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Gliders of the Forest
When English aviation pioneer Sir George Cayley launched his first gliding airplanes in the early 1800s, and put subsequent inventors on the path to achieving successful powered flight, it's unlikely that he'd ever seen a "flying" lizard. Yet Cayley's "governable parachute," which in 1853 glided 900 feet carrying his coachman, does bear passing resemblance to gliding reptiles such as the flying lizards of Southeast Asia.
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| Flying squirrels are nocturnal and rarely seen by people. (photos.com) |
Before Cayley, inventors including Leonardo da Vinci sought to build airplanes with flapping wings, modeled on birds. In flapping flight, wings provide both lift and thrust. Cayley's key insight was to separate the two functions—stable airplane wings provided lift, while a separate mechanism provided thrust. (He predicted, correctly, that powered flight would not be possible until the invention of a lightweight engine.)
In 1853, Cayley's governable parachute was pulled downhill until it was going fast enough to achieve lift. Today, hang gliders launch themselves off cliffs or other high points and let gravity provide the necessary velocity. That's pretty much the same way that gliding animals, including so-called flying lizards, do it.
Gliding animals leap or fall from a tree and use anatomical and behavioral features to form "wings" that allow them to descend at a gentler angle than if they were simply falling. Once airborne, explains Robert Dudley, a leading authority on gliders, "All of these animals can reorient and control where they go to some extent. It's controlled aerial behavior." Dudley is a professor in the department of integrative biology at the University of California at Berkeley and a scientist at the Smithsonian Tropical Research Institute in Panama.
Just as gliding principles proved far easier to apply to developing airplanes than true, flapping flight, so too it appears that gliding is easier for animals to evolve. Flapping flight has evolved independently only four times: in bats, birds, insects, and pterosaurs—reptiles that went extinct about 65 million years ago. In contrast, gliding has evolved independently at least 30 times. Among the non-avian vertebrates, there are "flying" lizards, frogs, snakes, squirrels, possums, and even fish.
Except, obviously, for flying fish, all the known gliders live in forests and are arboreal. This just makes sense. Animals that live in trees occasionally fall: They slip, are startled, are pushed, or attempt an ill-advised jump. Individuals that have a bit more control over the speed or direction of a fall, versus simply crashing to the forest floor at high speed, are more likely to survive and pass on their genes. Over time, adaptations for full-fledged gliding may evolve, particularly if gliding confers other advantages, such as increasing the efficiency with which animals move among the trees, or enabling them to more easily flee from predators.
Curiously, though, gliding vertebrates have not evolved to the same extent in all forests. African and Central and South American tropical forests are home to only a handful of gliding species, mostly squirrels and frogs. Southeast Asian tropical forests, however, are glider heaven.
The Makings of a Glider Heaven
Dudley believes that several features of Southeast Asian tropical forests encourage the evolution of gliding. One is that the forest canopy is typically higher than in African or Central and South American tropical forests. Trees in the Dipterocarpaceae family make up much of the Southeast Asian forest, and they commonly reach heights of about 200 feet or more. Trees that tall are rare in African and American tropical forests, which usually have a canopy about 100 to 150 feet high. Tall trees are good for gliding because the higher up an animal is when it starts a glide, the more horizontal distance it can cover, which makes for a longer trip. Furthermore, a gliding animal needs to attain a certain falling velocity in order to generate sufficient lift. The higher the tree, the more time and space the animal has to reach the necessary velocity during the initial dive.
Calm air also makes for good gliding. Southeast Asian forests tend to be calm, although not necessarily more so than other tropical forests. "There's not much wind below the canopy in any tropical forest," Dudley says. "It's windy at the top, but there's a gradient and it diminishes pretty quickly," although he adds, "All of these gliders can compensate for short-scale wind fluctuations."
Another structural feature of Southeast Asian forests that might encourage the evolution of gliding is the lack of connection between trees. Jim McGuire, who is an assistant professor of integrative biology at the University of California at Berkeley and a curator of herpetology at the university's Museum of Vertebrate Zoology, explains, "The trees themselves tend to have very tall, unbranching trunks until you get way up into the crown; it's like a forest of poles. The crowns are less connected than in other forests. There are fewer vines and things that span the crowns of two trees, which would provide continuous connection, so you either have to jump or glide to get from one tree to another." The alternative—climbing down to the ground, walking to another tree, and climbing up—exposes animals to predators on the ground and is less energetically efficient.
Louise Emmons, an ecologist at the Smithsonian's National Museum of Natural History, and her colleague Alwyn Gentry first speculated in 1983 that the relative abundance of lianas, or vines, might influence how arboreal animals move through tropical forests. They noted that most gliders, which do not have prehensile tails that can grip objects, are Asian, while most prehensile-tailed animals are Central and South American (Neotropical); few of either are African. As McGuire pointed out, fewer vines linking the trees in Asian forests may encourage the evolution of gliding. In the Neotropics, there are more vines, but they tend to be fragile and often break, so animals using them to travel between trees may need the extra support offered by tails that cling to stronger branches. African tropical forests, on the other hand, have the greatest density of vines forming bridges between trees, and these vines are quite strong, so special adaptations like gliding and prehensile tails are less important.
