This Spider Lassos Her Prey from the heavens above! (Monteverde Cloud Forest)
As described by French naturalist, George Buffon, in his 18-century compendium, Histoire naturelle, sloths were originally thought to have a dreadful existence. He characterized them using such words as “slowness, stupidity, neglect of its own body, and habitual sadness.” He finalized by saying “they are imperfect products made by nature, which, scarcely having the ability to exist at all, can only persist for a while, and shall then be effaced from the list of beings” that to have “one more defect would have made their existence impossible” (2). On the contrary, it seems these animals have adapted relatively well to their environment, their characteristic slow methodical movements serving a variety of purposes, and their odd behaviors benefiting themselves and other organisms.
Found within the forest canopies of Costa Rica, the three-toed (Bradypus) and two-toed sloth (Choloepus) both share a number of superficial similarities though they are from two different genera that split over 40 million years ago; one of the most remarkable examples of convergent evolution that has occurred (1). Studies based on various measurements of the skull, lower jaw, hyoid and dentition, exhibited the two-toed sloth is related to the extinct members of the Megalonychidae family (the extinct ground sloth from N. and S. America); whereas the three-toed sloth is positioned as the sister-taxon to all other sloths (1). Both the brown-throated three-toed sloth (Bradypus variegatus) and Hoffman’s two-toed sloth (Choloepus hoffmanni) found in Costa Rica belong to the Mammalian order Xenarthra (Edentata), which also includes armadillos and anteaters (2). Edentates maintain the lowest body temperature among the placental mammals, with temperatures ranging between 82 and 90 degrees Fahrenheit for the Bradypus spp., depending on the ambient temperature (2). They (Bradypus sp.) have been found to warm themselves in the sunlight, and on cloudy days their body temperature may be only 5 degrees Celsius more than the ambient (5).
Physical differences exist between the three and two-toed sloths. The hands and feet of the three-toed sloth have three functional digits (7). Distal phalanges are curved and covered by long narrow claws and hairy volar pads (7). There is also fusion of the proximal and intermediate phalanges; the claws are separate, but the digits are syndactylous (7)(9). The three-toed sloth has been found to be active during the day and night, showing decreased activity before and after sunrise (3). Often their movements are slow and methodical, with an average a speed of 0.1 to 0.2 miles per hour on terra firma (2), though there have been a few reports of relatively rapid movement associated with fear or aggression. In another species of three-toed sloth, Bradypus griseus, it was recorded that an individual moved 10 feet to the main trunk of a tree in less than one minute during a rainstorm (4). The movement observed appeared to be rapid, not forced or precise like its “normal” movement (4). In 1989, two males were observed fighting in a Cecropia tree; it was witnessed that each moved 2 to 3 times the normal speed during the encounter (8). There have been reports of helplessness and vulnerability of sloths while on the ground; most of these reports were of Bradypus sp., but as noted more recent studies have shown this may be incorrect (6)(7). Three-toed sloths are found high in the forest canopy (24-30 meters) (7). They move through trees by hanging upside down from all four feet, and placing one hand over the other, feeding on leaves from the Cecropia and other trees (9). They have long, coarse, thick hair that is tan/yellow-brown in color; each hair has a groove running through each hair, which allows for blue-green algae to grow (9). The alga gives the fur a greenish tinge, and helps to camouflage them (9). Their front legs are longer than their hind limbs; they have slender bodies and round heads (2)(9). They reach about two feet in length, and have 9 cervical vertebra, which allows them to rotate their head 270 degrees, or a full three-quarters of a turn (2). There sense of sight and hearing are poor; their external ears are reduced in size (9). They are strong swimmers (9).
