The Berkeley Science Review: Articles

Easy Glider
What it takes to fly without wings (view PDF)
by Adrienne Davich

“There is an art…or rather, a knack to flying,” says Douglas Adams, the British writer and radio personality famous for his Hitchhiker’s Guide to the Galaxy. “The knack lies in learning how to throw yourself at the ground and miss.” Curiously, Adams’ definition leaves out one element usually thought of as essential to flying—wings. While his intentions were clearly humorous, Adams was perhaps more correct than he realized: from squirrels and lizards to some tropical ant species, a surprising number of wingless creatures do have a knack for avoiding the ground. They glide.

Today, researchers at UC Berkeley are studying gliding organisms and accumulating clues as to how and why nature favored this adaptation. Not until recently, with the advent of high-speed, high-resolution video cameras, was it possible to observe the movement of individual body parts on an airborne animal or insect. Armed with these innovative techniques, researchers are now looking at the specific morphologies and movements of gliders. Such research is elucidating the biomechanics of aerial behavior in real time, in turn providing insight into the evolutionary patterns behind gliding flight.

Leapin’ Lizards
A glider is like a plane without an engine, or a bird without the ability to flap its wings. But like powered flyers, good gliders can move a significant distance horizontally without losing much height. To do so, they maximize the surface area of their bodies and adopt specific shapes, or morphologies, to generate lift that counteracts the downward pull of gravity. Researchers at Berkeley are studying how body morphology contributes to gliding flight, and how such morphologies might have evolved in the first place.

To study the evolution of flying in a lineage, scientists typically look at the phylogeny of that group—that is, they try to find its common ancestor. Next, they determine the group’s functional morphology, how it has made flight possible, and how it has evolved over time. Finally, they scour the fossil record for empirical evidence to support their findings and to formulate a theory of why flight evolved in that particular lineage.

While this conventional evolutionary approach can be useful, when it comes to Draco lizards, a genus of gliding arboreal lizards from Southeast Asia, it can’t be fully applied. Ranging in mass from three to thirty-five grams (from the weight of, say, a pencil to a plum), Dracos are the only living lizards to have evolved elongated ribs; these ribs support a wing-like flap of skin, the patagium, which provides the surface area needed to glide. Unfortunately for researchers interested in the evolution of these traits, there is no fossil record for this group (or any extant lizards with a similar morphology), making it difficult to chronicle how their ability to glide evolved. Fortunately, by focusing instead on how variation in Draco body morphology affects gliding behavior, Berkeley professors of integrative biology Jim McGuire and Robert Dudley have been able to fill in some details about how flight might have evolved within this group.

When McGuire began studying Draco in 1995, he was intrigued by the relationship between its body size and gliding performance. His interest was piqued in part by the group’s incredible diversification and coexistence: multiple species of Draco were found in the same habitat. It made sense that to coexist, different species must use different types of limited resources in order to prevent competition. Since the only major difference across the various species was body size, McGuire wondered whether size played a role in the type of resources each species consumed, and whether this had anything to do with the lizards’ gliding abilities.

At that point, he teamed up with Dudley, and for the next several years they measured the gliding performance of twenty-nine species of remote Malaysian Draco ranging across the full size distribution. They knew from the outset that Dracos scale isometrically—that is, as their body mass increases, their proportions stay the same. Thus, their body mass increases at a greater rate than their surface area. Since surface area is critical to generating lift, one would expect the big lizard species to be relatively poor gliders compared to the smaller ones.

To conduct gliding trials, McGuire and Dudley erected two tall poles 9.3 meters apart—a six meter takeoff pole and a four to five meter landing pole. They positioned the lizards on the takeoff pole and then prodded them, either by agitating them with a bamboo rod or tapping the pole, so they’d glide toward the landing pole. A video camera recorded each trial, and by taking into account the position of the camera relative to the poles, McGuire and Dudley were able to calculate the approximate height lost over a standard horizontal glide distance and the angle of each glide. They also used a digital anemometer to measure wind speed and a compass to determine wind direction. Together, these measurements allowed them to determine the velocity of each glide adjusted for wind conditions.

It turns out that the large Draco compensate for their extra mass by diving faster and farther: the first part of their glide is a steep ballistic dive to generate velocity, and then they level out into a flat glide. Practically speaking, though, the larger Draco are more limited than smaller ones in their access to resources within the forest. Since they need a higher initial altitude to perform a successful glide, they are likely to remain in the upper reaches of the rainforest canopy, whereas the smaller Draco may scavenge far below. This partitioning of the forest habitat may in turn explain how multiple Draco species can coexist: by living in different vertical layers of the forest, they draw on different resource pools.

