Why Komodo Dragons Haven’t Conquered the World

Komodo dragons live on a handful of islands in Indonesia, but their reputation has spread far and wide. Reaching lengths of up to 10 feet, the razor-toothed monitor lizards hunt deer in packs and have even attacked humans on occasion. But actually, they are real homebodies, according to a study published Wednesday in Proceedings of the Royal Society B.

A decade of observations at 10 sites on four islands has revealed that the dragons essentially never leave the valley where they were born. It is not that they can’t. They are capable of traveling many miles and through rough terrain, if necessary. They just do not seem to feel like it.

Many island species, as it happens, show a marked tendency to stay close to home. It feels like a bit of a paradox: Their forebears may have arrived on that island through some great feat of survival or exploration, but the present generation prefers familiar comforts.

“Once they colonize an island, despite these incredible feats of long-distance dispersal, they decide, ‘Enough is enough!'” said Tim Jessop, professor of ecology at Deakins University in Australia who led the study.

The causes of this behavior are likely to differ depending on the species and the situation. But it is puzzling: If animals stay in one place for many generations, they run the risk of inbreeding, facing resource scarcity and other dangers that moving elsewhere could allow them to avoid.

Is the problem that komodos are not confident navigators? Over the course of the study, researchers moved seven adult dragons from their home territory. Some were transported as much as 13.7 miles away on the same islands, while others were ferried across a slip of water just over a mile wide to another island.

Within four months, the Komodo dragons transplanted overland all turned up again at home, clearly capable of making a journey. The dragons on the new island — much closer to where they started, and capable of swimming back — stayed put. Swimming home just did not seem to be worth the effort, apparently.

One explanation for this sedentary behavior, Jessop proposed, is that once you are isolated on an island, any mistakes could be extremely costly. Having a whole continent to move across, with a landscape that changes relatively slowly, would make exploration less risky.

But a Komodo dragon that moves to a new island or a new island valley might find that it is out of luck if, for instance, it is unable to mate with any of the locals it encounters in its new home. There may also be survival benefits to being intimately familiar with one’s surroundings, like knowing exactly where to find prey.

That said, DNA data indicates that dragon populations show signs of inbreeding, and they are vulnerable to local shortages of food and natural disasters.

“They stay put almost irrespective of how bad it gets,” Jessop said. “It’s a bit bewildering.”

And yet Komodo dragons have their ways of making this life work.

“They are quirky animals,” Jessop continued, noting that the juveniles have a habit of climbing trees to escape cannibalism from their elders. They may look like ruthless top-level predators, but their goals as island creatures are considerably more modest that you may realize.

“Really what they’re trying to do,” Jessop said, “is not rock the boat.”