The Rainforest Canopy


July 30, 2012

The incredible diversity of food sources and unique niches of the canopy trees support a wide variety of animal species. Animals often congregate around a flowering tree, which makes trees in this stage some of the best sites for viewing wildlife. In places like these, where food is abundant, animals set up territories, but since canopy leaf cover affects visual territorial displays, most animals rely on sound signals. Thus some of the loudest animals of the world are canopy dwellers. Many primates emit howls and screams, while birds use song to let other animals know that they are intruding on their space.

Common paths, often leading to fruiting trees, where many animals may pass in the course of the day are well-worn and often free of epiphytes; these form highways in the trees. Similarly, areas in the canopy free of vegetation form flight corridors used by numerous species, especially the birds of prey which often attack their prey from below. These flight paths are embedded in the memory of bats and birds.


Despite the huge abundance of canopy leaves, few mammals are properly equipped to eat them. Cellulose, the material of which cell walls are made, is difficult to digest, so leaf-eating animals must have large stomachs to hold their meal while it is being broken down. A large stomach is often accompanied by a large body which can be detrimental to canopy dwellers who depend on branches to support their weight. Similarly, very few birds specialize in leaf-eating because the extensive digestive system adds weight that hinders flight. Interestingly, more Old World mammal species, especially primates, rely heavily on leaves for the bulk of their diet, but few New World primates have the physiological adaptations to digest fibrous cellulose. Forest canopy mammals outfitted to feed on leaves include sloths, howler monkeys, orangutans, and chimpanzees.


Sloths (pictures) are unique mammals highly adapted to life in the canopy. They belong to the Endentata (= "without teeth") order, although they do possess cheek teeth, and are endemic to Central and South America. There are six to seven species represented by two distinct groups: the three-toed sloth and the two-toed sloth. Three-toed sloths feed almost exclusively on cecropia leaves, while two-toed sloths live high in the canopy feeding on a wide variety of leaves and fruits. Three-toed sloths range from Honduras to Argentina and are known locally by natives as "Ai" for their shrill call. Two-toed sloths or "unau" inhabit forest areas from Nicaragua to Bolivia and northern Brazil. Both species are predominantly nocturnal. To facilitate water runoff, the hair of both species grows from the stomach to the back since sloths spend most of their time hanging from branches upside down. The slow movement and moist climate of the hair stimulates the growth of green algae in the fur of the sloth, giving it a greenish color which helps it camouflage from predators like the harpy eagle. Sloths are well evolved as a leaf-eating mammal with stomachs divided into many digestive compartments that contain cellulose-digesting bacteria. A low metabolic rate combined with minimum movement and 15 hours of daily sleep help the sloth conserve energy. Leaf-eaters in general must consume large quantities of their food to satisfy their nutritional requirements. Leaves may last up to one month in the sloth's intestines and feces and urine are passed about once a week when the sloth descends to the ground at habitual places.

The sloth's fur is an entire ecosystem of its own: one study found more than 950 beetles on a single sloth, living off the algae growing in its fur. The fur is also home to a certain species of moth which is dependent on the sloth's descent for its life cycle. When the sloth reaches the ground, the moth quickly lays its eggs in the sloth's dung and returns to the sloth's fur. After the eggs hatch and the caterpillars become moths, the moths, in some way or another, claim another sloth.

The routine descent of the sloth raises an interesting question—if sloths are so clumsy and slow on the ground why do they put themselves at risk to terrestrial predators when they could easily defecate from the trees? The answer lies in the intricacy of the rainforest ecosystem. By defecating at the base of their host cecropia tree, the sloth provides the tree with precious fertilizer, a rare but vital commodity in most rainforests.

Howler monkeys (pictures) are another New World canopy animal that relies heavily on leaves. Howler monkeys have earned the distinction of the loudest animal, according to the Guinness Book of World Records, with their raucous howls that can be heard clearly at distances over 10 miles away. This record is quite believable when a troop of noisy howler monkeys approaches. The male howler monkey, closely related to the spider monkey, is equipped with a special voice box which enables it to vocally defend its small territory without physical confrontation. Howler monkeys live in troops of 5-20 animals (average), in which females and juveniles make up the greatest percentage. These stocky, black primates, weighing up to 20 pounds (9 kg), are also known to eat leaves, although this species usually turns to canopy leaves only when fruits are in short supply.


In the Old World, a few canopy primates turn to leaves. The orangutan, Asia's largest rainforest primate, and the third largest primate in the world, is one of these. The orangutan was once found throughout Asia, but is now limited to northern Borneo (the Borneo orangutan species) and Sumatra (the Sumatra orangutan species). Male specimens of the Sumatran form may reach 5 feet (1.6 m) and 400 pounds (180 kg) with an arm span exceeding 6.5 feet (2 m). The Borneo form is much smaller. Orangutans occupy the mid-strata of the forest canopy where they feed on leaves, fruits, and young shoots, and occasionally may take a bird egg or two. Orangutans are not social animals, but solitary creatures that do not form lasting pairs. Fascinatingly, the unsociable behavior of orangutans is not instinctive but learned. Mothers enforce a regime of strict separation by dragging youngsters away from each other and leaving them alone in the forest. Solitary behavior is of the utmost importance for survival in the rainforest where food trees are widely scattered. A pair or a group of slow-moving orangutans would find little more food than an individual, but have to divide it among more mouths. Orangutans share 98 percent of our genetic makeup and, like humans, participate in prolonged care for their young. Like humans each individual has a distinctive face. From birth, the orangutan undergoes variations in facial structure during the course of its lifetime: at birth its face is bare, juveniles are bearded, and adult males have skin pouches on their cheeks. Orangutans build sleeping nests, 16-80 feet (5-25 m) off the ground, each night and never return to an earlier nest.

Orangutans are highly threatened by the illegal trade in endangered species and widespread deforestation in Indonesia and Malaysia. Scientists estimate that of the remaining 45,000-60,000 orangutans in the wild, more than 1,000 are poached every year as pets or sources of bushmeat.

Chimpanzees (pictures | news), genetically, the closest living relation to man, are also leaf eaters, although they also feed on shoots, seeds, bark, fruits, and (less frequently) insects, fish, reptiles, and small mammals. Chimpanzees are highly threatened in their native West and Central Africa by destruction of habitat and hunting as a source of bush meat. Chimpanzees, up to 5.5 feet (1.7 m) when erect, are strong animals that dwell both arboreally and terrestrially.

Adult male orangutan in Sumatra, Indonesia. Click image for more photos of orangutans.. Photo by Rhett A. Butler

Review questions:

  • How do animals communicate in the canopy?
  • Why do relatively few animals eat leaves as the staple of their diet?
  • Where does the orangutan live and why is it endangered?

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