Adaptations for Gliding
Gliders have evolved various anatomical mechanisms for gliding, all of which seem to increase the surface area of their bodies, which increases lift. For instance, a dozen or more species of frogs (genera Rhacophorus and Polypedates) and almost as many geckos (genera Ptychozoon, Cosymbotus, and possibly Luperosaurus, which have physical adaptations for gliding but have not been observed doing it) stretch out large webbed feet, or use flaps of skin on their arms and legs—the most common approach to gliding. Flying squirrels, a group scientists call the Pteromyini, glide with the help of a parachute-like membrane called a patagium that stretches between the forelimbs and hindlimbs. (The membrane in bats' wings is also called a patagium, but they use it for true flight rather than gliding; see "Illuminating Secret Shadows" for more on bats.) Flying squirrels heavier than two pounds, such as the wooly flying squirrel (Eupetaurus cinereus) of Pakistan, Afghanistan, and India, have an additional membrane between their hindlimbs and tail called a uropatagium.
Colugos, also known as flying lemurs, have a similar but more extensive membrane that stretches between the neck, limbs, and tail, and encloses the tips of fingers, toes, and tails. These Southeast Asian mammals are not rodents, as flying squirrels are, nor are they lemurs, which are found only in Madagascar. Instead, the two extant species—the Phillipine colugo (Cynocephalus volans) and the Sunda colugo (Galeopterus variegates)—form their own order, the Dermoptera, whose nearest relatives may be tree shrews (which they don't particularly resemble).
Like all gliding mammals, colugos are nocturnal, spending days sleeping in trees and nights foraging for fresh tree leaves. According to Greg Byrnes, a Ph.D. student at the University of California at Berkeley who studies Sunda colugos, "Most of the time you see them upside down," hanging from tree limbs in a similar posture to that of sloths. When they want to glide, they push off with their hindlimbs and stretch out their forelimbs. "On average, they probably glide about 35 meters [about 115 feet], but I've seen them glide much, much farther—100 or 120 meters [about 330 or 390 feet]," Byrnes says. "They move their tails around an awful lot when they are gliding. If they want to turn, they'll bank or collapse part of the membrane, folding their limbs on that side of the body. They'll collapse part of the membrane to squeeze through a small space and they'll open it up again."
Colugos glide from tree to tree in search of food. "On an average night they might glide eight to ten times," Byrnes says. "It's a very sporadic thing. They'll sit in a tree for an hour or more." Twice, Byrnes has seen a colugo land on the ground, a few feet short of the intended tree. "When they land on the ground, they're pretty helpless," he says. "They don't really look like they know how to run like a regular animal. They sort of flop to the tree and climb up it."
Flying Lizards
The 45 species of Draco—the so-called flying lizards—live throughout Southeast Asia, from southern China to Sumatra to the Philippines and almost to New Guinea. Their wings are formed by elongated ribs that support patagia. The lizards open and close these wings using modified intercostal muscles (the small muscles between ribs). Smaller wings extend from the throat. Both pairs of wings are used for mating and aggression displays, and for gliding.
Like all vertebrate gliders, when the lizards first jump off a tree, they fall steeply to gather sufficient velocity to generate lift, then level off to a more gentle angle.
McGuire says that at the end of a glide "they make a braking maneuver, gain a little bit of altitude, and land softly on the tree of their choice. It's easy for them to glide between ten and 30 meters [about 30 to 100 feet], but they can go much farther depending on where they start." He says maneuverability is difficult to quantify, but adds, "I've seen a flying lizard take off from the top of a coconut tree and do three spins around the tree and land lower on the trunk."
McGuire and Dudley collaborated on a study of how glide performance varies with size among these lizards, which range in weight from three to 35 grams (0.1 to 1.2 ounces). "The ten-fold variation in body size has to have some implication for locomotor performance," McGuire says. All the lizards are essentially scale models of one another; from an engineering standpoint, this creates a problem for the bigger lizards. "In general, larger things need relatively larger wings to create aerodynamic forces. You have to offset a body mass with surface area," Dudley says. (This is what the uropatagium does for larger flying squirrels.) Larger lizards have more mass per unit of surface area than do smaller lizards, so McGuire and Dudley hypothesized the glide performance of larger lizards would be worse.
The scientists captured Draco lizards and conducted gliding experiments in the Malay Peninsula and Borneo. In an open field, they erected two poles—a take-off pole about six meters high (nearly 20 feet) and a landing pole about four or five meters high (about 15 feet). The poles were a little more than nine meters (about 30 feet) apart, which is about the distance between trees in a typical Southeast Asian tropical forest. In the wild, flying lizards glide between trees, never coming to the ground except to lay eggs. With this in mind, McGuire and Dudley figured the lizards would use the landing pole as a target. They encouraged the lizards to jump by tapping the take-off pole, and the lizards did indeed glide to the landing pole.
McGuire and Dudley found that the larger species of Draco did tend to have steeper glide angles than their smaller relatives, losing more height over a glide of a similar horizontal distance. This finding might shed light on how more than one species of flying lizard can inhabit the same patch of forest without competing for limited resources.
