each bite well. The lizard gulps food as fast as possible so that it can finish off its meal before another lizard comes along. One 110-pound dragon was observed to devour a 68-pound pig within 17 minutes. Others have been known to down 5 pounds of meat per minute and consume up to 80 percent of their empty weight. (You aforementioned 200-pound reader, now that you’ve brought down that bull, try eating 160 pounds of raw steak at a sitting.) All that meat is swallowed in huge mouthfuls--a whole 33-pound pig, or an entire hindquarter of a goat, in one gulp. The monitors must have cast-iron stomachs, for they’ll swallow a goat’s head whole (including horns and teeth) or a horse’s hooves or a whole porcupine. Because the lizard swallows all the bones of its prey, it’s a superefficient feeder, utilizing a higher percentage of a carcass than even hyenas or vultures. Only the contents of the prey’s stomach and intestines are spurned by the monitor’s unfastidious palate.
Two advantages of the Komodo monitor’s teeth are notable in those feats of hunting and gluttony. First, the teeth have serrated (sawtooth) edges, like the teeth of some dinosaurs and big sharks. Meat fibers are caught between the serrations, and a chunk of flesh is shredded or torn off as the lizard pulls back its powerful head. Second, meat caught between the teeth becomes a breeding ground for virulent bacteria. Hence the Komodo monitor’s bite is notoriously infectious: bitten people or animals may die later of infection, even if they survive the initial attack. Although this consequence is loathsome to us, it’s useful to the lizard: there’s no other big native scavenger in its geographic range, so a victim that escapes only to succumb later may well end up as food for the same lizard anyway. No such advantage accrues to big predators in other parts of the globe, who share their turf with specialized scavenging species. For example, a giraffe that ran off to die of a lion’s bite would probably be found and consumed by vultures and hyenas, not by the lion.
Let’s now return to the question: Why did this largest of lizards evolve on the unlikely islands of Komodo, Flores, and their neighbors? Today those islands are home to no native mammals larger than rats. All the big prey of living Komodo monitors--goats, pigs, deer, horses, and water buffalo--are domestic or feral animals introduced by human herders within the last few thousand years. What were monitors eating before that? Surely the availability of rat-size prey didn’t drive the evolution of such a powerful predator.
In answer, Flores has yielded fossil bones of two other species of prey: elephants. (Strictly speaking, they weren’t most closely related to modern elephants but rather to extinct mammoths and mastodons). One of the Flores elephant species was nearly full-size; the other was a half-size pygmy. Those pachyderms and their young would have been worthy prey, sufficient to explain the evolution of a big predator.
Other large and small elephants used to live on at least two other Indonesian islands, Timor and Celebes, until they and the Flores elephants became extinct, probably due to the zeal of newly arrived human hunters. Timor fossil beds have also yielded bones of an extinct large monitor, so Timor elephants too may have spurred the evolution of a giant lizard like the Komodo dragon. On Celebes, meanwhile, you can find the world’s longest living snake, the reticulated python. Once it may have dined on young pygmy elephants; it now has the distinction of being the sole snake proved to have eaten a human. Thus, on at least three Indonesian islands--Flores, Timor, and Celebes--the role of top carnivore became filled by giant reptiles (lizards or snakes) instead of the big mammalian carnivores found elsewhere in the world. Why?
Could the answer be that monitors swim well and could reach Indonesian islands from the Asian mainland, while flightless mammals couldn’t? Yet elephants themselves reached these islands by swimming, and tigers failed to colonize the islands despite being good swimmers. It makes one wonder whether some factor other than swimming ability favored big carnivorous reptiles. Could they enjoy some advantage over their mammalian equivalents in island situations?
That’s the conclusion suggested by stunning fossil discoveries in Australia, whose lack of big mammalian carnivores has always been puzzling. Today Australia has plenty of kangaroos and other big herbivorous marsupials that could have provided food for big carnivores. In the Pleistocene Era, which ended 10,000 years ago, Australia was endowed with even bigger prey, including not just giant kangaroos nearly ten feet tall but also giant diprotodonts (marsupial equivalents of rhinos). Australia today seems quite able to support the evolution of a wolf-size carnivore, and it should have supported tiger- and bear-size carnivores in the past.
But in reality Australia’s sole modern native carnivorous mammal of any considerable size--topping 30 pounds, let’s say--was a doglike marsupial called the thylacine, or Tasmanian wolf (which, though it became extinct in Australia itself some 3,000 years ago, held on in Tasmania until 1936). Even it reached only 50 pounds and was puny compared with tigers. Only two other carnivorous marsupials reached the 10- to 30- pound range: the Tasmanian devil (looking like a miniature bear) and the catlike spotted- tailed quoll. Going back to the Australian Pleistocene adds only two more now-extinct mammalian carnivores, albeit remarkable ones: a leopardlike marsupial called Thylacoleo, and (incredible as it may seem) a partly carnivorous kangaroo.
