Written by L. Lee Grismer on 12 Feb 2007 with 1 comment. Contribute!
PART II -WING SURFACE AREA AND FLIGHT PATTERNS
A Black-bearded Gliding Lizard (Draco melanopogon) leapt from a tree and glided 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, a 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".
What made that crucial life or death difference between the skink and the gliding lizard was a wing. In fact, gliding lizards (genus Draco) are Southeast Asia's most adept flying reptiles and their wings have clearly contributed to their widespread distribution (India to eastern Indonesia) and radiation (over 40 species).
Oddly enough, the Draco wing is just a marvelously rearranged rib cage. The opposing ribs, whose ends normally meet in the center of the body to form a bony basket (i.e., rib cage) to protect the heart and lungs, have become disconnected. The ribs have lengthened and now lie bunched-up alongside the body when not flexed. Muscles in the chest that normally lift and elevate the rib cage to draw air into the lungs, now pull the ribs forward and outward, opening up the wing. The amount of skin along the sides of the body has increased and now functions as the flying surface of the wing that the lengthened ribs support. Although this wing is not capable of generating power, it does provide lift to extend the length of the glide and provides considerable in-flight maneuverability.
Gliding lizards use their wings for more than just flight.
These expansive surfaces also function as prime advertising space for communicating to other gliding lizards, and each of Malaysia's 11 species have their own, unique wing pattern. When not flying or displaying, gliding lizards are commonly seen sitting head-up on the trunks of trees with their wings inconspicuously folded against their bodies. They spend many of their daylight hours running up and down trees feeding on ants, which make up the vast majority of their diet. However, when it's time for a change of scenery, they simply leap from the tree, extend their ribs to open their wings, and glide to the next tree.
The extent and speed of the glide depends on a couple of factors: the height of the lizard on the tree and the surface area of the wing relative to the weight of the body. As it turns out, not all Draco are created equal. For example, the smaller, frail, Black-bearded Gliding Lizard, with its pencil-neck and light bulb-shaped head, has a very light body and big wings. Therefore, it attains lift immediately after leaving the tree at relatively slow speeds and, it is capable of considerable in-flight maneuverability.
On the other hand, the Orange-bearded Gliding lizard (Draco fimbriatus) is the Arnold Schwarzenegger of Draco with its thick neck and heavy, robust body. It has relatively small wings, meaning its in-flight maneuverability is poor but its glide speed is fast. In fact, when this species jumps off a tree, it has to dive straight down for a period of time with its wings folded against its body until it picks up enough speed to open them up and glide. And when it does, it looks a jet rocketing through the forest. Consequently, Orange-bearded Gliding Lizards are often found high up on some of the tallest trees in the forest where they can safely dive in order to gain the momentum necessary to initiate the glide whereas the Black-bearded Gliding Lizard is usually found on the lower portions of the tree.
As it turns out, this is one of the reasons these two species can live together in the same forest, or on the same tree for that matter. Their flight anatomy helps separate them ecologically and keeps them from directly competing with one another for some of the rainforest's resources. In certain areas of the forest, up to eight different species of Dracomay occur together.
Generally, when there are closely related species with unique, restrictive life histories living in the same area, the potential for competition is high and so the resources must be carefully partitioned. Dracodo this in such a way that you rarely see more than two species in close proximity.
For example, Common Gliding Lizards (D. sumatranus) and Spotted Gliding Lizards (D. maculatus) are common in open and disturbed areas; Five-banded Gliding Lizards (D. quinquefasciatus) are usually found in dense forest with relatively small, closely-spaced trees; Giant Gliding Lizards (D. maximus) are somewhat restricted to riparian areas; the smaller Yellow-bearded Gliding Lizards (D. haematopogon) and larger Blanford's Gilding Lizard (Draco blanfordi) occur at higher elevations than most other species; and the Black-bearded Gliding Lizards and the larger Dusky Gliding Lizards (D. formosus) are pretty much habitat generalists in lowland forests.
Sorry. We've been getting a lot of junk comments lately. So commenting has been turned off for now
L. Lee Grismer
L. Lee Grismer also contributed 2 other articles in this section:
- The Flying Reptiles of Tropical Asia: Evolution Takes a Leap (part 3)
- The Flying Reptiles of Tropical Asia: Evolution Takes a Leap (part 1)