Friendly Dragon

Up close the green iguana may look like a formidable beast, but a researcher has discovered that it's quite the gregarious lizard

  • Doug Stewart
  • Aug 01, 2004

AS A BOY IN THE 1950s with an interest in reptiles, Gordon Burghardt was watching an old Japanese monster movie on late-night TV when he recognized a cast member. "I thought, ‘Gosh, that’s an iguana!’" An iguana with extra scales and spikes glued on in order to impersonate a dinosaur, but an iguana nonetheless.

Even then, the idea of posing a docile, vegetarian lizard like the iguana as a stand-in for a rampaging dinosaur struck Burghardt, today a professor of psychology and ecology and evolutionary biology at the University of Tennessee in Knoxville, as absurd. Dinosaurs and iguanas belong on two distant branches of the reptilian family tree. But over the past 20 years, his own research has convinced Burghardt that iguanas and dinosaurs may not be so different after all.

"There’s evidence in the fossil record that dinosaurs moved in flocks or herds, the way birds and mammals do," he says, "and some people have said, ‘You’d never find that in mere reptiles today. They’re too slow and stupid.’" Burghardt, a leading authority on iguanas, snorts at the "mammalocentric bias" of such thinking. Iguanas and other lizards are neither slow nor stupid. And Burghardt has discovered that young iguanas in the wild also interact, cooperate and explore their environments in ways that biologists had never expected.

Iguanas are lizards of the Iguanid family, which encompasses the algae-eating marine iguanas of the Galápagos Islands, the Caribbean’s colorful, endangered rock iguanas, and the chuckwallas and desert iguanas of Mexico and the southwestern United States.

The most common species is the green iguana (Iguana iguana), the focus of Burghardt’s fieldwork at the Smithsonian Tropical Research Institute on Barro Colorado Island, six square miles of protected tropical rain forest in Panama’s Lake Gatún. The green iguana is native to a band of territory stretching from Mexico to southern Brazil. It is also a favorite of zookeepers and pet owners, and populations descended from escapees now flourish in parks and forests in South Florida and other warm pockets of the United States.

Newly hatched green iguanas, once they unroll their tails, are typically 7 inches long and bright green. A full-grown male can stretch 6 feet from snout to tail—more than half of that is tail—weigh 25 to 30 pounds and exhibit colors from orange to blue-green, with patches of pink, green-yellow, or reddish brown depending on the animal’s location and mental state. In Panama, green iguanas spend most of their time in trees close to water, basking on branches and munching on foliage.

Being arboreal is a key to their survival. When threatened, iguanas scurry into denser vegetation—or simply drop out of trees onto the ground or into water. With their tough scaly hides, the leaping lizards can survive falls from 50 feet or more. They’re also accomplished swimmers, using their long, flattened tails for propulsion, and can stay underwater for up to 40 minutes in a near-torpid state. In a pinch, iguanas can even scamper across the water’s surface, thanks to air bubbles trapped under the scaly webbing between their toes.

Female iguanas, which reach sexual maturity at the age of two to three, swim back to the sites where they hatched to lay eggs of their own. So far, Burghardt hasn’t figured out how first-time mothers manage to find their way to natal beaches. His hunch is that the lizards somehow remember visual landmarks they glimpsed as hatchlings.

Though adults tend to be solitary, young iguanas can be surprisingly gregarious, Burghardt has found. "When they’re young, they’ll lie with their heads resting on one anothers’ backs and their tails lying on different animals," he says. This sleeping arrangement may help juveniles stay warm, but Burghardt thinks it’s more likely to warn the dozing reptiles of nocturnal predators, which include crocodiles, ocelots, raptors and snakes. "If one iguana in the pile gets disturbed and jumps away, it alerts the whole group." He’s not prepared to say the lizards are displaying affection when they sleep together. "But that doesn’t mean it doesn’t feel good when they’re doing it."

A NEWLY HATCHED green iguana (left) is bright green and about 7 inches long. A full grown adult displays colors ranging from orange to blue-green, with patches of pink, green-yellow or reddish brown. Captured by remote-controlled camera, an adult female (left) peers into the breeding tunnel of another female.

Not all young iguanas live in groups, but Burghardt and his students in Panama have discovered that those that do grow more quickly than do solitary iguanas. Foraging as a group, he suggests, is an easier way to locate food (leaves, shoots, fruit and flowers, typically) than operating solo.

A more unexpected form of group behavior, discovered by former student Jesus Rivas, is brothers protecting sisters. Iguana hatchlings seem to be aware, perhaps by smell, of who their siblings are. "When a predator threatens a group of juveniles, males will actually jump on top of their sisters," Burghardt says. "They’re putting themselves at risk because if someone gets eaten, it’s more likely to be one of them."

He suspects this is inherited behavior. Any given female hatchling has a better chance of passing genes along to future generations than does any given male hatchling since, as adults, only a few high-ranking males within a population will do most of the mating. "So in evolutionary terms," says Burghardt, "it makes sense that young males should be more willing to put themselves at risk than females"—provided, of course, the benefiting female has many of the same genes as the male taking the risk. Burghardt wouldn’t be surprised if such brotherly altruism turns up in other reptilian species because, unlike birds and mammals—animals that usually live with one or both parents, which are protective—most reptiles are abandoned by their mothers before they hatch.

