Two-Toed Sloth (Choloepus hoffmanni)

Sloths spend their lives hanging from trees—eating, mating, and even giving birth.

The sloth is the world's slowest mammal, so sedentary that algae grows on its furry coat. The plant gives it a greenish tint that is useful camouflage in the trees of its Central and South American rain forest home.

Sloths are identified by the number of long, prominent claws that they have on each front foot. There are both two-toed and three-toed sloths.

All sloths are built for life in the treetops. They spend nearly all of their time aloft, hanging from branches with a powerful grip aided by their long claws. (Dead sloths have been known to retain their grip and remain suspended in the air.) Sloths even sleep in trees, and they sleep a lot—some 15 to 20 hours every day. Even when awake they often remain motionless, and two-toed sloths are generally silent. At night they eat leaves, shoots, and fruit from the trees and get almost all of their water from juicy plants.

Wallaby Macropus (Notamacropus)

Wallabies, like their close kangaroo relatives, have long tails for balance and large feet and strong legs for jumping great distances.

Wallabies are members of the kangaroo clan found primarily in Australia and on nearby islands. There are many wallaby species, grouped roughly by habitat: shrub wallabies, brush wallabies, and rock wallabies. Hare wallabies are named for their size and their hare-like behavior.

All wallabies are marsupials or pouched mammals. Wallaby young are born tiny, helpless, and undeveloped. They immediately crawl into their mothers' pouches where they continue to develop after birth—usually for a couple of months. Young wallabies, like their larger kangaroo cousins, are called joeys. Even after a joey leaves the pouch, it often returns to jump in when danger approaches.

Walrus (Odobenus rosmarus)

Both male and female walruses have tusks and have been observed using these overgrown teeth to help pull themselves out of the water.

The mustached and long-tusked walrus is most often found near the Arctic Circle, lying on the ice with hundreds of companions. These marine mammals are extremely sociable, prone to loudly bellowing and snorting at one another, but are aggressive during mating season. With wrinkled brown and pink hides, walruses are distinguished by their long white tusks, grizzly whiskers, flat flipper, and bodies full of blubber.

Walruses use their iconic long tusks for a variety of reasons, each of which makes their lives in the Arctic a bit easier. They use them to haul their enormous bodies out of frigid waters, thus their "tooth-walking" label,

Warthog (Phacochoerus africanus)

Although warthogs look fierce, they are actually herbivores who prefer to flee rather than fight.

Warthogs are members of the same family as domestic pigs, but present a much different appearance. These sturdy hogs are not among the world's most aesthetically pleasing animals—their large, flat heads are covered with "warts," which are actually protective bumps. Warthogs also sport four sharp tusks. They are mostly bald, but they do have some sparse hair and a thicker mane on their backs.

Though warthogs appear ferocious, they are basically grazers. They eat grasses and plants, and also use their snouts to dig or "root" for roots or bulbs. When startled or threatened, warthogs can be surprisingly fast, running at speeds of up to 30 miles (48 kilometers) an hour.

Warthogs are adaptable and are able to go long periods without water, as much as several months in the dry season.

When water is available, warthogs will seek it and often submerge to cool down. They will also wallow in mud for the same purpose—and to gain relief from insects. Birds also aid these hogs in their battle with insects; oxpeckers and other species sometimes ride along on their warthog hosts, feeding on the tiny creatures invading their hides.

Water Buffalo (Bubalus bubalis)

Both male and female water buffalo wear the species’ signature backward-curving horns, although the females’ horns are significantly smaller.

The water buffalo, or Asian buffalo, as it is often called, is the largest member of the Bovini tribe, which includes yak, bison, African buffalo, various species of wild cattle, and others.

Standing 5 to 6.2 feet (1.5 to 1.9 meters) tall at the shoulder, wild water buffalo are formidable mammals with sparse gray-black coats. Males carry enormous backward-curving, crescent-shaped horns stretching close to 5 feet (1.5 meters) long with deep ridges on their surface. Females are smaller in size and weight, but they also have horns, although they are proportionately smaller.

Water buffalo spend much of their day submerged in the muddy waters of Asia’s tropical and subtropical forests. Their wide-splayed hoofed feet prevent them from sinking too deeply in the mud and allow them to move about in wetlands and swamps. These marshes provide good cover and rich aquatic plants to forage on, although water buffalo actually prefer to feed in grasslands on grass and herbs.