The two-toed sloth is strictly nocturnal, unlike the three-toed (3). In the two-toed sloth, the digits have been reduced down to two on the hand and three on the feet, with distal phalanges being strongly curved, covered by claws and thick skin (the claws are separate, but the digits are syndactylous)(6). They are slightly larger, measuring slightly more than 3-4 feet (9). Their fur is grey-brown, it is paler on the head, and also turns a greenish hue due to algae. External ears are also reduced in size, and the forelimbs are longer than the hind limbs as in the three-toed sloth. The two-toed have 6 cervical vertebra rather than the 9 like the Bradypus. They feed on leaves, fruit, buds and even small invertebrates (9). Their body temperatures also vary substantially, though they do not bask in the sun like the three-toed sloth (9). They are also methodical in their movements, but have been reported to move almost as fast on the ground as they do in trees (6). They are capable of aggressive self-defense as well, slashing with their claws or biting with their large canine teeth (three-toeds are lacking canine teeth) (9).
Both species average 18 – 20 hours a day of rest, and generally move to a new tree about every day and a half. Their slow movement is associated with their slow metabolic rate, which is about half the rate of other animals their size (12). Sloths have a ruminant-like stomach and a long digestive tract that aide in digesting the leafy material, but results in slow digestion (11). It sacrifices heavy muscle mass and mobility to maximize body size in proportion to its weight (11). The slow movement also helps to protect them, allowing them to blend in with the forest canopy. Their main predators are jaguars and Harpy Eagles. Analysis of a Harpy Eagle nest revealed remains of both three-toed and two-toed sloths (10). It is interesting to note that there were equal numbers of each, especially since one is strictly nocturnal; it was expected that more three-toed sloths would be found since they are active during the day (10).
Males and females reach sexual maturity about 3 years of age. Females give birth about once a year to one offspring; the gestation period lasts 6 months. The young will ride on the mother’s chest for the following 6 months after birth (13). They will emit a contact call (the three-toed emit a high pitch whistle, whereas the two-toed emit a low pitch bleat) if they are separated from the mother (13). It has been noted that females may not pursue a fallen young due to the dangers presented from predators responding to the cries (12).
Theses peculiar mammals perform activities that may benefit their survival, and others survival. As mentioned previously, there is a symbiotic relationship between the sloth and algae (algae grows on fur, which helps to conceal the sloth from predators). Also within the fur, several species of insects carry out full lifecycles, feeding on the algae (14). The sloth moth (Cryptoses choloe) has been found living in the hair of the two-toed sloth (C. hoffmanni) and the three-toed sloth (B. infuscatus). Once a week the sloths descend to the forest floor to defecate and urinate at the base of a tree; they dig a hole, relieve themselves and then cover the hole with leaves and debris. The gravid female moths will oviposit in the feces; when the eggs hatch, the larvae will feed and pupate on the dung (14). After a few weeks the adults will emerge and fly off to find a sloth, where they mate and carry out the rest of the lifecycle (14). Defecation at the base of the tree also puts nutrients back into the soil that directly benefits the tree, which may also benefit the sloth in the future (15).
Aviarios del Caribe Sloth Refuge
The brown-throated three-toed sloth and Hoffman’s two-toed sloth are not listed as endangered, but their inflexibility in terms of habitat makes them susceptible to deforestation, habitat fragmentation and human encroachment (8). The protection of the tropical rainforests in Central and South America are important in maintaining these species. A facility located in Costa Rica is working to save the habitat these sloths thrive in, while also working to rehabilitate injured or orphaned sloths back into the wild. Aviarios del Caribe Sloth Refuge is a privately owned 120 hectare sanctuary that includes the delta of the Rio Estrella, that was founded by Judy and Luis Arroyo (17). The center was created in 1972, and was included in the Regiment of National Parks and Privately Owned Protected Areas in 1996, when the land was found to be an important migratory pathway for many birds (17). It protects marshland ecosystems and tropical forests, which are home to river otters, river turtles, 300 bird species, monkeys and sloths (16). It is the only sloth refuge/rehabilitation center in the world; young are raised here and prepared for rehabilitation into the wild. It is hoped that the center will attract students and university staff to use Aviarios as a source for research (17). The center also established the “Buttercup Foundation” (named after the first orphaned three-toed sloth cared for by the Arroyo’s) in 1991. The proceeds are used to purchase more land for wildlife protection, to create school programs for children of all ages throughout Costa Rica, create an ongoing wildlife veterinary scholarship fund for local vet students to travel abroad to study exotic wildlife medicine to improve veterinary practice in Costa Rica, and to create a scientific research center to study the sloths of Aviarios (17).
