On land, animal habitats are strongly influenced by climate, the combination of precipitation and temperature conditions experienced in a region. At or near the equator, year-round moisture and warmth generates a constant supply of food. Further north or south, seasonal changes become much more pronounced, shaping the type of animals that live in different habitats and their strategies for survival (see Animal Distribution).
Tropical and subtropical forests are home to by far the largest number of animal species on land. These animals include the majority of the world's insects, most of its primates, and a large proportion of its birds. Tropical forests have existed longer than any other forests on earth and their plants and animals have evolved an elaborate web of interrelationships.
Much of the exotic animal life of tropical forests is still poorly known, and new species are constantly being discovered. The majority of these newly identified animals are invertebrates, but larger animals have also come to light during the 20th century. Major discoveries have included three large but secretive plant-eating mammals: the okapi, discovered in Central Africa in 1900; the kouprey, discovered in the forests of Cambodia in 1937; and the sao la, which was identified in forests bordering Laos and Vietnam in 1993.
Unlike tropical forests, temperate forests provide animals with an abundance of food during spring and summer, but a dearth during the winter. In this habitat, animals have evolved several different strategies for avoiding starvation during the winter months. Food hoarders, such as squirrels and jay birds, bury surplus food during the fall, and dig it up again when other food supplies run out. Other forest animals, such as the common dormouse, avoid food shortages by hibernation, a period of inactivity when body temperature is lowered. A third group of animals—composed chiefly of birds, but also including some bats and insects–migrates to warmer regions before the winter begins and returns again in spring. In boreal forests, which are found in the far north, the seasonal swings are more extreme. Here only a few species stay and remain active during the winter months.
For land animals, the most testing habitats are ones that experience intense drought or extreme cold. Desert animals cope with heat and water shortage by behavioral adaptations, such as remaining below ground by day, and also by physiological adaptations. North American kangaroo rats, for example, can live entirely on dry seeds without ever drinking liquid water. They do this by losing very little moisture from their bodies and using all the "metabolic water" that is formed when food is broken down to release energy.
In tundra and on polar ice, winter air temperatures can fall to below -40° C (-40° F), which is far colder than the temperature of the surrounding seas. The smallest inhabitants of tundra, which include vast numbers of mosquitoes and other biting flies, spend winter in a state of suspended animation and are kept alive by chemical antifreeze within their tissues. The few animals that do remain active on land or ice during winter, such as seals and male emperor penguins, rely on a thick layer of insulating fat to prevent their body heat leaking away. Without this fat, they would die within a matter of minutes.