ADAPTATIONS enabling the green iguana to thrive in its habitat include (clockwise from top center): long fingers and claws, which allow the reptile to climb trees to eat leaves and escape predators; keen eyes that provide excellent vision; water-resistant skin, which—along with swimming and diving abilities—help the lizard escape predators and travel to nesting beaches; and the males’ spines, which turn bright orange during the breeding season and are used to threaten rivals. Green iguanas also have excellent senses of hearing and smell, and the coloring of their skin—tough enough to prevent cuts and scratches—helps the lizards blend into their surroundings, camouflaging them from predators.

Not that iguana moms are casual about their offspring’s future well-being. Like sea turtles and many other reptiles, green iguanas lay their eggs in underground burrows (in clutches ranging from fewer than a dozen eggs to more than 70). In Panama, Burghardt found that pregnant females are extremely picky about where they deposit their eggs. They prefer a tiny, uninhabited islet called Slothia, which they reach by swimming nearly a mile from the main island of Barro Colorado. There, year after year, females compete for nesting sites on the same cramped, 8-foot-by-10-foot patch of barren ground. To a human it may not look like much. To an iguana with eggs to lay, however, it’s perfect: lots of sun to help the eggs incubate, an absence of predatory mammals (though birds and crocs can take a toll) and sandy soil loosened by years of burrowing.

Burghardt once kept vigil for several weeks in a small wooden blind on Slothia next to the egg-filled clearing, waiting for hatchlings to emerge. One day, he finally spotted a tiny green head pop out of a hole and drop down again. More heads appeared at other holes, sometimes two or three together. Then one of the hatchlings, bolder than the others, crawled out, calmly stepped over to a neighbor, and licked its head as if to say, "You can come out now. The coast is clear."

As Burghardt watched in amazement, these young iguanas interacted repeatedly. "One would take the lead, and the others would follow. They’d lick each other and hang out together." On other occasions, he’s seen hatchlings scamper up to the top of a bluff and look out over the water as if scouting the best route and scanning for predators. "Then they’ll wander down to the shore together and swim across in groups," says Burghardt.

Watching iguanas alone and in groups, Burghardt says he’s noticed individual personalities—in the way an animal approaches a clearing, reacts when captured or digs a burrow, for example—which he takes to be evidence of cognitive complexity. "There’s a lot more potential for behavioral complexity in these animals than most people have given them credit for."

Burghardt and his team also have discovered signs of cognitive ability among monitor lizards, the iguanas’ carnivorous counterparts in the world of big lizards. "We’ve documented problem-solving in young monitor lizards and play behavior in Komodo dragons," he says. Where a leaf-eating iguana shows its mental agility mainly in bouts of inquisitive, exploratory behavior, a monitor lizard displays actual learning and manipulation of objects, as would be expected of a prey-chasing meat eater.

Next, Burghardt plans to focus his research on exactly what kinds of social bonds exist among juveniles hatched from the same clutch—and whether young iguanas that live in groups are related in the first place. "Using genetic testing, I’d like to check if these groups are siblings or just random groups of animals. Who’s related to whom? And how do female hatchlings behave differently, especially around predators?"

He’s counting on his study subjects being around long enough to finish the work. Though green iguanas still have healthy populations throughout most of their range, habitat loss and human predation—for food and the pet trade—are a threat to their future welfare. Over the long term, says Burghardt, greater understanding and appreciation of these ancient-looking reptiles may help save them from the fate of the dinosaurs they remind us of.

Doug Stewart has also written about natural history for Time and Smithsonian.

Adult Behavior
Sex and the Single Lizard

While juvenile green iguanas turn out to be unexpectedly companionable, adults tend not to mix. Males, which take five years to reach sexual maturity, try to establish territories they can defend during the month-long mating season. While guarding their turf, they’ll go for a month or more without eating.

A male iguana threatening a rival is an unforgettable sight. The spiny crest running along its back from neck to tail bristles impressively, while its dewlap—a large fold of skin under its throat—expands and turns dark. A male feeling "mannish" will also exhale loudly with a kind of hiss. An unwelcome intruder will either slink away or assume a similar posture, whereupon the two males may begin batting heads together. Female iguanas, too, will sometimes put on threat displays while defending a territory, although typically what’s at stake is a good sleeping perch or a contested egg-laying site.

Whether a male iguana is proclaiming a territory as all his or wooing a female with an attention-getting courtship display, a key part of the show is the head bob. Oddly enough, each male apparently has a unique way of bobbing his head. During courtship, females may judge a male’s fitness as a mate in part by rating how fetchingly he performs his routine. A male who passes muster may keep a harem of a half-dozen females in his territory during mating season. Females aren’t passive participants in the mating game, however. They frequently move from one male’s territory to another’s, and males have only a 50 percent success rate mating with females even within their territories, because they can mate just once a day during the breeding season.

Tough Customer

If a green iguana’s tail breaks off, or is bitten off by a predator, it grows back with no permanent damage.

Lizard Biology

Green iguanas are cold-blooded; they cannot produce their own body heat. The reptiles also are diurnal, or active during the day.

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