Some of the early ancestors of today's sloth were known to be the size of elephants and even larger (Steele, 2002). They have very bad hearing (Baker, 2003) and, though they are mammals, they have difficulty regulating their body temperature (Aiello, 1985). Variation in surrounding air temperature also dictates activity patterns. They tend to be more active and feed more during the daytime throughout the cooler, dry season rather than during the hotter, wet season (Chiarello, 1998).
Sloths spend the majority of their lives in the canopy within rainforests in Central (two-toed) and South America (three-toed). Owing to their extremely slow metabolism, the sloth does not like to expend a lot of energy moving. A mere ten acres for Choloepus and only five acres for Bradypus encompass their home range. They spend most of their day (up to 15 hours) sleeping (Steele, 2002) and allot the rest of their time for eating, moving from tree to tree, and lounging around. Differences in daytime and nighttime travel, however, varied considerably for the three-toed sloth with an average travel distance during the day of seventeen meters compared to five meters at night. Throughout the dry, cooler season, the sloth spends more time foraging for food and less time resting when compared to activity during the hotter, wet season (Chiarello, 1998). While the two-toed sloth changes trees daily the three-toed sloth will reside in one tree for several days at a time.
Though younger sloths can partially stand, this type of posture is not one that they are best suited for. Movement on the ground is quite awkward and is only enabled when the sloth drags itself forward on its belly by its arms (Steele, 2002). Outside of ridding themselves of excrement and subsequently burying it, they rarely venture down from their comfort zone within the trees. This activity, again due to the slow metabolism, only occurs about once a week (Baker, 2003). Without the mobility that accompanies residing in the forest canopy, the sloth is very vulnerable to predators such as the harpy eagle, jaguars, ocelots, and snakes on the forest floor as it can only move a mere twelve feet per minute. The design of their hair pattern from stomach to back is unique in that it directs the flow of rainwater so that it drips off onto the forest floor. This interesting trait of the sloth also allows it to be a great swimmer (Steele, 2003).
Sloths, being herbivores, have a diet that consists mainly of twigs, leaves, and fruits. Of the two types, the three-toed sloth tends to be a bit more choosy when it comes time to eat. It can take up to a few days to digest one meal (Steele, 2002) and it is not uncommon for food to remain in the stomach for up to a week (Baker, 2003). The main source of water is found within and running down the tops of leaves (Steele, 2002), the sloth's favorite of which is from the Cecropia tree (Baker, 2003).
Female sloths generally give birth to one offspring a year, most frequently during the rainy season. In congruency with the rest of their lives, sloths give birth while hanging upside down from the limb of a tree from six to eleven months after conception. The newborn then climbs onto its mother's stomach by using its claws (Steele, 2002). Infant sloths are born with teeth, hair, claws, open eyes, and they weigh from 340-400g. They are born with the ability to vocalize and can grasp on to objects firmly. They generally begin eating solid foods between fifteen and twenty-seven days of age and have been witnessed hanging from tree limbs within twenty to twenty-five days of their birth (Meritt, 1985). Offspring typically ride on their mother's stomach until they are about nine months old (Steele, 2002). The average life span of the sloth is twenty years (Baker, 2003).
Sloths are solitary animals but it is not improbable to come across two or more female sloths living together. Outside of this type of case, they only come together in order to mate. Sexual maturity is reached between the years of four and five for males and around the age of three years for females (Steele, 2002). After the birth of offspring, other females have been known to lick and provide assistance for newborns. On occasion, however, and depending on the sloth, some mothers have been known to hiss, claw, and strike at visitors demonstrating protective maternal instincts.
Because of the solitary behavior of sloths, the majority of what is known of their social structure is demonstrated in the interactions between mother and offspring. As the infant matures and becomes more adventurous, the mother is always in close proximity. If the wandering infant runs into another sloth, it always vocalizes for its mother runs back toward its mother for protection. If the path to its mother is blocked, it bleats until its mother responds. This attentiveness and response to distress lasts from the time the offspring can move about on its own until it is about six months old. As the young sloth ages and becomes more independent, the relationship between mother and offspring changes from one of dependence to friendship. Contrary to the behavior of other animals, there is never a time between mother and offspring when the mother forces the young out on its own. There is no strong or pronounced separation. Mother and offspring have been known to associate years after birth without any aggression (Meritt, 1985).
The identifying behavior and subsequent classification of sloths as being arboreal mammals directs attention toward possible adaptations conducive to the ability to remain suspended in trees. Lacking any hand-like structure and possessing only claws leads one to question how this adaptation is advantageous. After all, the sloth can only grab onto a limited range of tree limbs (Mendel, 1985). This unique structure of the hands and feet does not allow the sloth to assume the standing position or even support themselves in any similar manner. It has been proposed that the suspensory behavior of the sloth could be due to the efficiency of energy expended in these arboreal movements. The claws present on the hands and feet surprisingly allow for a wide range of motion and positions. These joints allow the feet to pivot almost a full 180 degrees and they also allow them to hook tree limbs at right angles to original support (Mendel, 1985).
