Sloths

Sloth, a word used in English for over 400 years to mean slow, was used in the 17th century to name six species of slow-moving mammals native to the tropical rainforests of Central America and northern South America.  Those in the genus Choloepus have two toes on their forelimbs and include Linnaeus’s two-toed sloth, Choloepus didactylus, and Hoffman’s two-toed sloth, Choloepus hoffmanni.  In a second genus, Bradypus, they have three toes on their forelimbs and include the pygmy three-toed sloth, Bradypus pygmaeus, the maned sloth, Bradypus torquatus, the pale-throated sloth, Bradypus tridactylus, and the brown-throated sloth, Bradypus variegatus. 

Sloths are 24″ to 30″ in length, weighing 8-17 pounds.  They have rounded heads with tiny ears, and their forelimbs are almost twice as long as their hind limbs. Three-toed species have a small, stubby tail.  Limbs are adapted for hanging and grasping, while strong claws help to support the weight of the animal.  About half their body weight is from the undigested food in their stomach, which is constantly filled.  Sloths have an unusual number of vertebrae in their necks, either six or nine compared to most mammal species that have seven.  This allows them to almost look backwards, being able to turn through a 270˚ arc.

Everything about the sloth is slow: their daily routine, their feeding, their movements, and their metabolism.  They are nocturnal, eating at night and spending most of the day hanging motionless, while sleeping.  Their low-energy, leaf-based diet supports a slow-paced lifestyle.  Sloths are primarily folivores, feeding on leaves and fruit of several tree species.  They have a multi-chambered stomach with symbiotic bacteria to help digest fibrous vegetation in a slow process that can take several weeks to fully process a single meal.

Sloths move very slowly, even when threatened, averaging just 13′ a minute.  They rarely leave their arboreal habitat, except to defecate.  This happens only once every eight days, when they go down to the ground.  They are extremely awkward and exposed during these short periods.  However, sloths are agile swimmers and often use this ability to move between locations.  Having a low metabolism allows them to hold their breath underwater for up to 40 minutes.  Slow movements also provide protection from predators that hunt by sight, looking for movement among the trees, including ocelots, jaguars, and harpy eagles.

Sloths have long, shaggy hair that is home to a host of other organisms.  Each strand is grooved, able to retain rain water, and hosts a symbiotic green algae.  The algae gets shelter and water from the hair and provides nutrients to its host by absorption through the sloth’s skin.  The algae is also a source of food for several other species including mosquitos, ticks, mites, beetles, and several moth species whose entire lifecycle depends on sloths.  When a sloth goes down to the forest floor to defecate, female moths lay their eggs in the dung.  Moth larvae feed and pupate in the dung, leaving several months later to fly up into the trees to find a mate in the sloth’s hair and begin a new cycle of life.

Sloths have several predators, but their long, sharp claws make formidable weapons if they are attacked.  However, their biggest threats come from poaching, deforestation, and electric wires as more forest acreage is being opened to development.  Habitat destruction is a serious threat to the world’s rainforests, and supporting efforts to slow down or stop this process provides the best assistance for sloths and other rainforest species.  You can observe these unique and interesting animals at many zoos, or learn more at your local natural history museum, including the South American display at the Jurica-Suchy Nature Museum at Benedictine University.