Back in the late 1800s, scientists stumbled on mysterious, deep, helix-shaped burrows that they dubbed Daimonelix, or devil's corkscrews. Some of these date back to 255 million years, and ideas about who the excavators were ranged wildly from giant sponges to plant vines to beaver-like critters called Palaeocastor, New Scientist explains. Eventually, skeletons and claw marks indicated that it was, in fact, the extinct beaver from 20 million years ago. But without a modern counterpart, we’re still not sure why these deep, spirally burrows were created.
Well, déjà vu! Researchers have spotted structures similar to devil’s corkscrews at the northern end of Western Australia, and they were made by the yellow-spotted monitor lizard (Varanus panoptes). Not only are these the first helical reptile burrows known, they’re also the deepest nests ever made by vertebrates. The findings were published in the Biological Journal of the Linnean Society last week.
A team led by Sean Doody from the University of Tennessee, Knoxville, studied 52 of these monitor lizard burrows. They’re plugged up with soil, and at the very end, there’s a nest chamber for eggs. The burrows ranged from one to 3.6 meters (3 to 12 feet) deep, with the average at 2.3 meters (7.5 feet). For comparison, giant sea turtles and crocodilians, New Scientist reports, nest only about half a meter (1.5 feet) below ground.
They found that, after the top meter, the soil in the burrows become more moist with depth. That suggests that deep nesting may be an evolutionary response to egg desiccation during the monitors’ 8-month incubation period, which goes into the dry season. Or perhaps the lizards may be avoiding shallow nesting because of the temperature fluctuations over the course of the day. Even small changes could be detrimental to developing embryos. “Our data show that this species may have the most stable incubation environment of any reptile,” they write.
The team thinks that prehistoric helical burrows were also used for nesting or rearing young. But why a corkscrew? "It is costly to build a helix compared to excavating a straight burrow, so there must be a pay-off," Doody tells New Scientist. It may have to do with deterring predators or promoting drainage during heavy rainfall in the early part of the nesting season. For now, it remains a mystery.