The sloth is an animal that uses its claws to hang from branches.
Credit: © Michael Fogden, Bruce Coleman, Inc.

Slow down and wait a minute! Today is International Sloth Day, a day to slow our speed and appreciate the world’s slowest mammal. While many species evolve to eat more, sloths have done the opposite! They evolved in a way that allows them to eat less and survive just by slowing down.

A sloth is a mammal that has a slow and peculiar way of moving. Sloths spend nearly all of their time in rain forest trees in Central and South America, where they travel upside down, hanging from branches with their hooklike claws. Hanging upside down requires almost no energy for a sloth. They can fall asleep in this position and may even stay suspended in the trees for some time after they die. There are two main groups of sloths. One is two-toed and the other is three-toed.

All sloths have small heads, and their noses are blunt. They have peglike teeth. Two-toed sloths also have large sharp teeth at the front of the mouth. Both measure 15 to 30 inches (38 to 76 centimeters) long and weigh 5 to 23 pounds (2.3 to 10.5 kilograms). Their long, coarse fur grows in the opposite direction as that of other mammals, from the stomach towards the back. This allows rain water to easily drain off the body as the sloth hangs. The fur ranges from grayish to brownish in color, which makes them hard to see among the branches.

Sloths turn green in the rainy season from algae that grows in their fur. This helps the sloth blend into the rain forest and protects it from large birds of prey, such as the harpy eagle, and big cats. Sloth fur also provides a home to a variety of invertebrates (animals without backbones) — some of which are found nowhere else on earth. A single sloth can host more than 100 moths and other insects within its fur.

Sloths get little energy from their diet, feeding mostly on leaves. Two-toed sloths may also eat fruits and flowers. They need relatively little food and have a lower rate of metabolism than do other mammals of similar size. Metabolism is the process by which living things turn food into energy. In order to save energy, sloths do not regulate their body temperature like other mammals. They have a lower body temperature than most mammals, which varies with the environmental conditions.

A sloth can take up to 30 days to digest a single leaf. As a result, they have a constantly full stomach. Sloths climb down to the forest floor to defecate (eliminate wastes) about once a week. They can lose up to a third of their body weight in one sitting. Sloths are surprisingly good swimmers. During the rainy season they can swim about three times faster than they can move on the ground.

Although commonly grouped together, the two types of sloths are actually very different animals with very different lifestyles. Two-toed sloths are slightly larger, more active, have a broader diet, and are generally faster-moving than the three-toed sloth. They have brown hair with a long, pinkish, piglike snout. Three-toed sloths have gray hair, a white face and a dark mask around the eyes. Two-toed sloths are primarily active at night, while three-toed sloths are active throughout the day and night. Although almost all mammals possess seven cervical (neck) vertebrae as standard, sloths are one of the few mammals that do not. Two-toed sloths retain only five to seven cervical vertebrae, while three-toed sloths have eight or nine. This unusual trait enables three-toed sloths to turn their head through 270 degrees. This allows them to look for predators and to see the world right side up, while hanging upside down. Sloths can live up to about 30 years.

August 20, 2013

The first new species of mammal identified in the Western Hemisphere in 35 years has been unveiled by a scientist who spent 10 years trying to prove that a specimen he found in a museum drawer was, in fact, a new kind of raccoon. The new animal, which has reddish-orange fur and a bushy tail, is called a olinguito, a Spanish word meaning little, adorable olingo. Olingos are members of the raccoon family, with brownish fur and larger bodies and ears than the olinguito. At only 2 pounds (0.9 kilogram), the olinguito is the smallest raccoon known to scientists. Kristofer Helgen of the Smithsonian Institution in Washington, D.C., led the team that found the olinguito in the cloud forests of the Andes Mountains in Ecuador and Columbia.

The olinguito of Central and South America is the first canivore discovered in the New World in 35 years. (Mark Gurney)

Although new to science, the olinguito is not new to scientists. That is, scientists had previously observed the animal in the wild, collected pelts for museum collections, and even displayed the animal in zoos. But they never realized that it was a separate species. Helgen first saw the olinguito in the specimen collection at the Field Museum in Chicago while working to count the number of olingo species. After spending some years trying to prove that the specimen represented a new kind of raccoon, Helgen and other scientists traveled to the Andes to search for the animal in its natural habitat. Further studies, including genetic testing, confirmed that the animal was a previously unknown species.

