Falling Leaf, Flying Dragon

In the canopy of a Malaysian rainforest, a little lizard scuttles to the end of a branch and launches itself into the air. It doesn’t, however, fall to its doom. Instead, it extends two flaps of skin from its flanks, supported by unusually long ribs. The flaps look and work like wings, allowing this lizard—the aptly named flying dragon—to glide to safety. They are so adept in the air that they almost never come to the ground. Why bother, when they can travel for 20 to 30 metres between treetops, without losing much altitude?

There are 42 species of flying dragons, or Draco as they are formally known, and they all glide on extended flaps of skin or patagia. But Danielle Klomp from the University of New South Wales thinks that there’s more to the patagia than gliding. They are also beautifully coloured and Klomp has shown that, in at least one species, these hues match those of falling leaves from the local area. This, she says, is no coincidence. She thinks that the lizards have evolved to mimic falling leaves, to avoid the attention of birds.

“The locals we would chat to would often describe the lizards as looking like falling leaves,” she says. “We spent a lot of time walking around the forest trying to find them, and we often confused a gliding lizard for a falling leaf out of the corner of our eyes.” She would also find fallen leaves on the floors of many different rainforests that looked like the patagia of the dragons that lived in the area. This called for a more systematic study.

Klomp focused one species—Draco cornutus—which lives in Borneo and comes in at least two varieties. The individuals that dwell in coastal mangrove forests have rusty red patagia, and the dominant trees there jettison similarly coloured leaves. Elsewhere, in the lowland forests, the lizards’ patagia are a dark greenish-brown, and so are the falling leaves of the local trees.

The resemblance is striking to human eyes. To quantify it, Klomp collected both dragons and leaves from the two forests, and analysed the light reflecting from all of them. She showed that the contrast in colour was smallest when she paired the dragons with falling leaves from their own habitat, and higher when she compared them to standing leaves, or falling leaves from a different area.

Flying dragons glide around four times an hour and although they excel at it, they aren’t more manoeuvrable than birds. With plenty of hungry beaks around, it behoves them to have some way of avoiding attention. Mimicking falling leaves is one possible solution and not a far-fetched one, either. Some birds might do the same. The black fairy hummingbird, for example, does a weird gliding flight whenever it leaves its nest. It opens its wings and tail so that its body is horizontal to the ground, and it spins on its way down, recovering just a couple of metres before crashing. The movement looks a lot like a falling leaf.

Other scientists have suggested that the flying dragons use their brightly coloured patagia as billboards for signalling to mates. But Klomp’s team have filmed many of these lizards in the wild, and their 30 hours of footage rarely shows the animals using their wings in displays.

But absence of evidence isn’t evidence of absence, and Jim McGuire at the University of California in Berkeley, who has studied these lizards extensively, has often seen the males displaying with their patagia (here’s some video). They’ll sometimes open just the wing that’s closest to the female.

Other lines of evidence support the idea that the dragons communicate with their patagia. In most species, males have more vividly coloured wings than females, even though both sexes would presumably benefit from mimicking leaves. The colours are almost always species-specific too, and different species with distinct colours often live in the same area amid the same trees.

“There’s no doubt in my mind that patagial colours play an important role in species recognition,” says McGuire. “If it’s possible to evolve a colour pattern that would at once be conspicuous to [other Draco individuals] and simultaneously cryptic to predators, this would be a win-win. However, it’s also possible that Draco could be mimicking something other than leaves, like unpalatable [stick insects] or butterflies. And, of course, Draco may not be mimicking anything at all.”

To support her hypothesis, Klomp needs more data. So far, she has only compared wings and leaves in two populations of Draco from one species. Anecdotally, she has seen that several other species resemble like their local leaves but “this needs to be done properly,” she says. She also wants to test her prediction that species that live in more open habitats, or in places with a single dominant tree species, might benefit more from mimicking leaves.

Reference: Klomp, Stuart-Fox, Das & Ord. 2014. Marked colour divergence in the gliding membranes of a tropical lizard mirrors population differences in the colour of falling leaves. Biology Letters http://dx.doi.org/10.1098/rsbl.2014.0776