Herps in the Sky? – Part 2

True flight is something that is restricted to birds, bats and insects. It is distinguished from gliding and parachuting in that flying animals are able to produce thrust, to sustain their upward path. This is done by way of the “flight stroke”. It is an important thing to note that no herps are capable of true flight. However, some herptiles have evolved ingenious methods of gliding to get around their respective habitats.

This is Part 2, of the four-part series of posts, we discuss the various South-East Asian herps that are capable of gliding or parachuting. Click here for Part 1!

PART 2 – GLIDING GECKOS

IMG_6964
A Kuhl’s Gliding Gecko in Bintan. Photo Credit: Law Ing Sind

Before this discussion proceeds, it must be noted, that there is a difference between true gliding and parachuting. Parachuting simply means a descent that’s slowed by increasing surface area exposed to air resistance. In contrast, gliding requires more direct control over the aerial locomotion. The distinction is arbitrary, but it is generally agreed that a descent with an angle lesser than 45° is considered gliding, while an angle greater than 45° constitutes parachuting[1].

Geckos are fantastic climbers and they have amazing anatomy that enables them to climb almost any surface. But even then, geckos do fall. Interestingly, much like cats, geckos are able to right themselves in mid-fall to ensure that they land on their feet. The time it takes for them to right themselves is the fastest air-righting response ever recorded! A group of researchers from UC Berkeley actually put this to the test! In this brilliant TED Talk by Prof. Robert Full, he describes how the research team discovered some pretty amazing things.

SO HOW DO THEY DO IT?
The researchers conducted experiments on the Flat-tailed House Gecko (Hemidactylus platyurus), one of the most common geckos in South-East Asia. They discovered that the geckos (much like the Flying Snakes we discussed in Part 1) followed a series of action to right themselves[2].

Step 1: Upon losing grip of a surface, the gecko adopts a spread-eagle posture.

Step 2: The gecko flicks its tail, so that it points downward, forming a right angle to the rest of the body.

Step 3: The gecko then turns the tail around the axis of its body, until it points upwards. Due to the Conservation of Angular Momentum, the body turns in the opposite direction.

Step 4: Once the gecko has made a 180° turn and it’s right side up, it stops turning the tail and continues falling in spread-eagle posture.

It was initially thought that these geckos were simply slowing their rate of descent (i.e. parachuting). However, experiments have shown that they are actually capable of some degree of controlled gliding! Such behaviour has been observed in several genera, including HemidactylusLuperosaurus and most notably, Ptychozoon.

Ptychozoon, the Gliding Geckos, is a genus of highly arboreal geckos that are endemic to South-East Asia. Out of the eight species that have been described thus far, only one, Kuhl’s Gliding Gcko, (P. kuhli), can currently be found in Singapore. However, it has only been recorded on Pulau Tekong. (Historically, Horsfield’s Gliding Gecko (P horsfieldii) has been recorded on Mainland Singapore, but has since been presumed extinct.)

Photo of a Kuhl's Gliding Gecko by Bernard Dupont
Photo of a Kuhl’s Gliding Gecko from Perak.  Image Credit: Bernard Dupont, Flickr
Despite their rarity, research has been done on this group of cool (pun intended) geckos! As can be seen in the above photos, their bodies are covered in extensive fringing, including large flaps (patagia) on their sides and webbing between their fingers. This increases the surface area of their body that is exposed to air resistance. In turn, this effectively slows their descent to the ground. It is noteworthy that the patagia of the gliding geckos are passive and are not structurally supported, as they are in most other gliding vertebrates[3].
But the geckos do not just fall with style. Several studies done on P. kuhli and its sister species have demonstrated that the geckos are actually capable of gliding. One group of researchers noted that the geckos often begun with a vertical drop, but adopted a gliding posture after falling about 1-3m. The gradient of the glide would then decrease before rising slightly and eventually landing[4].
SO WHAT?
In the TED Talk above, Prof Robert Full talks about Biomutualism and Biomimetics. Often, human innovation is inspired by nature. And sometimes natural curiosity is generated by means of innovation. Multiple fields advancing one another in a reciprocal fashion. Engineering that is inspired by biology cannot be sustained without conserving the natural blueprint. If these animals go extinct, there is nothing that can be learnt from them. So the organisms from which these ideas are drawn are invaluable. And it’s important to preserve them.
REFERENCES
[1] – Oliver, J. (1951). “Gliding” in Amphibians and Reptiles, with a Remark on an Arboreal Adaptation in the Lizard, Anolis carolinensis carolinensis Voigt. The American Naturalist, 85(822), 171-176. <link>
[2] – Jusufi, A., Goldman, D., Revzen, S., & Full, R. (2008). Active tails enhance arboreal acrobatics in geckos. Proceedings of the National Academy of Sciences, 105(11), 4215-4219. <link>
[3] – Russell, A., Dijkstra, L., & Powell, G. (2001). Structural characteristics of the patagium of Ptychozoon kuhli (Reptilia: Gekkonidae) in relation to parachuting locomotion. Journal of Morphology, 252-263. <link>
[4] – Marcellini, D. L., & Thomas E. Keefer. (1976). Analysis of the Gliding Behavior of Ptychozoon lionatum (Reptilia: Gekkonidae). Herpetologica, 32(4), 362–366. <link>
MORE INFORMATION
[1] – Ecology Asia Factsheet on Smooth-backed Gliding Gecko <link>
[2] – LKCNHM Factsheet on Kuhl’s Gliding Gecko <link>
[3] – Tetrapod Zoology Blogpost on Ptychozoon <link>
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