In the jungles of Southeast Asia, it's not just the birds, bats and bugs that fly – some of the lizards take to the skies, too! They're called Draco lizards, and they have wide skin membranes supported by special elongated ribs on either side of the body, allowing them to glide through the air. For years, scientists thought these membranes simply opened by themselves, but an incredible new study has found that the lizards actually reach back with their arms and manually spread their "wings".
In March of 2015, Maximilian Dehling of Germany's University of Koblenz took a trip to the small Indian village of Agumbe, where he observed and photographed the lizards in flight.
His photos revealed that shortly after the animals leapt from trees, rib muscles began to unfurl their gliding membranes (called patagia) – but they didn't open them all the way. Remarkably, the lizards would then reach back with their arms (in mid-air!), grab the membranes and spread them all the way forward. When they neared their landing site, the lizards would let go and brace for impact.
Examining museum specimens up close, Dehling found that Draco lizards can perform this action because they are able to bend their wrists much more flexibly than even closely related species. This allows them to grasp firmly onto the patagia and complete their two-part "wings".
This, Dehling says, may allow the animals to use their forearms for aerial manoeuvring. "[These lizards] can show abrupt changes of direction in mid-air. There are even reports of specimens doing a barrel roll," he told me via email.
Precisely how the lizards guide their flight is still unclear. "The movements of the forelimbs certainly contribute a great part to the manoeuvrability, but other body parts likely contribute as well, including the hind limbs, the head, the tail and especially the [central body], which can be arched and straightened and also bent sideways."
How exactly the lizards use their arms in flight might be the biggest question, and Dehling's interpretations are sure to drum up discussion. The study must still go through peer review before official publication, so other experts will soon have the chance to weigh in and offer additional insights.
Draco lizards aren't the only gliders out there. Gliding geckos, for example, have their own patagia that work like parachutes, expanding passively when wind billows up under them. And in gliding mammals like flying squirrels, the membranes are attached to the limbs like a wingsuit.
But no other animal does it like Draco. If they are indeed using their arms for steering, this would make them the only aerial animals that catch the air with one body part and steer with another.
To investigate lizard aerodynamics a bit more, I spoke with Colin Palmer of the University of Bristol, who makes use of his background in naval architecture to study the evolution of flight (Palmer was not involved in this Draco study). He revealed that a wing with an arm at the front is actually less aerodynamically efficient than just a flat membrane. "But there are all sorts of benefits to having a limb at the leading edge of the wing," he explained, including not just the ability to steer, but also the benefit of added strength and support. "It's all a trade-off."
Keeping their arms and patagia separate is also beneficial when it comes time for running and climbing. These lizards get the best of both worlds: they have wonderfully adapted forearms for clambering up trees, unencumbered by any flappy membranes, but when it comes time for flying, their arm becomes the front of a glide-worthy wing.
The diversity of gliding creatures extends deep into the past, so this new research may also have palaeontologists reconsidering how they view gliders in the fossil record. Ancient reptiles like Icarosaurus had expanded ribs like Draco, while the unusual Coelurosauravus had its own unique wing bones. And then there's the delightfully bizarre Sharovipteryx, whose main gliding membranes were not on its arms but on its hind legs!
In 2008, Palmer and his colleagues examined the aerodynamics of two of these prehistoric gliders, named Kuehneosaurus and Kuehneosuchus. The team's interpretations, he says, may have been slightly different had they known these latest findings. "It suggests they were maybe more aerially capable that we would have thought," he told me. "Our models had the limbs held forward [away from the patagia] because that's what we thought modern lizards do."
More work remains to be done for us to fully understand just how Draco lizards soar. According to Dehling, further research will be needed to explore how these lizards perform aerial manoeuvres, and how different body parts contribute to their mid-air acrobatics. This work, he said, will require experimental approaches. I, for one, hope those experiments come with more photos!
Top header image: Patrick Randall, Flickr