Tropical rainforests are just as rich in animal life as in plants, although the animal biomass is lower than that of plants. Although only a fraction of the total number of species in these forests has been discovered and classified, we know that there are at least twice as many species of mammals and birds in tropical forests as in temperate forests. Assuming that this relationship also holds for insects and invertebrate animals, the tropics contain at least three million species of animals, two-thirds of the world’s total. All of these species evolved in forest habitats and so they are closely adapted to the unique conditions within these forests. Only a few small groups of organisms (conifers, aphids, salamanders) are more numerous in temperate regions than in tropical areas.

Each of the three major tropical forest areas has its unique set of species. Most leaf-eating species live in Africa and Asia, while America has most of the frugivores (fruit-eaters) and insectivores (insect-eaters). More bird and bat species live in the American tropics than in either Southeast Asia or Africa, and the Amazon basin alone contains half the known species of freshwater fish. Three hundred species of mammals are also found there, an enormous number. Borneo, within Malesia, has more than 200 described mammal species, 350 species of birds, 200 species of reptiles and 80 species of amphibia (and probably many more which have not as yet been described), as well as half of the known species of fish (freshwater and saltwater) (Payne, 1995). Many rainforest mammals are arboreal (45% of non flying and non gliding mammals in Borneo, compared to 15% in temperate forests in the eastern United States), and most are nocturnal. Although the vast majority of animal species are insects, we have few clues as to the actual number of insect species in tropical rainforests – certainly the true number will be in the millions. Because of the great diversity of animals, there are relatively few individuals of any particular species, so that members of any one species have to maximize all avenues of reproduction and communication.

Since there are so many tropical species, one can ask how each can survive the intense competition in the rainforest. As is true of plants, animals are “compartmentalized” in their spaces and life styles within the forest, each species existing in its own niche. Animals may specialize in time or in space; they may be nocturnal or diurnal; they may live in the canopy or on the ground, on water or vegetation or on other animals; they may differ in feeding preferences, reproductive patterns, and so on. Scientists conducting a study of neotropical bats on Barro Colorado Island, Panama (Whitmore, 1998), discovered nine bat “guilds,” which were defined by feeding preferences, mainly with regard to the size of their insect prey. In Gabon, Africa, five species of nocturnal lorises segregate themselves by both food preferences and living space. Some live in the canopy (one insect-eating species, one frugivorous one, one insectivorous one) and two species live in the undergrowth (one insectivorous, the other frugivorous). Thus they never have to compete directly for space or food supply. In Ecuador, 74 species of frogs and toads have been found in one area of rainforest, most of which differed in reproductive modes. Some species lay their eggs on water, others on vegetation, in cavities in trees, in depressions in the soil, in foam nests, or on the backs of the females. Yet others have staggered reproductive periods, so that only a few species breed simultaneously, thus avoiding competition among the young for food, space and water (Whitmore, 1998).