Weddell Seal (Leptonychotes weddelli)

Weddell seals are known for their docile nature and are easily approached by humans.

Weddell seals spend much of their time below the Antarctic ice. They have the southernmost range of any seal, but find the chilly waters rich with the prey they seek. These seals do not migrate often and are commonly found within a few miles of their birthplace.

By swimming under the ice, these seals can often avoid their main predators—orcas and leopard seals. The environment helps their own fishing as well. When feeding below the ice, they may dive beneath their prey. As the seal rises, the fish above it are backlit by the ice above and easily spotted in silhouette. Weddell seals can also use air to collect a meal. They have been known to blow air into cracks in the ice. The surprise tactic puts small fish to flight, which the seal then devours. Cod and silverfish are favorites, though these seals also eat small crustaceans, octopuses, and other marine creatures.

Western Lowland Gorilla

Gorillas are very intelligent and have been taught simple sign language in captivity. Like chimpanzees, gorillas have been observed using tools in the wild.

Western lowland gorillas are endangered, but they remain far more common than their relatives, the mountain gorillas. They live in heavy rain forests, and it is difficult for scientists to accurately estimate how many survive in Cameroon, Central African Republic, Congo, Equatorial Guinea, Gabon, Angola, and the Democratic Republic of Congo.

Western lowland gorillas tend to be a bit smaller than their mountain cousins. They also have shorter hair and longer arms.

Gorillas can climb trees, but are usually found on the ground in communities of up to 30 individuals. These troops are organized according to fascinating social structures. Troops are led by one dominant, older adult male, often called a silverback because of the swath of silver hair that adorns his otherwise dark fur. Troops also include several other young males, some females, and their offspring.

The leader organizes troop activities like eating, nesting in leaves, and moving about the group's three-quarter- to 16-square-mile (2- to 40-square-kilometer) home range.

White Rhinoceros (Ceratotherium simum)

White rhinos graze on grasses, walking with their enormous heads and squared lips lowered to the ground. Both black and white rhinoceroses are actually gray. They are different not in color but in lip shape. The black rhino has a pointed upper lip, while its white relative has a squared lip. The difference in lip shape is related to the animals' diets. Black rhinos are browsers that get most of their sustenance from eating trees and bushes. They use their lips to pluck leaves and fruit from the branches. White rhinos graze on grasses, walking with their enormous heads and squared lips lowered to the ground.

White rhinos live on Africa's grassy plains, where they sometimes gather in groups of as many as a dozen individuals. Females reproduce only every two and a half to five years. Their single calf does not live on its own until it is about three years old.

White-Eared Kob (Kobus kob)

The Sudd wetlands of southern Sudan are home to more than 800,000 white-eared kob, best known for their annual migration.

These graceful antelopes of central Africa’s well-watered savannas and floodplain grasslands are best known for their annual migration, which produces one of the natural world’s great spectacles.

The Sudd wetlands of southern Sudan, nourished by the White Nile, are home to an astounding number of white-eared kob—more than 800,000 animals. When joined by tiang antelopes and Mongalla gazelles they form enormous migrating herds of more than 1.2 million individuals. Thickly packed columns of these animals in motion can stretch a staggering 50 miles (80 kilometers) long and 30 miles (48 kilometers) across. Such herds rival the Serengeti’s teeming masses of wildebeests for the title of the world’s most massive—and awe-inspiring—mammal migration.

White-Tailed Deer (Odocoileus virginianus)

White-tailed deer spend their summers grazing in the meadows and move into the forests during winter for protection from the elements.

White-tailed deer, the smallest members of the North American deer family, are found from southern Canada to South America. In the heat of summer they typically inhabit fields and meadows using clumps of broad-leaved and coniferous forests for shade. During the winter they generally keep to forests, preferring coniferous stands that provide shelter from the harsh elements.

Adult white-tails have reddish-brown coats in summer which fade to a duller grayish-brown in winter. Male deer, called bucks, are easily recognizable in the summer and fall by their prominent set of antlers, which are grown annually and fall off in the winter. Only the bucks grow antlers, which bear a number of tines, or sharp points. During the mating season, also called the rut, bucks fight over territory by using their antlers in sparring matches.