Who thought snakes and geckos could fly? L. LEE GRISMER concludes his look at the flying reptiles of Asia's tropical rainforests in this final chapter on the unexpected aerodynamic skills of snakes and geckos.

Written by L. Lee Grismer on 10 Mar 2007 with 0 comments. Be the first!

PART III - THE FLIGHT DYNAMICS OF GECKOS AND SNAKES

Gliding Lizards aren't the only flying, reptilian denizens of Asia's rainforest. Geckos have proven there's more than one way to make a lizard wing. And as if geckos weren't weird enough already with their big heads, bulging lidless eyes, ability to vocalize, and elaborate hands and feet, many geckos can add flight to their résumé.

As exemplified in Flying Geckos (genus Ptychozoon), their wings lack the elaborate thoracic (chest) modifications of Gliding Lizards and are composed solely of a large flap of skin along their flanks. These flaps remain rolled across the belly until the lizard jumps off a tree at which time they become passively opened by air during the descent. Accompanying the body flaps are extended flaps along the sides of the head, neck, and tail; back sides of the hind limbs; and extensive webbing on the hands and feet.

During flight, when all the flaps and webbing are splayed and extended, they serve to provide more of a parachuting effect rather than generating lift as does the wing of a Gilding Lizard. Nonetheless, I have seen Kuhl's Flying Gecko (Ptychozoon kuhli) display remarkable in-flight maneuverability and actually change directions 180° during flight. Very similar flying surfaces have evolved independently in an unrelated species known as the Frilly Gecko (Cosymbotus craspedotus). The Frilly Gecko is smaller than most Flying Geckos and has not been observed to demonstrate the same in-flight maneuverability or ability to glide from tree to tree. It usually uses its parachuting capabilities to glide from one section of a tree to a lower section of the same tree to avoid predators.

Geckos (family Gekkonidae), however, are cryptic species that attempt to go undetected during the day and are active at night. So all the displaying and leaping about common to many arboreal agamids, does not happen in geckos. In fact, their color patterns generally match the substrate on which they reside enabling them to go unnoticed.

Interestingly, the Flat-tailed Gecko (Cosymbotus platyuurus), a species closely related to the Frilly Gecko, presents us with another example of intermediacy. It has the same folds of skin along the head, body, limbs, and tail as the Frilly Gecko but they're not nearly as well developed. It lays these flaps out on the trunk of the tree to prevent the curvature of the body from casting a shadow where it contacts the trunk, so as not to give away its location. If these flaps passively open up like they do in other geckos when this gecko jumps from one branch to another and this imparts even a small advantage by extending the length of the jump, then it will be selected for. This is a likely example of what evolutionary biologists refer to as preadaptation—where a structure evolves to be used in one context (camouflage) and becomes modified later on to be used in a completely different context (flight).

Because of their lack of limbs and other appendages, one might think that the most unlikely group of vertebrates to evolve flight mechanisms would be snakes. But in Peninsular Malaysia, there are three, closely related species of snakes that have the ability to glide for considerable distances. These are the Tree Snakes (genus Chrysopelea).

Their flight modification involves a simple little notch on each side of each belly (ventral) scales where their ends turn upwards along the lower sides of the body. This notch works like an expansion joint, allowing Tree Snakes to flatten out and widen their bodies. The flat, wide body works sort of like a parasail and its undulatory movements during flight, analogous to a spinning frisbee, stabilize the body to prevent it from overturning. Prior to launching themselves from branches, Tree Snakes hang the uncoiled forepart of their body off the branch in a "J-loop". Then, by rocking the body upward while simultaneously springing outward by rapidly straightening its coils and releasing their hold on the branch, the snakes take flight.

Intermediate conditions to this style of flight are found in related, semi-arboreal snakes such as Bronzebacks (genus Dendrelaphis) whose notched ventral scales help anchor them to the substrate while climbing vertical surfaces. They also expand their rib cage as a defence mechanism to expose brightly coloured markings on their scales. So the three main ingredients; arboreal behavioral, notched ventral scales, and a behavior involving the expansion of the rib cage, already existed in this group of related snakes. Perhaps this is another example of preadaptation.

So back to the earlier questions of "why flight, and why here in Southeast Asia?" I think we've pretty much answered, "why flight" so lets look at "why in Southeast Asia?" With the exception of birds and bats, when you compare the number of flying vertebrates in Asian and Australasian rainforests to those in the world's other major rainforests ecosystems, namely Central and South America and Africa, a striking pattern emerges—these other rainforests essentially have no flying vertebrates.

There are no really solid or generally agreed upon hypotheses as to why. But the most common explanation suggests that it has to do with Asian rainforests lacking the extensive system of vines and understory vegetation that in other rainforests connect adjacent trees to one another. Additionally, the crowns of trees are often discontinuous. In fact, in species such as the Kapur (Dryobanalops aromatica), the crowns of adjacent trees repel one another. Therefore, it is more difficult for arboreal species to get from one tree to the next without coming down to the ground, walking across the forest floor, and running the risk of being preyed upon.

So they fly.

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