Even when these extinct species are added to the surviving ones, though, Australia’s mammalian carnivores still make a pitiful show compared with the several dozen big carnivore species living together on the other continents. There we find not just big cats, bears, and hyenas but also a wealth of medium-size wolves, jackals, cats, weasels, and mongooses and their kin. What prevented Australia from evolving large mammalian carnivores?
Part of the answer is that Australia’s isolation from the rest of the world kept out tigers and bears, leaving marsupials and rats as its main flightless mammals. Are marsupials and rats simply no good at evolving into big carnivores? In fact, Australia’s marsupials did evolve into equivalents of a dog and leopard; and in South America marsupials produced bear-size predators, including one like a saber-toothed cat. In the West Indies rodents evolved to the size of bears. Apparently, then, Australia is just unsuitable for supporting very big carnivorous mammals. Why? And what replaced them?
One hint is modern Australia’s richness in large lizards and snakes, including about ten species of pythons and ten of lizards weighing over ten pounds, bigger than all but a few snakes and lizards in North America. Those modern survivors are impressive enough, but even they undersell Australia’s past fruitfulness at evolving big reptiles. In the Pleistocene, Australia had not one but four species of crocodile: the notorious big saltwater croc and the smaller freshwater croc that still survive; a now-extinct big freshwater croc; and (weird as it sounds) an also-extinct land-going croc weighing over 100 pounds. To Australia’s existing big snakes were formerly added several very big ones. They included a constrictor worthy of your worst nightmares: 20 feet long, 12 inches thick, and over 110 pounds. They also included, from the Australian site of Riversleigh, a big python for which a snake taxonomist with a sense of humor coined the Latin name Montypythonoides riversleighensis.
All those big snakes and crocs made up a fearsome gantlet that Australia’s big marsupial herbivores had to run. If they escaped those dangers, they still had to face the fiercest reptile of them all, Megalania, a huge monitor lizard that dwarfed the Komodo dragon: the monster measured up to 23 feet long, with a weight variously estimated from 1,300 pounds up to a ton or more. Should your toes curl when you see a photo of a Komodo dragon, try running through your mind this even worse nightmare. You’re dozing peacefully in a forest clearing when you suddenly become aware that you’re not alone. You look up to see, to your horror, a giant one-ton lizard advancing on you, rapidly flicking its forked tongue in and out of its mouth. It lunges toward you. You jump up and turn to flee. You trip over a vine. You feel the monster’s breath behind you, then an awful hot flash of pain--AAAARGHHH!
All right, now, calm down. Let’s reflect on this nightmare, which may actually have befallen some of the first aboriginal settlers of Australia 40,000 years ago. In Australia, as on Flores, Timor, and Celebes, evolution filled the top predator niche with cold-blooded reptiles, not with warm-blooded mammals. Why?
Australian paleontologist Tim Flannery recently offered the following plausible explanation: We have to stop thinking of Australia as the smallest continent. Instead, think of it as an island, like Flores, Timor, and Celebes. Yes, Australia is bigger than those three islands, but effectively not nearly as big as it looks on the map. Remember that most of Australia is dry and can support living things only in low numbers. Making things worse, Australia’s soils are notoriously thin, mineral-poor, and unproductive: only 10 percent of Australia’s area is considered arable even by Australian farmers, and American farmers would sneer at most of that 10 percent. Finally, El Niño cycles cause Australia’s climate to fluctuate greatly: infrequently good, usually bleak, often truly lousy. All this makes Australia effectively small and, like those Indonesian islands, unable to support numerous populations of animals--including our own species. Those are the obvious reasons that Australia, with nearly the same area as the continental United States, supports barely one-fifteenth of our human population. Why might an environment capable of feeding only modest numbers of animals favor reptiles over mammals as top carnivores?
The answer is easy to grasp on reflection. It involves a concept familiar in ecology: the ecological pyramid. That means two things. First, species relate to each other ecologically in chains, with each species eating other species (usually smaller ones) at the next step down the chain. Second, the number of individuals and their aggregate biomass (total weight) decrease as one goes up the chain--the chain is actually a pyramid. That’s because it takes a population of many individuals of species B to support a few individuals of species A, if A eats B. For example, Yellowstone National Park supports far fewer grizzly bears than elk, while elk biomass is in turn dwarfed by the biomass of the plants on which elk feed. As a result, mammalian top carnivores like tigers live at far lower numbers than do beetles and rats because tigers stand at the top of the ecological pyramid, and also because they’re big: 500 pounds of feline flesh suffice to make 100 house cats but only one tiger.
