DRAFT #1 The Brown-throated Three-toed Sloth (Bradypus variegatus)

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The Brown-throated Three-toed Sloth (Bradypus variegatus)

The brown-throated three-toed sloth (Bradypus variegatus) is one of the most common types of sloth found in the rainforests of the New World (Emmons 1990). Being one of the slowest moving creatures on Earth has not stopped the sloth from thriving within its forest habitat. Its physiological traits serve its lifestyle well and allow it to be one of the few mammals of the forests to stay off the endangered species list (Chan 1999, Emmons 1990 and Goffart 1995).

The sloth, weighing nine to ten pounds and ranging only about two feet in length, can easily be mistaken for a part of a tree due to its slow movements along the branches (Chan 1999). The Bradypus variegatus usually has pale brown fur with dirty white patches on its back. The fur itself is long, coarse and wavy with a greenish tint. The chest and throat of the Bradypus variegatus are brown which is inferred by its common name. The male Bradypus variegatus is distinguished by an orange patch of fur running from their neck to in-between the shoulder blades. Its tail, unlike most other tree dwelling animals, is short and stumpy, no more than about three inches in length. The limbs of the sloth are split between two very different leg lengths. The forelegs are long and bent awkwardly in comparison to the much shorter and straighter hind legs. Each of the four limbs end in three long hook shaped claws (Emmons 1990). Although many other types of sloth live in the same habitat as the Bradypus variegatus, several key differences can be used to tell them apart. The pale-throated three-toed sloth (Bradypus variegatustridactylus) is distinguished, as the name implies, by its pale throat and chest instead of a brown one (Chan 1999, Emmons 1990, Goffart 1995). The maned three-toed sloth (Bradypus variegatustorquatus) is different from all other sloths because of the long black mane located on the back of its neck. The two-toed sloth (Choloepus ssp.) lacks the white patches on their backs as well as tails. They also have only two claws on their front feet (Emmos 1990).

Physiologically, the sloth is a unique creature. Its abnormally slow movements make it appear to be easy prey for other animals, yet the sloth has the ability to swim at a decent rate. Its metabolic rate is an oddity; due to its slow movements the rate at which it burns calories is extremely slow, which indicates that its digestive and renal systems are honed to be compatible with such a strange animal (Kircher 1999). The eyes of the Bradypus variegatus are of particular interest given that the Bradypus variegatus is both nocturnal and diurnal, while most other sloths are active only at night. Few animals are truly able to function well all times of the day (Goffart 1995). To compensate for this abnormality the slothÕs eyes gain the ability uses extreme lens apertures to move in all types of lighting conditions, but gives up the close range focus. To grab on to branches, the sloth moves by feel rather than sight and deliberates on each movement before executing that motion.

According to American Heritage Dictionary ÒslothÓ stems from Middle English slowth, from slow. The word sloth implies laziness, and the Bradypus variegatus exemplifies this quality by doing approximately ten percent of the work a normal mammal of similar size performs in a day. A sloth will sleep or rest about twenty hours a day, although contrary to popular belief the sloth does not hang upside down while resting. The Bradypus variegatus is especially known for squatting in-between the forks of trees. Its sluggish movements can reach speeds around 0.5 to 1.0 km/hr when moving through the trees. In comparison to a common cat, which can reach speeds up to 40 km/hr, the sloth barely seems to move at all. If a sloth needs to move on the ground, its speed is drastically reduced to about 0.05 km/hr (Goffart 1995). The slothÕs lack of ability to move on the ground is due not only to its irregular feet that have curved claws for grabbing but also the lack of muscles required to erect itself on all fours. Without the capability to move on the ground or at any great speed, it is fortunate that the sloth is a strong animal for its size (Mendel 1985). Its strength allows it to be highly acrobatic, suspended horizontally for several minutes by a single leg, and to also to be a powerful swimmer (Emmons 1990 and Kricher 1999). This does not mean this slow animal can move quickly in the water, it moves twice as fast climbing through the trees, but it is just rather surprising that it can swim at all. There have been many suggestions over the years as to why the sloth is such a slow moving creature ranging from the fact that it has a small heart to just having slow brain function. The most logical reasoning is that being slow helps protect the sloth from hunting eyes because it appears to be an inanimate object (Goffart 1995).

