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Feb. 14, 2012

It's a Bird, it's a Plane, it's...a Gliding Mammal?

by Kara Rogers

Click to enlarge images

Sugarglider (Petaurus breviceps) Photo credit: Anke Meyring

Gliding mammals sail silently from one tree to the next, maneuvering to their destinations with extraordinary precision and control, often in complete darkness. This unique ability is found in only about 60 species of mammals in the world, but those species include marsupials and placental animals, two very distantly related groups distinguished by the vast evolutionary differences in their reproductive systems. As a result, gliding mammals serve as a fascinating example of convergent evolution -- when similar physical and functional traits occur in unrelated species.

All gliding mammals share several key features, the most notable of which is the presence of a thin membrane of skin that runs between the forelimb and hind limb on each side of the body. When the animal leaps into the air and extends its feet outward, these membranes, known as patagia, become stretched by air pressure, essentially turning the animal’s body into a rectangular wing that produces lift and sustains gliding. While in the air, gliding mammals actively regulate aerodynamic forces, though often through unique mechanisms. For instance, whereas one species may rely primarily on tail movements to control orientation and trajectory during a glide, another species may rely on a combination of tail, arm, and leg adjustments.

Malayan flying lemur (Cynocephalus variegatus). Photo credit: Nina Holopainen

Some of the best-known gliding mammals are flying squirrels, which belong to the order Rodentia and are found in temperate and tropical forests in Asia, northern Europe, India, and North America. There are about 44 species of flying squirrels, of which the northern flying squirrel (Glaucomys sabrinus) holds the record for the longest glide, at almost 90 meters (295 feet). In addition to patagia, flying squirrels possess other specialized gliding adaptations, including a small shaft of cartilage on each wrist that helps control the patagia during gliding and a bushy, usually flattened tail that acts as a rudder for steering. The woolly flying squirrel (Eupetaurus cinereus) of south-central Asia is the largest of the group, weighing as much as 2.5 kg (5.5 pounds) and measuring about 45 to 60 cm (about 1.5 to 2 feet) in length from the head to the base of the tail. The lesser pygmy flying squirrel (Petaurillus emiliae) of Borneo is believed to be the smallest, though data on its size is lacking.

Another well-known gliding mammal is the sugar glider (Petaurus breviceps), which is one of six species of so-called wrist-winged gliders (genus Petaurus) of the order Diprotodontia that inhabit the forests of parts of northern and eastern Australia, New Guinea, and nearby islands. Wrist-winged gliders differ from flying squirrels in several ways, perhaps the most significant being that gliders are marsupials and flying squirrels are placental mammals. In addition, the attachment site of the patagia of wrist-winged gliders is the fifth finger of each forelimb, which differs from the cartilaginous attachment structure of flying squirrels. The sugar glider is the smallest member of Petaurus, weighing 80 to 170 grams (2.8 to 6 ounces) and measuring 15 to 21 cm (6 to 8 inches) in body length. The largest of the group is the yellow-bellied glider (Petaurus australis), which weighs 450 to 700 grams (1 to 1.5 pounds) and measures around 30 cm (1 foot) in body length.

Other gliding mammals include the greater glider (Petauroides volans) and the feather-tailed glider (Acrobates pygmaeus), which are members of Diprotodontia, and the colugos (or flying lemurs, though they are not related to true lemurs) of Southeast Asia, which belong to the order Dermoptera. The two extant species of colugos are the Philippine flying lemur (Cynocephalus volans) and the Sunda flying lemur (Galeopterus variegatus). These animals have a patagium that extends from the shoulders to the tip of the tail, and they have webbed feet. Thus, colugos have more developed gliding capabilities compared with flying squirrels and gliding marsupials. These capabilities led to the proposal in the 1980s that colugos were very closely related to bats, but this idea has since been rejected by DNA analyses.

The physical differences and variations in aerodynamic control among gliding mammals are the result of independent evolutionary events. In fact, gliding evolved independently at least nine times in mammals, and each time, it came about through a series of increasingly sophisticated adaptations that provided greater control over aerial descent. While there is no clear explanation yet as to why some arboreal mammals took to aerial descent as a mode of locomotion in the first place, scientists suspect that habitat structure, predators, aerodynamic and landing forces associated with leaping or jumping, and energy expenditure during foraging may have been important factors.
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Kara Rogers is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is a member of the National Association of Science Writers and author of Science Up Front on the Britannica Blog. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.

About Kara Rogers

Kara is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is the author of Out of Nature: Why Drugs From Plants Matter to the Future of Humanity (University of Arizona Press, 2012).

The views expressed are those of the author and are not necessarily those of Science Friday.

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