McGuire says the larger lizards, being worse gliders, need to generate more velocity to achieve lift—which means they need to have a longer initial dive. "The expectation is that they would not be able to initiate a successful glide from as low on the trees as smaller ones," McGuire says, so perhaps lizards divide up their habitat vertically, with larger lizards living higher up. This has not been conclusively demonstrated, but there is anecdotal evidence that it is the case.
Draco is the only lizard genus with such obvious wings, but McGuire says, "There are probably a lot of arboreal lizards that can generate lift while falling." By orienting their bodies and limbs, animals can produce a glide even with no obvious physical modifications. For example, green anoles (Anolis carolinensis), which live in the southeastern United States, have been shown to glide at a ratio of one horizontal foot to one vertical foot.
Flying Snakes
If it's surprising to learn that animals without patagia can manipulate aerodynamic forces with their limbs and tails, consider flying snakes. How can an animal with no wings, no webbing, and no limbs possibly generate lift?
That's what Jake Socha, a researcher at Argonne National Laboratory near Chicago, wanted to know when he began studying snakes in the genus Chrysopelea for his Ph.D. research a decade ago. He spent time at the National University of Singapore, the Singapore Zoological Gardens, and the Bukit Timah Nature Reserve (also in Singapore) filming the snakes as they jumped off ten- to 15-meter-tall (about 35 to 50 feet) towers or buildings. Socha worked primarily with C. paradisi and C. ornata.
One at a time, he placed snakes on a branch at the top of the tower; often a snake would jump immediately to get away from Socha. If the snake didn't jump, Socha would tap on the branch or the snake's tail to encourage it.
Socha analyzed his videotapes and photographs and found that the snakes were flattening out in the air. When a snake is preparing to jump, it hangs in a loop from a branch. "It jumps up and away from the branch, and as it's jumping it starts to flatten out from the head all the way to the tail," he says. The width of the body doubles, and it becomes slightly concave underneath. After taking to the air, the snake draws its entire body into an "S" shape (in the horizontal plane). "It undulates, sending these waves down its body starting from head and going to tail," Socha says. He doesn't know exactly how the undulations help with the gliding. "We don't think that it aids in lift generation," he says. "We think it helps balance the snake in the air so it doesn't tumble over."
When the snakes flatten out, they not only change shape, they also increase their surface area. "Lift is all about deflecting air downward," Socha says, "and the more surface you have, the more air you can deflect downward." The snakes are able to increase surface area by moving their ribs and "unfurling" tiny folds of skin between their scales.
The snakes start off in a fairly steep dive of about 50 or 60 degrees, then level off as they speed up and produce more lift. The flattest part of the dive that Socha measured was about 12 to 13 degrees from horizontal. He says they tend to land tail-first on the ground. He hasn't worked out exactly how they land on branches, although he's seen a lot of them do it. "One thing is for sure—they don't hit their heads!" he says.
How far they can go depends on how high they start. The best performance of C. paradisi that Socha measured from the ten-meter tower was a glide of 21 horizontal meters (nearly 70 feet). The snakes can also maneuver, taking sharp turns in midair. Once, Socha says, he was working on a tower next to some woods. He strung up a sheet alongside the tower to block the snakes' view of the woods. One snake jumped off the tower and as soon as it passed the end of the sheet, it turned sharply toward the trees.
Although Socha is not currently doing research with flying snakes, he is taking a faculty job at Virginia Tech in Blacksburg, Virginia, in the fall of 2008 and plans to spend some of his research time there studying Chrysopelea. He hopes to learn how the snakes turn while gliding; he says some of the species of Chrysopelea don't seem to be able to turn as well as others. "I would definitely like to examine that from an aerodynamic view," he says. "What does the body do and how does it create the force required to make the snake turn?"
He has also become interested in the ecology of gliders. He'd like to implant some snakes in the wild with radio transmitters and track them. "I'm dying to do an ecological study," he says. "There has been no systematic account of when and why and how often they [glide]." His studies thus far have been in a controlled setting; he's only seen a snake glide in the wild once. He was on a wildlife observation tower in the forest and he saw a flying snake on the edge of the tower. When he moved toward it, it jumped off and glided to some trees. "That was definitely an instance of predator avoidance—it was trying to escape from me—and I presume this is one of the reasons they use it in the wild."
Future Fliers
There may be dozens or even hundreds of species of arboreal vertebrates in Southeast Asia and elsewhere whose gliding behavior has not yet been discovered by scientists, because these species lack obvious physical modifications to call attention to their abilities. Some of these animals may eventually evolve physical characteristics that make them better gliders. It's unlikely that any of them will evolve into fliers, however.
Take Draco, for example. Even though they already have wings, the wings have scant musculature. The wings of birds, bats, and pterosaurs are all modified forelimbs and, as such, come with relatively powerful muscles of the sort that would be needed for active flight.
"To become a glider is difficult, but evolving active flight is far, far more difficult," Socha says. "And part of the evidence of that is that it has only happened four times in the history of life."
—Mary-Russell Roberson last wrote about oak forests in the July/August issue.
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