The Understory


July 31, 2012

As a result of the lack of abundant ground growth, the tropical rainforest supports few large-bodied herbivores and consequently an even smaller population of large predators. The majority of ground-dwelling animals are small to medium-sized creatures that feed on fallen fruits and seeds, saplings, and small prey.


Because much of the vegetation is above the reach of forest animals, tropical rainforests support only a small biomass of large herbivores, especially in comparison with surrounding savanna areas. Much of their sustenance comes from the consumption of fallen fruits, seeds, and flowers.


Whole niches have opened for species that feed exclusively on fallen matter. Convergent evolution has resulted in species with similar adaptations and appearance on each forested continent, even though none belong to the same family. This niche is filled in Asia by the mouse deer; in Africa by the duikers; and in South and Central America by the agouti. All three animal groups are similar in structure—about the size of a rabbit with thin legs and sharp claws on their hooves—and similar behavior—all three are nervous animals due to their highly sought and appreciated meat.

There are at least six species of duikers, a type of small antelope, which feed only on fallen fruit and seeds. The African rainforest is able to support this diversity in any one location because each species has adapted to feed on a unique type of seed. For example, one species is able to eat huge fruits by having a flexible jaw. Another species has special teeth which enable it to crack some of the hardest seeds in the rainforest.

Agoutis, acouchis, and pacas, all cat-sized, primarily nocturnal, rodents of the New World, are examples of the incredible rodent radiation in the New World. New World rodents are important seed dispersers in tropical rainforests, like the squirrels of temperate regions, in that they collect seeds and bury them in scattered stashes over a relatively wide range. A good portion of the seed caches go unclaimed and the seeds germinate, now safely distanced from the parent tree. Agoutis and pacas are active in the same forests, but avoid direct competition by feeding at different times: agoutis tend to be more diurnal (day-active), while Pacas tend to be nocturnal.


Mammal life in the tropical rainforest is as abundant and diverse as the other life forms there. However, small mammals are the rule and larger mammals are far less common than in temperate forests and the African savannas. The scarcity of large mammals is in part due to the lack of leaves at ground level on which to feed. Only a few large mammal species exist in tropical rainforests: the okapi, the elephant, the pygmy hippo, the bongo, and the gorilla of Africa; the tapir, the rhinoceros, the forest deer, and the elephant of Asia; and the tapir of South America.


The bizarre Okapi is a primitive giraffe, about the size of a horse, found in a small region of rainforest in Zaire bordered by the Ituri, Uele, and Zaire rivers. It has striped legs like a zebra to help it blend into the shadows of the forest and a long neck to reach leaves above the floor. The okapi is solitary or lives in pairs and feeds on foliage that is shunned by other forest creatures. The okapi was only discovered by Western science in 1899 and less than 10,000 individuals exist today. During the civil strife in the Democratic Republic of Congo (formerly Zaire) in the late 1990s and 2000s, the Okapi Faunal Reserve suffered much encroachment from fleeing refugees and militias, but the okapi population fared well. However in 2012, guerrillas attacked the headquarters of the reserve, killing rangers and more than a dozen okapi.

The gorilla is the world's largest primate, weighing up to 660 pounds (300 kg), having an armspan exceeding 6.6 feet (2 m), and standing six feet (1.8 m) on all fours and even taller when erect (females usually weigh less than 120 kg-265 pounds). Gorillas are represented by two subspecies: the western gorilla of lowland rainforests in Gabon, Cameroon, Nigeria, and Congo; and the eastern or mountain gorilla of the montane cloud forests in Uganda, Rwanda, and Congo (Zaire). Gorillas live in family groups consisting of one dominant silverback male, one to three sub-adult males, females, and juveniles. Gorillas build nests each night; males usually build theirs on the ground or low branches, while females build their quarters higher up in trees. Silverback males have virtually complete control over the group and dictate when to commence nest-building, when to forage, and how to accept intruders. Gorillas, despite a reputation for being brutal, are generally gentle, investigative creatures that feed on leaves, stems, and bamboo shoots [news and information on gorillas | photos].

African elephants are the largest land mammals of the world and are usually associated with the open savannas and plains of Africa. However, some African elephants (now considered a separate subspecies of elephant) do enter the Ituri rainforest of Zaire where they play an essential, although poorly understood, role as "architects of the rainforest." Elephants create clearings in the forest where they gather at mineral deposits and preferred drinking spots. After hundreds and even thousands of years of elephant trampling and digging, some of these clearings may reach several hundred yards across. The elephants keep these clearings open by trampling young colonizing plants and feeding on new leaves. These clearings attract other species which drink at the water hole and hunt prey exposed in the open area. Elephants, by feeding on young leaves in the forest, also allow more light to penetrate the canopy and reach the forest floor. Elephants not only clear areas, but also contribute to tree dispersal, in that several species of seed will only germinate after passing through an elephant's gut. [news and information on elephants | photos]


Like African elephants, Asian elephants play a similar role in the tropical rainforests of India and Southeast Asia. They have even been introduced into the world's third largest island, Borneo, when they were given as a gift to the sultan in the early 19th century.

Asia, like Africa, also possesses rhinoceroses, but those of Asia are more associated with the rainforest and are much smaller. Both the Sumatran and Java rhino are on the brink of extinction from habitat loss and hunting for their horn, which is ground up into a fine powder and used for medicinal purposes despite its composition: 100 percent keratin—the same structural material of fingernails. It is estimated that there are only 413-563 Sumatran rhinos left [news and information on rhinos | photos].

The Malay tapir is the largest of four species of tapir found worldwide. Related to horses and rhinos (Perissodactyla order), the tapir resembles a giant pig with a sawed-off elephant trunk, and has a short, thick body which may reach eight feet (2.4 m) in length. The tapir likes to be near water and is an excellent swimmer. It feeds on grass, leaves, and fallen fruits. Like the tapir of the New World, the Malay tapir lives alone or in pairs.

South America

There are three species of tapir in the Americas: two mountain tapirs of the Andes in Colombia and Ecuador, and the extremely rare Baird's Tapir ranging from Mexico to Ecuador. Tapirs are most closely related to horses and rhinos, despite their appearance, and have a mobile proboscis. Tapirs have recently become the focus of a new IUCN campaign because of their rapidly declining numbers.

Western Lowland Gorilla (Gorilla gorilla gorilla). Click picture for more gorilla photos. Photo by Rhett A. Butler

Review questions:

  • What role do elephants play in forest dynamics?
  • What is an Okapi?

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Continued / Next: Mammals of the Rainforest Floor