Meters beneath the copper and paved land of northwestern Australia, an entire community hides in the darkness. Geckos lay their eggs while centipedes and scorpions form. A snake slides deeper underground, away from the light. This underground household is taking advantage of an old burrow, uprooted from the ground by a huge lizard.
Now, a new study shows that two different species of Australian lizards dig matrices of these burrows in the ground and that the openings have a major impact on local biodiversity, providing shelter for a surprisingly wide variety of animal life. The findings, published Dec. 18 in Ecology, indicate that lizards are "ecosystem engineers," similar to beavers flooding streams with prey or seabirds that fertilize reefs with their guano, researchers say (SN: 7/11 / 18).
Sean Doody, an ecologist at the University of South Florida in St. Petersburg, began monitoring cat-sized lizards in northern Australia alongside colleagues at the Australian University of Canberra in Bruce and the University of Newcastle. The team tracked how invasive, poisonous cane frogs were negatively impacting reptiles.
Until recently it was not clear where the monitor lizards lay their eggs. Reaching burrows that were thought to contain their eggs produced nothing. Then Doody and his team began digging burrows from the yellow-spotted monitor (Varanus panoptes) and discovered that the holes had a tight helical shape, dipping into the ground about four feet – deeper than any other known vertebrate nest – with eggs at the bottom. . What’s more, the nests were part of a warren made up of dozens of twisted burrows, each made by a single monitor and arranged on the ground like dozens of vertically placed fusilli noodles.
“We kept digging up these things and started finding a lot of animals in most of them,” Doody says.
The team found arthropods, snakes, toads and other lizards in the nests of yellow-spotted monitors and goanna sand monitors (Varanus gouldii), which dig similar burrows. At first there were some creatures here and there, Doody says, but then the team found 418 Uperoleia frogs in a single warren. In all, the team found nearly 750 individuals from 28 different vertebrate species in a combination of 16 warriors made up of many individual nesting burrows and a handful of forage burrows, made when lizards dig for prey.
Some animals are using burrows to overwinter, Doody says. Others use them as shelters when creatures need to sleep during the hot dry summer. Others still catch prey there, while "probably some hide from predators. And some even lay eggs in the burrow."
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Surprisingly, Doody says, he and his colleagues found very few mammals using the burrows. With the “massive reptile smell” there they can walk away, he says.
The variety of non-mammals that use burrows is “incredible,” especially given the wide appetite of reptiles, says Sophie Cross, an ecologist at Curtin University in Perth, Australia, who did not participate in the research.
“(Monitors) will eat pretty much anything they can grab or dig from the ground,” she says. "I'm surprised so many animals use these burrows, since many of them would be easy prey for a lizard."
If smaller residents use the burrows at a different time than the monitors, the two groups can avoid conflicts. It looks like the monitors lay eggs over a few weeks and leave, letting them incubate during the eight-month dry season, Doody says.
Given the widespread use of burrows by wildlife, Doody is concerned about the broader ecological effects of the invasion of cane frogs in tropical northern Australia. Monitor lizards, naive to the powerful toxins of frogs, will eat amphibians, with lethal consequences. As a result, monitors are dying quickly, Doody says, and their wars are filling up, leaving less shelter for other animals using burrows. "You pass hundreds of animals using a warren system to zero."
In the future, Doody wants to investigate why some animals make helical burrows in the first place. The practice is rare, with creatures such as beach crabs, some extinct rodents and pocket goblins as some of the other examples.
Along with the new study, that research may be crucial to changing the public perceptions of reptiles, which can be slandered out of fear, Cross says. "It's great to see research like this highlight the importance that (reptiles) can have on ecosystems."