A sloth is always thought to have a full stomach. This declaration is made because the sloth can have food remains within its stomach for seventy to ninety hours. Sometimes, it can take over a week to pass through its entire digestive tract. Like many other herbivores, the sloth has four different stomachs, usually compared to those found within an ox, which uses bacterial fermentation to digest the food (Goffart 1995). The stomachs of a sloth fill almost the entire abdominal cavity and make up approximately 20-30% of the animals total body weight. The slothÕs lack of skeletal muscles and therefore minimal movements contribute to its incredibly slow metabolism. This slow metabolic rate is the reason behind the slothÕs full stomach and lack of need to defecate waste. A sloth can go for six to ten days without discharging urine or feces. When a sloth does defecate, it is no small matter. The Bradypus variegatus is said to sometimes take as long as an hour to complete the process and can expel around 250 g of feces and 800 - 1200 ml or urine. This amount if translated to a 70 kg human male is 27 kg of urine and feces or about 38% of their total body weight.

The sloth requires a very specific climate in which it can reside. Several factor can explain why the sloth is only found in tropic rainforests. The Bradypus variegatus depends heavily on its climate and for that reason has trouble surviving in captivity. The Bradypus variegatus is best suited in areas that are wooded with high humidity, near rivers, and open jungles (Chan 1999). The forests that fit this description span from Honduras down through Central America into the heart of South America around Peru (Chan 1999 and Emmons 1990). The reason a rainforest climate is required is that the sloth has difficulty keeping their body temperature in homeostasis with changing surroundings. The sloth has a rather low body temperature compared to other mammals and slight changes in the external temperatures can be damaging for the sloth. The sloth must seek the shade of the trees when the heat from the sun is too great and rely on the fact that the tropics will remain warm year round so they will not have to suffer the cold (Chan 1999 and Goffart 1995). The sloth needs the climate to be humid to keep water loss at a minimum. Sloths rarely drink water; most of the liquid they receive is from the food they eat. Since the water from food is not found in vast amounts, the sloth cannot afford to lose water to its environment. The rainforests also provide one last essential resource for the sloth, its food. The leaves of the Cecropia tree are the main diet for the Bradypus variegatus. To say that it is the only thing the Bradypus variegatus will eat is a common misconception. It may appear this way because the sloth is most visible within those trees to onlookers. The Bradypus variegatus must venture to lower branches on the Cecropia trees to eat the leaves, therefore leaving the upper canopies where human eyes can see them. The Bradypus variegatus will eat many types of tropic leaves but it refuses almost everything else. The three-toed sloths are very different from their two-toed relatives when it comes to eating; the two-toed sloths are not nearly so picky on what they will eat to survive. Their finicky eating habits make the Bradypus variegatus even more difficult to keep in captivity. Currently, the Bradypus variegatus remains off the endangered species list but its lack of ability to live anywhere else in the world may place it there in the near future. As deforestation strikes ever harder at the forests of Central and South America, the slothÕs habitat continues to become ever-smaller (Goffart 1995).

For being one of the earthÕs slowest mammals, one might assume that the sloth is easy prey for its natural enemies. The slothÕs lifestyle caters to its ability to avoid being killed by the great predators of the jungle, which include the jaguar, the anaconda, and the harpy eagle. The slothÕs slow and silent movement make it almost impossible to be visible from ground and the greenish tint within its fur gives it added camouflage (Mendel 1985 and Montgomery 1985). The slothÕs fur and movements do not prevent it from being attacked from the sky but they take special precaution to stay out of the sunlight at dawn and dusk when the eagles are hunting. The Bradypus variegatus has another interesting defensive technique which it uses to avoid detection. This technique involves climbing down to ground level, digging a hole in which to defecate and then burying it. By covering its waste, there is no smell to give away its position to a jaguar. Although sloths can defend themselves with their sharp claws, they are little match against the predators that hunt them (Montgomery 1985).