A closer look at these mammals challenges Buffon’s original thoughts about their degree of fitness. It appears that they have evolved in a manner that has allowed them to survive rather than in a way that would “efface” them quickly; though new threats exist for these species now that did not exist 100 years ago. Hopefully through the efforts of local residents, like the Arroyo’s, and further forest protection they will continue to thrive as they have over the last 40 million years.
1. Gaudin, Timothy. February 2004. Phylogenetic relationships among sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence. Zoological Journal of Linnean Society. Vol 140 (2): 255-305.
2. Gould, Stephen Jay. April 1996. Can we truly know sloth and rapacity? Natural History. Vol 105(4) pg 18 – 26.
3. Sunquist, M.E.; Montgomery, G.G. 1973. Activity Patterns and Rates of Movement of Two-Toed and Three-Toed Sloths (Choloepus hoffmanni and Bradypus infuscatus). Journal of Mammalogy. Vol 54(4): 946-954.
4. Brattstrom, Bayard. 1966. Sloth Behavior. Journal of Mammalogy. Vol 47(2): 348.
5. Nagy, K. A.; Montgomery, G.G. 1980. Field Metabolic Rate, Water Flux, and Food Consumption in Three-Toed Sloths (Bradypus variegatus). Journal of Mammalogy. Vol. 61(3): 465-472.
6. Mendel, Frank. 1981. Use of Hands and Feet of Two-Toed Sloths (Choloepus hoffmanni) during Climbing and Terrestrial Locomotion. Journal of Mammalogy. Vol. 62(2): 413-421.
7. Mendel, Frank. 1985. Use of Hands and Feet of Three-Toed Sloths (Bradypus variegatus) during Climbing and Terrestrial Locomotion. Journal of Mammalogy. Vol. 66(2): 359-366.
8. Greene, Harry. 1989. Agnostic Behavior by Three-Toed Sloths, Bradypus variegatus. Biotropica. Vol. 21(4): 369-372.
9. University of Michigan Museum of Zoology: Animal Diversity Web. Online http://animaldiversity.ummz.umich.edu/site/accounts/information/Xenarthra.html.Viewed on 5/16/05.
10. Galetti, M.; de Carvalho, O. 2000. Sloths in the Diet of a Harpy Eagle Nestling in Eastern Amazon. Wilson Bulletin. Vol. 112(4): 535-536.
11. The Three-toed sloth. Online at http://www.infocostarica.com/fauna/3sloth.html. Viewed on 5/14/05.
12. Baker, Christopher. Mammals. Online at http://photo.net/cr/moon/mammals.html. Viewed on 5/14/05.
13. Montgomery, G.G.; Sunquist, M.E. 1974. Contact-Distress Calls of Young Sloths. Journal of Mammalogy. Vol. 55 (1): 211-213.
14. Waage, J.; Montomery, G.G. 1979. Cryptoses choloepi: A Coprophagous Moth that Lives on a Sloth. Science, New Series. Vol. 193(4248): 157-158.
15. Klien, Lori. East does it for the two-toed sloth. The California Native International Newsletter. Online at http://www.calnative.com/stories/n_sloth.htm. Viewed on 5/13/05.
16. Baker, Christopher. Moon Travel Planner. Aviarios del Caribe Sloth Refuge. Online at http://www.moon.com/planner/costa_rica/mustsee/aviaros_caribe.html. Viewed on 5/14/05.
17. Aviarios del Caribe. Online at http://www.ogphoto.com/aviarios/index.htm. Viewed on 5/16/05.
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