Gliding and Parachuting

Many vertebrates have independently evolved the ability to glide. In spite of their name, "flying" squirrels (Glaucomys volans and Glaucomys sabrinus ) glide, not fly, from tree to tree. The little marsupial known as a sugar glider (Petaurus breviceps ) can also glide from tree to tree. Many other animals practice parachuting. The difference between parachuting and gliding is control. Parachuting is simply slowing the rate of descent with little or no attempt to control direction. Flying squirrels carefully steer themselves as they glide from the crown of one tree to the trunk of another.

Cynocephalus is a group of medium-sized gliding mammals closely related to bats. These mammals hang upside down in trees, leaping into the air to glide in search of fruit to eat. The group Exocoetus contains the "flying" fishes. Although a true glider, Exocoetus can extend the range of its glide by flapping its pectoral fins. Flying fishes glide when startled, as by a predator. They swim rapidly underwater, then launch themselves into the air, where they can glide over long distances. Rhacophorus is a tree-dwelling frog with expanded toe membranes that help it fall more slowly after it leaps off a tree. The frog is a true glider, as it can turn and maneuver while airborne.

All gliders and parachuters can increase the relative width of their bodies, thus increasing the surface area exposed to wind resistance. A few gliding frogs flatten their bodies and spread their limbs outward. Gliding snakes not only flatten their bodies, but also draw in the scales on the lower side of the body to form a kind of trough. Some flying lizards, such as Draco volans, have evolved the ability to glide using specialized ribs that spread out like a fan.

Gliding mammals, such as flying squirrels, have a fold of skin on each side of their bodies that extend from the front leg or front wrist back along the side of the body to the hind leg or the ankle. To glide, the squirrel climbs to near the top of a tree and launches itself toward another tree, spreading the fold of skin by holding out its front and rear legs. The glide angle is quite steep, but accurate enough that the squirrel securely lands well up on the trunk of the target tree and can climb back to a safe height above ground.

Parachuting lacks the implied directional control of gliding. To parachute, the animal launches itself into the air and controls its fall by spreading toes, limbs, and membranes. Parachuters usually fall to the ground or to a lower branch of a tree. Most gliding and parachuting animals are fairly small. Their surface area is large relative to their weight, so air resistance effectively slows them down. If an animal is small enough, it needs no special adaptation for parachuting. For example, an insect can fall from the top of a tall tree all the way down to the ground without harm. The insect is its own parachute.

Gliding and parachuting are not generally evolutionary steps toward flying. They are independent adaptations acquired by animals that live primarily in forests. However, birds may have evolved the ability to fly as an extension of running along the ground with short, gliding hops that became longer and longer over time, evolving eventually into true flight. Roadrunners (Geocòccyx californiànus ) regularly display this behavior. Although capable of flight over short distances, they prefer to run and occasionally glide.

see also Flight.

Elliot Richmond

Bibliography

Curtis, Helena, and N. Sue Barnes. Biology, 5th ed. New York: Worth Publishing, 1989.

Hildebrand, Milton. Analysis of Vertebrate Structure, 4th ed. New York: John Wiley & Sons, Inc., 1995.

INDEPENDENT EVOLUTION

Many completely unrelated organisms seemed to have evolved similar structures at different times. For example, mollusks, arthropods, and vertebrates all evolved eyes. However they didn't "borrow" the idea from each other. Each class of animal evolved the structure separately from the others. Grasping appendages, eyes, wings, powerful legs for jumping, and many other features of our animal world are simply good ideas that have been discovered many times through the processes of evolution.