A proposed reason for this amount of joint flexibility is to enhance the ability to reach for other limbs. This enables the sloth to expand its "feeding sphere" and even hang upside down suspended only by its feet to reach lower branches for food. Keeping these acrobatic skills in mind, one would think that the muscle mass of such an animal would be rather high. However, only about twenty-five percent of the total body weight of a sloth is muscle. Again, the lack of muscle is also due to the low metabolic rate which is compensated for by an increase in muscle efficiency. The overall interpretation for these adaptations is that they have all developed for the conservation of energy (Mendel, 1985).
Another interesting "adaptation" that the sloth possesses is the growth of algae on its hair. The structure of hair of both Bradypus and Choloepus is conducive to algae growth in that there are crevasses and cracks along the entire length of the hair. This structure allows for the unique, symbiotic relationship between the sloth and algae in that no other mammal has a feature like it. The hair structure between the two genera also differs from one to the other. While both genera have a soft undercoat of hair and a thick, coarser overcoat, neither have algae growing in the underfur. The growth of algae requires some moisture which results in dry hair being a grey or brownish color and wet hair having a green tinge. The presence of algae is already noticeable in sloths only a few weeks old (Aiello, 1985).
As mentioned earlier, the algae grows within cracks on hair that expands like a sponge as it becomes wet. It is for this reason that algae does not grow too deep in the cracks as they close significantly with swelling when wet. A few hypotheses have been proposed for the presence of algae on sloth hair. One such hypothesis proposes the idea that the sloth is more easily concealed with the green tinge and it is therefore used as camouflage(Aiello, 1985). Yet a second reason provided for the algae is the provision of nitrogen which the sloth definitely lacks. Two methods are possibilities for this transfer of nitrogen from algae to the sloth. One method is by the sloth licking its hair during auto-grooming. Though this may be possible, sloths have not been known to perform this action frequently outside of a rare brushing of their claws through their hair. The grooming process, when it has been observed, is very slow and, for the most part, ineffective. A second option is that the nitrogen moves into the sloth through the hair itself. The nutrient could travel down the hair to where it makes contact with the skin and could be absorbed into the body (Aiello, 1985).
Algae, however, is not the only life form found residing within the hair of the sloth. Nine hundred species of arthropods have been found living in a sloth's hair (Steele, 2002). The arthropods found on sloths can be divided into two categories: hematophagous guild which is comprised of biting flies, ticks, lice, and mites, and the coprophagous guild which is comprised of moths, mites, and beetles. The dependence of these arthropods on the sloth is due to the scarcity of sloth dung. Life cycles of the moths, beetles, and mites involve depositing eggs on dung which is used as a food supply once the eggs mature to the larval stage. Since sloths excrete so infrequently, these arthropods hitch a ride on the source itself until it deposits dung on the forest floor. The arthropods then proceed to lay their eggs. Not only does the sloth bare host to arthropods but many viruses as well that remain within the system for surprisingly extended periods of time. This interesting characteristic of the sloth could perhaps provide insight into a pathway taken by certain organisms as they become parasitic (Aiello, 1985).
The physiology, reproduction, behavior, social structure, and fascinating characteristics of sloths make them a one-of-a-kind kind of mammal. Their slow metabolism appears to affect movement, eating and sleeping habits, muscle mass, grooming, and niche within the rainforest and that just scratches the surface. The evolution and adaptations of the sloth and its limbs enable it to make the most efficient use of its body as well as its resources. Though it may be slow, its adopted lifestyle makes it an integral part of the ecosystem in which it plays a part. The life cycles of hundreds of other species would be affected if the survival of the sloth was ever called into question which could soon be the case. Their role in the rainforest is invaluable as is the role of every other organism.
Aiello, Annette. "Sloth Hair: Unanswered Questions." The Evolution and Ecology of Armadillos, Sloths, and Vermilinguas. Ed. G. Gene Montgomery. Washington: Smithsonian Institute, 1985.
Chiarello, Adriano G. "Activity Budgets and Ranging patterns of the Atlantic Forest Maned Sloth Bradypus torquatus." Journal of Zoology (London), v. 246 issue 1, 1998, p. 1-10.
Mendel, Frank C. "Adaptations for Suspensory Behavior in the Limbs of Two-Toed Sloths."The Evolution and Ecology of Armadillos, Sloths, and Vermilinguas. Ed. G. GeneMontgomery. Washington: Smithsonian Institute, 1985.
Meritt, Dennis A. Jr. "The Two-Toed Hoffmann's Sloth." The Evolution and Ecology of Armadillos, Sloths, and Vermilinguas. Ed. G. Gene Montgomery. Washington: Smithsonian Institute, 1985.
Steele, Christy. Animals of the Rainforest: Sloths. New York: Raintree-Steck Vaughn Publishers, 2002.
Waage, J. K. and R. C. Best. "Arthropod Associates of Sloths." The Evolution and Ecology of Armadillos, Sloths, and Vermilinguas. Ed. G. Gene Montgomery. Washington: Smithsonian Institute, 1985.
Baker, Chrisopher. Mammals. Luminal Path Corporation. 2003. 5 March 2004.
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