July 17, 2009

Neon Blue-Tailed Tree Lizard Falls Like A Feather

Scientists have discovered that a tiny lizard drifts slowly to the ground like a feather when it falls. 

Looking from the outside, the neon blue tailed tree lizards (Holaspis guentheri) seem adapted to flying, gliding or moving through the air as they go from branch to branch in the trees of the African forest.

There have even been stories about the African tree lizard gliding through the air. However, lacking the apparent adaptations necessary to upgrade their moves from a leap to a glide made it unclear if and how the reptiles stay aloft.

Fascinated by the aspects of lacertid locomotion, Bieke Vanhooydonck from the University of Antwerp along with her colleagues, Anthony Herrel and Peter Aerts, decided to find out whether the lizards really glide.

They may be strikingly colorful, but in terms of their behavior, morphology or ecology they are not quite so unique.

"Also, compared to other gliding lizard species, it does not have any conspicuous morphological adaptations to an aerial lifestyle, i.e. no cutaneous flaps, webbed feet etc," says Vanhooydonck.

"It made me very curious about whether these animals were really able to 'glide' and if so, how they were accomplishing it."

Once they brought undergraduate Greet Meulepas on board, they started filming the neon blue tailed tree lizards, gliding geckos (Ptychozoon kuhli) and the common wall lizard (Podarcis muralis). They observed the animals leaping from a platform 2 yards high to see if they could catch the neon blue tailed tree lizards take to the air.

Vanhooydonck and her colleagues publish their discovery in the Journal of Experimental Biology.

Unfortunately, filming the lizards turned out to be an incredibly difficult task.  

The team would startle the small animals into leaping off the platform, but then they were unable to control the animal's direction, meaning it may not be in view of the camera. But after weeks of persistence the team finally collected enough film.
Initially, the African lizard did not set itself apart from the common wall lizard in terms of its ability to glide. They both had the ability to cover horizontal distances of half a yard, while the gliding gecko could clear distances over a yard with the help of its webbed feet and skin flaps.

After taking the size of the lizards into account, there was indeed a huge difference in the wall lizard and the tree lizard.

The tiny tree lizard only weighs about one-third of the larger common wall lizard's weight and has one-tenth of the gliding gecko's mass.

After learning this, Aerts was then able to calculate the distance each lizard would travel horizontally if they fell like a stone. It became obvious that the tiny tree lizard was traveling 0.2 yards farther than would be expected if it were simply jumping off the platform. The tree lizard's fall was undeniably slowed and it landed much slower than the wall lizard.

The tree lizard was indeed gliding "“ but how?

One possibility they considered was that the lizard flattens itself as it glides to increase its surface area and give it more lift. The problem with that theory is that once the team analyzed the trajectories of the lizard, the tree lizard's shape was unchanging. And when Aerts calculated the amount of lift each lizard got as they descended, it was clearly impossible for the lizard to generate enough force to produce that kind of a lift. 

The team then realized that rather than increasing its surface area to generate lift, the tree lizard can glide simply because it is so incredibly light. The tree lizard's 'wing loading' (mass to surface area ratio) was identical to the gliding gecko, which uses skin flaps and webbed feet. So it is definitely its incredible lightness that gives it "wings".

Herrel contacted Renaud Boistel, Paul Tafforeau and Vincent Fernandez at the European Synchrotron Radiation Facility to scan all three lizards' bodies to see what makes the tree lizard so light.

When they saw the skeletons of the individual animals with X-rays, they found the bones of the tree lizard to be full of air spaces, making its skeleton light as a feather and perfect for gliding.

Although the lizard's light weight and ability to fall gently are connected, it remains unknown whether its air-filled bones are an adaptation for parachuting, or whether they evolved for another reason.

It is also unclear whether H.guentheri glides from tree to tree to escape predators or move about more efficiently.

"Because of [the lizards'] secretive lifestyle, it is very hard to observe them in the wild, but it seems plausible they use it as an escape response," says Vanhooydonck.

And that could be just how other gliding animals took the first evolutionary steps towards an aerial lifestyle, she says.


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