The rainforest is full of reptilian acrobats; L. LEE GRISMER explains how lizards, snakes, and geckos take flight under the canopy to escape prey and survive in the jungle.

Written by L. Lee Grismer on 16 Jan 2007 with 0 comments. Be the first!

PART I - REPTILIAN FLIGHTS OF FANCY OR SURVIVAL?

I can't count the number of times I've said to myself, "I sure wish I could fly". In particular, I remember standing on the very top of Gunung Kajang on Malaysia's Tioman Island just after a miserable, leech-infested, three-day climb to the summit. I was looking across a deep valley toward the twin peaks of Nenek- Semukut wishing I could spread my wings, glide between the peaks across the valley, land on the beach at Mukut, and crack open a nice, cold 100 Plus. I did not want to hike back down that mountain. Well, as it turns out, I'm not alone.

Flying has been a preoccupation with humans for time on end and it was certainly up there on the "to-do list" of Orville and Wilbur Wright around the turn of the 20th century. Sadly though, humans will never acquire the capability of unassisted flight and we'll have to continue appeasing and teasing ourselves with everything from hang-gliders to space shuttles.

In other vertebrates, however, flying is really no big deal. Some fish leap out of the water to escape predators, extend their enlarged pectoral fins, and glide for hundreds of yards. Many frogs have extensively-webbed, elongate fingers and toes that function as parachutes when they leap from the leaves and branches of trees to glide across the forest.

Several groups of mammals ranging from bats, to rodents, to colugos, have evolved many different ways to move through the air. And reptiles have been flying for millions of years. The earliest fliers were the Frigate Bird-looking pterosaurs. They were also the first reptiles capable of powered flight and ranged in size from chickens to giants having wingspans of over 16 metres. But of course the most successful flying reptiles of all time, are (not were) the dinosaurs and their modern-day representatives - birds. In Southeast Asia, however, the ability to fly in modern, non-avian reptiles has gone out of control and has evolved independently at least three, maybe four times in lizards, and once in snakes.

But why flight, and why here in Southeast Asia? Well, there are a number of hypotheses and we'll get to a few of them later on. But the over-arching theme throughout all of them is predator evasion. Think about it: you're less than six inches in body length, you have no formidable defenses, and you're living in a jungle. By default, that makes you some of nature's "fast-food". So if you want to avoid becoming the next McLizard in the Rainforest Happy Meal you had better develop an escape mechanism.

This always seemed logical to me but it really hit home one afternoon while I was taking a break along the banks of the Sungai Mentawak on Tioman Island. I was just about to resume my hike when I noticed a Bronzeback Snake (Dendrelaphis caudolineatus) crawling up the trunk of a large tree. This snake's ability to climb on flat, vertical surfaces had always fascinated me so I decided to watch for a while. Its coils were purposely looped in just the precise way so as to gain purchase on the irregularities of the tree's surface while it continued to effortlessly push itself upwards. Then I noticed it was ever so slightly moving its head from side to side and rapidly flicking its tongue - the telltale signs it was scent-trailing a lizard.

At that very moment, I saw a Black-bearded Gliding Lizard (Draco melanopogon) leap from the tree and glide through the green, filtered light of the forest to land on a different tree. I thought to myself "How cool is that, it really works". Then suddenly, the snake struck and grabbed a Spotted Forest Skink (Sphenomorphus scotophilus), which I didn't even see, right off the side of tree. While the Bronze-back Snake gnawed and manipulated the Forest Skink in its mouth to position it just-so before swallowing, it dawned on me that this particular little skink and I had something profound in common - and I swear I could hear him saying, "I sure wish I could fly".

In Southeast Asia, the Agamidae, the family to which Gliding Lizards belong, are arboreal, diurnal, conspicuous insectivores who signal to one another by puffing out their throats and expanding their chests so as to display their brilliant colour patterns. They also jump from branch to branch in search of prey and to escape predation. So, in a sense, they can already do everything necessary to fly-jump from one place to another and expand their rib cage. Now all that's necessary is to do them both at the same time and let natural selection run its course.

In support of this hypothesis is a remarkably obvious intermediate condition in the Green Crested Lizard (Bronchocela cristatella), a common, arboreal agamid in Southeast Asia. When threatened, Green Crested Lizards will leap from one tree to next, splay out their limbs, and expand their rib cages during flight. Although they have no trace of a wing, experiments demonstrated that increased surface area resulting from these alterations significantly decreases the angle of the drop and extends the length of the "glide". So it's not too difficult to see how natural selection may again, within the same selection regime and same related group of lizards, be developing flight in the Green Crested Lizard.

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