by Whit Gibbons

May 28, 2017

During the last quarter of a billion years or so, a variety of vertebrate animals have excavated deep, spiral-shaped burrows into the ground. Some of them went several feet straight down in a helix.

Early paleontologists called them “devil’s corkscrews” and spent many years speculating on the origin and purpose of these mysterious structures. The earliest reports are from the 1890s.

As often happens with new scientific discoveries, these odd constructions that had no modern counterpart prompted various explanations as to their origin and purpose. Among the proposals: They were the tunnels of giant sponges; they were passageways left by some form of vine; they were animal burrows. Almost a century after their discovery, spiral burrows in Nebraska were confirmed to have been made by an extinct beaver.

Lots of animals living today, including ants, mice and prairie dogs burrow into the ground. Almost every turtle in the world digs a nest in sand or dirt to lay its eggs. According to Bob Zappalorti, who has studied pine snakes living in sandy habitats in New Jersey, these big snakes dig horizontal tunnels more than 6 feet long and a foot deep to make a chamber for laying their eggs.

The gopher tortoises of the Southeast are noted for their underground burrows, to which they retire at night and during cold weather, that often extend more than 20 feet horizontally. But the true purpose of the enigmatic corkscrew tunnels of long ago kept scientists bewildered for decades after their discovery, in part because no living equivalent existed for investigation. But thanks to Sean Doody of Southeastern Louisiana University and his studies of a giant Australian lizard, scientists now know what kind of animal would dig a spiral staircase beneath the desert.

Sean has studied the ecology and behavior of yellow-spotted monitors in Western Australia. These enormous lizards, which are related to Komodo dragons, reach lengths of more than 5 feet. I remember seeing one in northern Australia standing on its back legs and supported tripod-like by its tail as it looked around at the customers in a restaurant with outdoor seating. I decided to eat inside.

Sean recently showed me photographs of what looked like an archeological dig in desert habitat. He and his colleagues were using shovels to carefully excavate a large area, several yards on each side and above their heads to investigate the nesting habits of the big lizards.

Sean and his crew were rewarded for their meticulous quarrying efforts by discovering corkscrew-shaped tunnels going straight down below the desert floor. At the bottom, they found a nesting chamber. They excavated and measured 52 of these spiral tunnels, which were capped at the top, preventing the entry of would-be predators. According to their scientific paper in the Journal of the Linnean Society, they are the “deepest known of any vertebrate, and by far the deepest of any reptile.” The deepest ones went straight down for more than 11 feet, like spiral steps in a lighthouse.

The research team concluded that egg survival is the explanation for the unusual architecture. The lizards nest in a region and season of near perpetual drought, with egg incubation taking 8 months. Desiccation is a continual threat to eggs in such an environment, and a deep nest chamber solves that problem. Also, upon hatching, the young lizards can make their way to the surface without being buried alive or expending energy digging out.

The researchers concluded that the mystery of the ancient corkscrews discovered more than a century ago has been solved. Devil’s corkscrews “were used mainly for nesting or rearing young, because helical burrows of extant vertebrates are generally associated with a nest.”

Imagine the time and effort needed for an animal to dig a long spiraling tunnel deep into the ground. Another of Mother Nature’s wonders. Imagine, too, the training, skill and dedication of scientists like Sean Doody and colleagues who help solve the mysteries of the natural world.

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