The Rainforest Canopy

CANOPY LOCOMOTION: Prehensile Tail versus the Ability to Glide

Because significant gaps exist between the branches of the canopy, animals of this zone must be able to negotiate these discontinuities by some means. The majority of canopy species climb, leap, or fly from tree to tree, and are equipped with appropriate mechanisms which enable them to do so successfully. Some species have undergone major adaptations which allow them to glide. The dominant form of canopy locomotion differs in each continental rainforest, a product of forest structure and evolutionary history.

THE AMERICAS

Primates (pictures | news) are some of the best known mammals outfitted with a prehensile tail and several New World primates have this fifth limb, including howler and spider monkeys. Spider monkeys of Central and South America owe their name to their long, supple limbs which give them impressive agility in tree habitats much like the gibbons of Asia. Although closely related to howler monkeys, spider monkeys have a slender body and exceed 13 pounds (6 kg). This monkey feeds on fruit, shoots, flowers, and occasionally insects and bird eggs.

The largest New World monkey is the muriqui or charcoal monkey, which was once distributed throughout Brazil's Atlantic forests, but is now restricted to a few small patches, since less than 5 percent of its original forest remains. Before the Portuguese arrived in Brazil, the muriqui population probably numbered around 400,000, but a 1987 census found only 386 animals and a 1993 census found 559. Habitat reduction is speculated to be the leading cause the muriqui decline, although extensive hunting has also had a detrimental effect. The muriqui is characterized by light golden gray fur, a dark hairless face, and a prehensile tail. This species has interesting reproductive behavior with several males mating with a single female in the course of a day. The most fit sperm is responsible for fertilization. The muriqui is an exception among primates and other animals in that the females leave the troop when they reach maturity. Usually, in animal populations, the males disperse and add variation to the gene pool.

Several members of the Endentata order have prehensile tails including mammal species that are not usually considered tree dwellers. Two species of anteaters (Myrmecophagidae family) and at least one species of porcupine are tree dwellers, equipped with prehensile tails. Even a carnivore, the nocturnal kinkajou, possesses a prehensile tail.

ASIA
In Asian rainforests, especially those of the island of Borneo, where taller trees are characteristic, gliding and brachiation are the predominant means of locomotion.

Among those species equipped with mechanisms allowing extended gliding are flying squirrels (several genera), flying lemurs (two species), geckos (two species - pictures), draco lizards (several species - pictures), frogs (several species - pictures), and the chrysopelea snake. Only one mammal species of Southeast Asia, the pangolin, has a prehensile tail; the pangolin is described later under Africa. (The marsupial cuscus of Sulawesi is also equipped with a prehensile tail.)

None of these gliding creatures can actually fly, but instead glide from tree to tree. In order to glide, the animal must climb to the upper regions of a tree and leap and glide at a downward angle to another tree. Many species have evolved a gliding membrane known as a patagium. The patagium consists of a loose flap of skin that is opened when the animal extends its limbs and sometimes its tail. When not in use, this skin hangs loosely on the sides of the animal and often makes for difficult walking and climbing.

The most well known, widely dispersed gliding animal is the flying squirrel. These squirrels are found almost worldwide in tropical, temperate, and even Arctic environments. Rainforest flying squirrels are found only in Southeast Asia, India, and Sri Lanka, where they are most active at night. Flying squirrels have been recorded gliding distances of over 650 feet (200 m). A lesser known glider is the flying lemur (though not actually a lemur) of Southeast Asia.

Surprisingly, at least four species of lizard have developed means for gliding. The two species of flying dragon are found in Sri Lanka, India, and Southeast Asia although the best known, Draco splendens, is from the Philippines, Malaysia, and Indonesia. Flying dragons live in trees all their lives except when they go down to the forest floor to nest, and they feed primarily on tree ants. This lizard can glide up to 325 feet (100 m), but usually not more than 65 to 100 feet (20-30 m) since forest trees tend to be closely spaced. Flying dragons are able to glide thanks to a patagium supported by its elongated ribs. Two species of flying gecko from Southeast Asia have a different style of patagium. Instead of having one large patagium supported by its ribs, the flying gecko has small skin flaps along its limbs, torso, tail, and head.

Even stranger than the gliding lizards is the Malayan flying frog which glides using the membranes between the toes of its limbs, and small membranes located at the heel, the base of the leg, and the forearm. Its color varies, although usually the frog's back is bright green with yellow belly and blue patches on the feet and shoulder. Its eggs are laid like many other canopy frogs, on vegetation overhanging water, so the tadpoles drop into the water when they hatch.

Perhaps the strangest gliding animal is the paradise tree snake from southern Thailand, Malaysia, Borneo, Philippines, and Sulawesi. It has the ability to parachute by stretching out its body sideways by opening its ribs so the belly is concave, and by making lateral slithering movements.

Brachiation is the form of arboreal locomotion characteristic of certain primates—especially the gibbon—where movement is accomplished by swinging by the arms from one branch to another. Such primates are anatomically adapted for this form of movement with their long arms and fingers and their mobile shoulder joints.

AFRICA

In Africa, where the forests are of intermediate height and limited liana growth, neither form of canopy locomotion predominates. The most notable species with a prehensile tail is the pangolin; all of Africa's mammals lack prehensile tails. The pangolin is an odd-looking creature—resembling an Old World cross between the New World armadillos and anteaters—having a body completely covered (except for the belly) with large, thick scales that render it inedible to predators when it curls up into a spiny ball. It has a long muzzle, small protected eyes, and strong arms and legs for digging and tearing. Its prehensile tail, like similar animals of the New World, has a finger-like sensor at the tip. There are about seven species of pangolin distributed in Africa, India, and Asia, of which six are found in rainforest regions. Within the rainforest some pangolins are canopy dwellers, while others prefer the ground. Ground dwelling species live in burrows, while arboreal pangolins live in tree hollows, have prehensile tails, and are good climbers. Regardless of what zone they prefer, all pangolins are excellent swimmers, are nocturnal, and feed on termites, ants, and larvae. Pangolins depend on their well-developed sense of smell to locate termites and ants. With their strong, sharp claws, pangolins tear open ant and termite nests and use their 10-inch long (25 cm.) tongues to capture insects. As they feed, pangolins pick up grit from the nest which is useful later in the stomach for grinding up the insects.

OTHER FORMS OF LOCOMOTION

One of the most common forms of moving through the canopy is simply scurrying along tree limbs using a tail for balance and leaping the small gaps between trees. Many monkeys, squirrels, and lemurs have bushy tails to assist in balancing. Other canopy dwellers, like lorises, sloths, and anteaters, simply use large claws to cling to canopy branches and move slowly in the trees.

Review questions:

• What are some ways that animals move through the canopy?
• What is a prehensile tail?

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