Yet any animal or plant species needs to surpass some minimum population size if it’s to survive. A population of only a few individuals may deteriorate through genetic inbreeding, or it may find itself left with no breeding-age females (or males) if the few individuals of one sex happen to die within a short time. Those are problems especially for top carnivores, since they live in such low numbers. For instance, the grizzly bear population of Yellowstone National Park, which is not much smaller than the Komodo monitor’s main home of Flores, consists of fewer than 300 bears. There’s serious concern whether that small a bear population can survive in the long run. Hence it’s no surprise that big carnivorous mammals like lions live on the big continents, not on islands or in Australia, where they could never sustain a viable population.
How do reptiles get around this basic problem? Simply by being cold-blooded rather than warm-blooded! Because mammals maintain their body temperature high and constant through their own metabolism, they need lots of food as fuel to stoke those metabolic fires. But cold-blooded reptiles don’t incur that expense, so they need barely one-tenth as much food as a mammal of the same size. Any of you who keep both an iguana and a rabbit as pets will have learned to appreciate the iguana’s much lower food bill. The more adventurous pet owner who keeps a 100-pound Komodo monitor will similarly learn to love it for its low meat consumption, only 30 pounds a month, whereas a wolf or leopard of the same size would devour about 300 pounds a month. Put another way, a given amount of meat on the hoof can support ten times more Komodo monitors than wolves or leopards.
In the light of this argument, any problems that tigers might have had in swimming to Flores wouldn’t be the sole reason--perhaps not even the main reason--that Flores now supports dragons instead of tigers. In the long run, tigers could never have survived on Flores, even if they had gotten there. But all those tasty elephants offered a great evolutionary opportunity to any big predator capable of maintaining a viable population there. Only cold-blooded predators, with their low food requirements, could do that. That’s why Flores (and possibly Timor) evolved the Komodo monitor; why Celebes today supports man-eating snakes; and why Australia, with its equally tasty giant kangaroos, evolved an even larger monitor along with all those big snakes and crocodiles.
I said at the outset that Komodo monitors teach us a broad lesson about evolution in general. We tend to equate evolution with progress: natural selection improves both the prey and the predator, in an endlessly escalating evolutionary arms race. As mammals, we’re especially impressed with the virtues of mammalian warm-bloodedness. The high metabolic rate of a wolf, after all, lets it pursue prey for miles, while a Komodo monitor is good for only a few hundred yards. Mammals are constantly ready to go, but the monitor doesn’t operate at night and needs the morning sun to get itself going. We warm-blooded creatures grow much faster than reptiles: that’s why farmers grow chickens and lambs instead of equally delicious turtles and iguanas. The former grow so fast that they’re ready for market in a few months, while the latter would take years or decades. How could any cold-blooded vertebrate have withstood the evolutionary onslaught of such superior warm-blooded animals?
Obviously there must be something missing in this reasoning, since cold-blooded vertebrates are still doing just fine. There are more species of reptiles alive today than there are of mammals. Cold-blooded vertebrates still dominate the oceans, rivers, and lakes and are abundant in many terrestrial habitats. Our error of logic becomes obvious on reflection: in doing a cost-benefit analysis, we weighed the benefits but ignored the costs!
Yes, warm-bloodedness yields big benefits, but it also incurs big costs. A mammal or bird is nearly ten times more expensive to maintain than a similar-size reptile, amphibian, or fish. And as we all know, the more expensive model isn’t always best. On a tight budget you buy a Honda Civic and stay healthy; you don’t buy a gas-guzzling Lamborghini and starve, though there’s no doubt which car can outrun the other.
In the biological world as in the shopper’s world, the benefits of the expensive model may or may not be worth its costs. Natural selection, rather than your budget and taste, makes the evaluation in each case. Yes, birds and mammals monopolize the Arctic, where no cold-blooded terrestrial vertebrate at all can operate. But if you look around in other terrestrial habitats, you’ll find a lot of those cheap-to-operate lizards. Before you sneer too loudly at them, take a look at the Komodo monitor on your next trip to the zoo. Reflect on how it pulls down all those supposedly superior warm-blooded deer. Imagine, as the last sight that you’ll ever see in your life, its forked tongue flicking in and out as it tenses itself to charge on you. Those primitive lizards are masters of their niches. And if we warm-blooded humans continue to push the world toward a nuclear or environmental holocaust, I’ll bet on those cheap-to- operate cold-blooded animals to be the sole survivors.