Evolutionary deductions can also be made from the data. According to McGuire, the original Draco gliders—the common ancestor of those living today—were probably either large and living in really tall trees or cliffs from where they could execute steep ballistic dives, or they were fairly small so they could take advantage of their higher surface area to body mass ratio. They could probably expand their ribs just a little bit to create more surface area for increased lift, but then were favored by selection such that their patagia got larger and larger over many generations. In the absence of a fossil record, such evolutionary hypotheses establish a framework for understanding how Draco may have started gliding.

The Ants Go Gliding One by One...
Another striking example of a glider that may shed light on the evolution of aerial behavior is Cephalotes atratus, a species of wingless ants living in the tropical rainforest canopy. Remarkably, these ants are the first documented case of wingless flight in any living insect.

Three years ago, Robert Dudley listened as his colleague Steve Yanoviak, a University of Florida entomologist, told him he’d discovered something amazing in the jungles of Panama. He’d found that wingless ants, when flicked off high tree branches, glided back to the trunk of the tree from which they’d fallen rather than spiraling aimlessly to the rainforest floor. Yanoviak said he’d witnessed the gliding behavior again, in the same ant species, in Peru. Since there were no documented cases of gliding flight in a living wingless insect, the discovery indeed seemed novel. In 2004, Dudley joined Yanoviak and Michael Kaspari, a University of Oklahoma entomologist, in studying this phenomenon. How were the ants maneuvering themselves, they wondered? What about the ants’ morphology facilitated aerodynamic behavior? And what could it reveal about the evolution of insect flight?

Dudley, Yanoviak, Kaspari, and others spent the next year or so using high-speed, high-resolution video cameras to document how wingless ants in the Peruvian rainforest glide. They used a rope and sit harness to pull themselves up to high, treetop branches. Sometimes they climbed with ants they’d collected elsewhere in the forest, and other times they resolved to find a colony once they reached the upper reaches of the tree. They painted the backs of the ant bodies white (for visibility against the dark forest background), dropped them from trees, and then recorded the ants’ descent on video camera. The painted ants repeatedly glided back to the tree trunk.

Upon further study, it turns out that these ants do what skydivers do—they use their limbs to control their posture and orientation. Their legs are slightly flattened at the ends, so like skydivers who can initiate a spin by slight right and left asymmetry of hand or arm posture, the ants can initiate torque that will rotate their bodies.

And it’s not just one ant species doing this. Dudley says, “it’s hundreds, and they’re not all related to each other.” Nor are ants the only wingless insects gliding. Biologists are now finding that many species across all insect groups are wingless gliders. They don’t have wings and never have—and yet, they glide.

The evolutionary pressure to adopt gliding flight may have been as simple as accidentally falling out of a tree. Or, as Dudley puts it, “If you’re an ant falling from the rainforest canopy, you don’t want to land on your head. You want to control your descent, try to land on something so you don’t break your antennae or another body part.” Put this way, gliding flight sounds like a reflex, the body and limbs responding to the force of gravity by creating aerodynamic surfaces that produce lift and adjust to drag. Directed aerial descent has another possible advantage as well—a falling ant may be able to return to its colony as opposed to being woefully lost on the forest floor far below.

Given how common gliding behavior is throughout insect species, it’s natural to ask whether these gliders fit somewhere on the evolutionary continuum between land-dwelling and flying insects. In other words, did today’s flying insects evolve from gliding ancestors? Dudley’s hypothesis is that wingless gliding flight preceded winged flight—that aerodynamic behavior in insects started without wings. Admittedly, “flight without wings” sounds a bit like putting the chicken before the egg, but research on gliding by Dudley and others has definitively shown that it’s possible.

Furthermore, the evolutionary history of insects offers some evidence that flying insects evolved from gliding ancestors. According to Dudley, the most “primitive” insects around today—those which most closely resemble the earliest insects on earth—are the Silverfish (Thysanura) and Bristletails (Archaeognatha). Both of these groups lack wings, and the Silverfish can glide, just like the Peruvian rainforest ants. This suggests that all modern winged insects may have evolved the ability to fly from wingless, gliding ancestors. So far, however, scientists have not yet pinpointed the transitional morphologies that would illuminate the evolutionary steps between skydiving silverfish and some of the earliest winged insects.

Eventually, by understanding how gliding played a role in the evolution of insect flight, researchers hope to gain insight into the evolution of flight in vertebrates. But in the meantime, they continue to investigate the mechanics of gliding, fascinated by the many species—insect, lizard, or otherwise—that throw themselves determinedly at the ground, but somehow, repeatedly, miss.

Adrienne Davich is a graduate student in journalism.

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