A Deep Dive Into Giant-Lizard Nests

After many futile hours of shoveling dirt under the scorching Australian sun, Sean Doody began to think that he had made an embarrassing mistake and was—quite literally—digging himself into a hole.

Doody is a herpetologist from the University of South Florida who has spent years studying Australia’s yellow-spotted goanna—a predatory monitor lizard with long claws, a whiplike tail, and a sinuous, muscular body that can reach five feet in length. Its range is as large as Europe but contains just 3 million people, so despite the goanna’s size, it is seldom seen and remains mysterious. Until recently, for example, no one knew where it laid its eggs. Doody spoke to Aboriginal Australians who would often catch pregnant females near what looked like burrow entrances. “But every time someone tried to reach in, they’d get up to their shoulder and hit a dead end,” he told me.

He found the same thing in 2012, when he and four colleagues dug into a goanna burrow themselves. From the surface, a tunnel that was only slightly wider than a muscular arm descended for about two feet before abruptly stopping. But when Doody traded his shovel for a spoon and gently pushed against the exposed dirt, he realized that the tunnel’s end was slightly softer than its walls. That meant the burrow was deeper than it seemed; the goanna that had created it had backfilled it with soil. Intrigued, Doody and his team dug deeper.

By the time they were three feet down, with nothing to show for their efforts, Doody was confused. When most reptiles bury their eggs, they do so less than a foot beneath the surface. Even big sea turtles dig shallow nests. But the backfilled goanna burrow kept going. Stranger still, its path began to twist, corkscrewing as it descended. “My partner, who is also a herpetologist, was on the phone, saying, ‘You know, Sean, reptiles don’t dig that deep,’” he told me. “I was starting to doubt myself.”

At five feet down, on the second day of digging, Doody was mentally composing an apology to his colleagues when one of them, lying flat on the ground with his head out of sight and his arm curled down the spiraling burrow, screamed out, “EGGS!” Doody and his team had finally found a nest; from it, they pulled out 10 eggs, all intact.

Since then, the team has analyzed dozens of goanna burrows, all of which had the same elaborate structure. A few animals also dig (or dug) helical burrows, including scorpions, pocket gophers, an extinct beaver called Palaeocastor, and a mammal-like reptile called Diictodon that lived 255 million years ago. But the yellow-spotted goanna’s nests are deeper than those of all these creatures—extending as far as 13 feet below the surface. “That’s a ridiculous depth,” Doody told me. He thinks that the yellow-spotted goanna faces a unique challenge. Its large eggs need to incubate for 8 months before hatching—a period that takes them through Australia’s brutal dry season, when several months might go by without any rain. At shallow depths, the eggs would cook and desiccate. Only in deeper soil, which is cooler and wetter, can they survive.

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To create her elaborate burrow, an expectant female goanna first digs out about two feet of soil and piles it in a nearby mound. Then she effectively swims downward, digging with her front legs while moving the loose sand behind her to backfill the tunnel she creates. This takes days, during which she is encased in soil and surrounded by just enough air to breathe. “We found one in the act, and she was lodged in there like a potato,” Doody told me. This might be why she moves in a corkscrew, he added: “Any sand that falls back down gets blocked by her body and the fact that she’s turning.” Once she reaches the right depth, she digs an open chamber the size of two clenched fists. She lays her eggs and digs her way back out, emerging seven to 10 days after she first submerged.

Several months later, when the baby goannas hatch, they ignore their mother’s helical burrow. Instead, they punch their own way out, scratching straight upward. “Sea turtles have to dig through nice, loose sand, and there’s 100 of them,” Doody said. “Here we have maybe eight hatchling lizards going through meters of resistant soil.”

The goanna burrows are not sparsely distributed. Many of these lizards nest together, creating large communal warrens. The team once found more than 100 separate clutches in an area the size of a small living room. The researchers’ reconstruction of the site looks like a spring mattress full of tightly packed coils. The goannas reuse these sites year after year, and as the separate burrows merge, collapse, and erode, the warrens become complex labyrinths full of open spaces.

These vacancies don’t stay empty for long. Doody’s team has found a diverse menagerie of animals sheltering and nesting within the goanna burrows. These include other lizards, snakes, scorpions, centipedes, beetles, ants, and a mouselike marsupial called the fat-tailed false antechinus. Many frogs endure the dry season by burying themselves in the backfilled burrows’ loose soil. “When we pulled out backfill, we’d pull out handfuls of frogs, over and over,” Doody told me. In one warren, he and his colleagues found 418 frogs. “The sheer number and variety of other animals relying on these burrows is astounding,” Jane Melville, a curator at Museums Victoria who studies reptiles and amphibians, told me.

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Goannas are typically characterized as top predators that will eat anything and everything they can manage. But Doody’s work shows that they create as well as destroy. They easily count as ecosystem engineers—animals that modify their environment in ways that create opportunities for the creatures around them. Beavers (which dam rivers), corals (which create reefs), and woodpeckers (which drill holes in trees) are all ecosystem engineers. But other than a few species of tortoise and turtle, reptiles are rarely considered among their ranks. “I think this may be due to their often-negative public perception,” Sophie Cross, a herpetologist at Curtin University told me. “Reptiles are not considered charismatic. They play important ecological roles, but these aren’t widely known.”

The yellow-spotted goanna, for example, is found across much of northern Australia, which makes it an engineer with “continent-wide importance,” Melville said. Doody thinks that other goanna species also reshape Australia’s landscape, including the country’s largest lizard—the eight-foot-long perentie. His student Kari Soennichsen has been radio-tracking these animals to find out where and how they lay their eggs. “It’s been miserable trying to find nests for these guys,” Doody said. “They’re secretive, and you have to be on them all the time.” And last year, as the team was gearing up for another field season, “COVID really kicked us in the butt.”

They can’t wait forever. Cane toads, which were ill-advisedly introduced to Australia in 1935, have been slowly spreading through the country’s northern region, and their poisonous secretions are lethal to would-be predators, including yellow-spotted goannas. In some areas, the lizards’ numbers have fallen by 90 percent. Doody started studying the species to learn the consequences of its disappearance, and his answer is more surprising and more galling than he could have anticipated. As the goannas vanish, so do the underground worlds they create.