The olinguito, whose scientific name is Bassaricyon neblina, is about the size of a hamster. Because of its well-developed canine teeth, the olinguito is considered a carnivore. Most carnivores are meat-eaters. However, like other members of the raccoon family, the olinguito eats other foods, particularly figs, insects, and plant nectar. Although tens of thousands of olinguitos live in the northern Andes, the people of the region apparently have never named it.

Additional World Book articles:

• Coati
• Ringtail

Polar bears are more distantly related to other bears than previously thought, according to new research. Scientists have long known that polar bears share a common ancestor with brown bears. Previous research had shown that the split between the two groups occurred relatively recently, about 150,000 years ago. But in new research, scientists found that the split took place much earlier, about 600,000 years ago. Thus, polar bears are more distinct from other bears than scientists thought.

The research helps scientists to understand the history of how polar bears have evolved (developed over many generations). Polar bears have many adaptations (features) that help them to survive on Arctic sea ice. These adaptations help to make them different from brown bears. Polar bears would have had to evolve these adaptations in relatively little time if they had split from the same ancestor as brown bears only 150,000 years ago.

The new research involved studying bear DNA, which carries hereditary information. Scientists compared the DNA of black bears, brown bears, and polar bears. Comparing the DNA enabled scientists to estimate the date at which polar bears and brown bears shared a common ancestor. Earlier research into polar bear origins used DNA from a different part of the cell. Most DNA is housed in the cell’s nucleus (a body at the center of the cell). However, a small amount of DNA is housed in mitochondria (tiny bodies in the cell that burn chemical energy). The earlier research used DNA from the mitochondria to estimate the first appearance of polar bears. The new research used DNA from the nucleus. This information provided more data about polar bear origins.

The hardy polar bear lives along the frozen shores and in the icy waters of the Arctic Ocean. Polar bears have a thick, white coat that blends in with the ice and snow. They swim strongly by paddling with their front legs and stretching their head forward. (Marvin E. Newman, DPI)

Scientists suggest the different estimates for the first appearance of polar bears may have been caused by later matings between polar bears and brown bears. Such mating is known to occur, especially when polar bear populations are under stress. Thus, the mitochondria results may record breeding between polar bears and brown bears about 150,000 years ago. In fact, the research suggested that polar bears have gone through several so-called bottlenecks, periods during which relatively few polar bears survived. These bottlenecks may have corresponded to reduced sea ice or other climate changes that made it more difficult for polar bears to survive. Polar bears are currently considered vulnerable to extinction because of a loss of sea ice caused by climate change, and scientists have shown that some polar bears and brown bears have breed in recent years. The research was published online in the journal Science.

Additional World Book articles:

The Great Meltdown (a special report)

Nov. 23, 2011

A comparison of the whiskers on a small South American marsupial with those of modern mice and rats have led researchers in the United Kingdom to an interesting theory about the evolution of mammals. Whiskers, say scientists from the University of Sheffield, may have represented an important milestone in the development of the first mammals. The marsupial, called the grey short-tailed opossum, is similar in many ways to an early mammal that lived about 125 million years ago. Scientists believe that the ancestors of modern rodents and marsupials split from a common ancestor several million years before that.

Most mammals other than humans have long, stiff hairs, commonly known as whiskers, growing around mouth or other parts of the head. These highly sensitive touch organs are also called vibrissae or tactile hairs. Around the tactile hairs lie many nerves, which transmit signals to the brain when the whiskers brush against objects. “Whisking” help animals to feel their way through narrow or dark places. It also helps animals, particularly mice and rats, build maps of their surroundings.

A mother opossum carries its young on its back. Baby opossums stay in their mother's pouch for about two months after birth. They remain near her for several more weeks. E. R. Degginger

The University of Sheffield researchers found that both modern mice and rats and the grey short-tailed opossum whisk rapidly from side to side if the animals are traveling in a straight line. As they turn, however, their whiskers follow the turn. If the whiskers contact an object or a surface, the whiskers on the opposite side sweep around to collect more information.

The researchers proposed that the first mammals developed whiskers to help them survive in a challenging environment. At that time, mammals were small, tree-dwelling creatures that hunted at night in the shadow of the dinosaurs that dominated the land areas of Earth. In addition to learning about whisking, the scientists are using their findings to help to develop robots with artificial whiskers that could be used in search-and-rescue operations at disaster sites.

Additional World Book articles:

• Animal (Adaptations for sensing the environment)
• Perception
• Senses