Animals, like plants, are limited in the location and size of their populations by many factors. Competition, predation, availability of water, food and habitat – all will affect where animals are found. Perhaps the most important of these factors is food. Animals will be prevalent where there are adequate sources of nutrition. In tropical rainforests, foods are sporadically distributed throughout the forest, since few species are clustered. Nor is animal prey. Therefore, some animals, such as deer, pigs, and primates, must migrate to satisfy their nutritional needs. Many animals depend on a variety of fruits, which are produced at different times of year in the forest. Some plant species, such as figs, produce fruit all year, and provide a major source of food. When prime food supplies decline, animals may switch to less desirable alternative foods. For instance, when fruits are scarce, some primates will shift to eating palm nuts (capuchin monkeys, with strong jaws), or strangler fig fruits and insects (squirrel monkeys). Some animals, like the tamarins of South America, find a major food resource in plant nectars, and will follow the flowering of their preferred plant species.
Little is known about other factors which regulate animal distribution. Some animals are prevalent on mineral-rich soils. In Africa, many herbivores are found where soils are rich in sodium, phosphorus and magnesium. The locations of rhinoceros and elephant populations may be determined (at least partially) by the availability of these minerals. In Southeast Asian forests, where leaves are often low in sodium, animals are abundant near mineral-rich salt licks. The proboscis monkey of Southeast Asia lives in forests which grow on soils with a high mineral content. Perhaps only plants growing on this type of soil provide adequate minerals and food for their breeding and survival.
Different types of habitats are required by various kinds of animals. Some animals, such as many birds, require undisturbed primary forest. Other animals like tapirs do well in disturbed forests and secondary growth. Orangutans, found only on Borneo, require forest cover, but can survive even in secondary forest. They prefer lowland swampy areas, and are more rare in hardwood (dipterocarp) forests.