Even in times of mating, the sloth is a solitary creature to the end. They are extremely intolerant of each other even if they are mates. The females will attempt to attract a male during the rainy season with a distinct Òaa-eeeÓ sound, which calls the male over to start a courtship. This courtship lacks emotion and is very short (Chan 1999 and Greene 1989). Once the mating is over, the two sloths will not stay near each other for very long. The female sloth has a single offspring each year, the gestation period lasts four to six months. During the pregnancy, the mother will not build any sort of nest in which to keep the newborn. Once the baby is born, it clings to its motherÕs abdomen. After four to five weeks, the baby is able to eat solid foods and no longer needs its mother. At that time, the mother will leave the child to fend for itself (Goffart 1995 and Montgomery 1985).

The Bradypus variegatus has several oddities that make it rather unique. Because of its slow movements, it acts as an easy excess point for many types of insects to cling onto its fur for survival. The slothÕs body functions as a host as it protects, provides a food source and place to reproduce. The feces of the sloth are the perfect place for beetle and moth larva to mature. By burying the pellets, the sloth ensures the insect offspring protection. At this time, there is no conclusive evidence that suggests that the sloth gains anything by playing host to these insects. It appears that the sloth is indirectly just keeping balance within the ecosystem. The Bradypus variegatus is unique in that it is the only mammal existing or extinct known to naturally accumulate algae on its body. A few weeks after birth, the slothÕs fur becomes covered in the algae and takes on a greenish tint. The plant and sloth have a symbiotic relationship that involves the plant gaining shelter and the sunlight in a forest that is normally kept dark by the vast canopies. The sloth gains camouflage, which helps protect it from many of its predators. The final oddity is found in the slothÕs immune system. Because the sloth has such a slow population turn over rate, its immune system is very flexible. If any strain of virus common to the forest becomes fatal to the sloth, it will kill the entire population in one fell sweep. Instead of being killed by the diseases of the rainforests like yellow fever, the sloth keeps the disease within its system and is not harmed by it. This makes the sloth extremely useful to doctors allowing them to run tests on these diseases in living mammals without killing them. The information can be used to create antibodies and vaccines to the diseases (Montgomery 1985).

Sloth is a very peculiar creature, in some ways seeming to be too dull to even consider, and at others times fascinating because of its odd ways of survival. The sloth is one of natureÕs slowest creatures and if not investigated can be overlooked, but its strange lifestyle seems to be the reason it avoids natural section taking its toll on an unfit creature. It is strange to think that a mammal unable to walk on solid ground can thrive in todayÕs world, yet it not only survives but it is useful to the ecosystem for which it lives as well as to humankind.

Literature Cited

Chan, Christine. "The Biogeography of the Brown-throated Three-toed sloth (Bradypus variegatus)." San Francisco State University Department of Geography. Oct. 1999. 28 Apr. 2007 .

Emmons, Louise H. Neotropical Rainforest Mammals. Chicago, IL: The University of Chicago Press, 1990.

Goffart, M. Function and Form in the Sloth. Elkins Park, PA: Franklin Book Company, 1995.
Greene, Harry W. "Agonistic Behavior by Three-toed Sloths, Bradypus variegatus ." Biotropica 21.4 (Dec. 1989): 369-372.
Kricher, John. A Neotropical Companion. Princeton, NJ: Princeton University Press, 1999.
Mendel, Frank C. "Use of Hands and Feet of Three-Toed Sloths (Bradypus variegatus) during Climbing and Terrestrial Locomotion." Journal of Mammalogy 66.2 (May 1985): 359-366.
Montgomery, Gene G., ed. The Evolution and Ecology of Armadillos, Sloths, and Vermilinguas. Washington and London: Smithsonian Institution Press, 1985.

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