1902 Encyclopedia > Sloth

Sloth




SLOTH. The general characters by which the family Bradypodidae are distinguished from the rest of the order Edentata have been given in the article MAMMALIA (vol. xv. p. 384). The sloths, as the animals of this family are called on account of the habitual sluggishness of their to them with the simple hook-like organs to which the terminations of all their limbs are reduced. When they are obliged from any cause to descend to the ground, which they rarely, if ever, do voluntarily, their limbs, owing to their unequal length and the peculiar conformation of the feet—which allows the animals to rest only on the outer edge—are most inefficient for terrestrial progression, and the sloths crawl along a level surface with considerable difficulty. Though generally slow and inactive, even when in their natural haunts, they can on occasions travel with considerable rapidity along the branches, and, as they do not leap, like most other arboreal creatures, they avail themselves of the swaying of the boughs by the wind to pass from tree to tree. They feed entirely on leaves and young shoots and fruits, which they gather in their mouth, the fore-limbs aiding in dragging boughs within reach, but not being used as hands, as they are by monkeys, squirrels, &c. When sleeping they roll themselves up in a ball, and, owing to the dry shaggy character of their hair, are very inconspicuous among the mosses and lichens with which the trees of their native forests abound; and the concealment thus afforded is heightened in some species by the peculiar greenish tint of the outer covering,—very uncommon in mammals. This is not due to the colour of the hair itself, but to the presence upon its surface of an alga, the lodgement of which is facilitated by the fluted or rough surface of the exterior of the hair, and the growth of which is promoted by the dampness of the atmosphere in the gloomy tropical forests, as it soon disappears from the hair of animals kept in captivity in England. Sloths are nocturnal, silent, inoffensive, and solitary animals, and produce usually but one young at birth. They appear to show an almost reptilian tenacity of life, surviving the most severe injuries and large doses of poisons, and ex-hibiting longer persistence of irritability of muscular tissue after death than other mammals.

IMAGE: Two-toed sloth (Choloepus hoffmanni).

The sloths were all included in the Linnean genus Bradypus, but Illiger very properly separated the species with but two claws on the fore-feet, under the name of Choloepus, leaving Bradypus for those with three.





Genus Bradypus.—Three-toed sloths. Teeth usually 5/4 on each side ; no tooth projecting greatly beyond the others ; the first in the upper jaw much smaller than any of the others ; the first in the lower jaw broad and compressed ; the grinding surfaces of all much cupped. Vertebrae : C 9, D and L 20 (of which 15 to 17 bear ribs), S 6, C 11. All the known species present the remark-able peculiarity of possessing nine cervical vertebrae, i.e., nine vertebra? in front of the one which bears the first thoracic rib (or first rib connected with the sternum, and corresponding in its general relations with the first rib of other mammals); but the ninth, and sometimes the eighth, bears a pair of short movable ribs. The arms or fore-limbs are considerably longer than the hind legs. The bones of the fore-arm are complete, free, and capable ol pronation and supination. The hand is long, very narrow, habitually curved, and terminates in three pointed curved claws, in close apposition with each other; they are, in fact, incapable of being divaricated, so that the hand is reduced to the condition of a triple hook, fit only for the function of suspension from the boughs of trees. The foot closely resembles the hand in its general struc-ture and mode of use. The sole is habitually turned inwards and cannot be applied to the ground in walking. The tongue is short and soft, and the stomach large and complex, bearing some resem-blance to that of the ruminating animals. The windpipe or trachea has the remarkable peculiarity among mammals—not unfrequent among birds and reptiles—of being folded on itself before it reaches the lungs. The mammas are two and pectoral in position.

"Ai" is the common name given in books to the three-toed sloths. They were all comprised by Linnaeus under the species Bradypus tridactylus. More recently Dr Gray has described as many as eleven, ranged in two genera, Bradypus and Arctopithecus ; but the distinctions which he assigns both to species and to genera do not bear close examination. Some are covered uniformly with a grey or greyish brown coat; others have a dark collar of elongated hairs around the shoulders (B. torquatus); some have the hair of the face very much shorter than that of the rest of the head and neck; and others have a remarkable-looking patch of soft short hair on the back between the shoulders, consisting when best marked of a median stripe of glossy black, bordered on each side by bright orange, yellow, or white. There are also structural differences in the skulls, as in the amount of inflation of the pterygoid bones, which indicate real differences of species ; but the materials in our museums are not yet sufficient to correlate these with external characters and geographical distribution. The habits of all are apparently alike. They arc natives of Guiana, Brazil, and Peru, and one if not two species (B. infuscatus and B. castaneiceps) extend north of the Isthmus of Panama as far as Nicaragua. Of the former of these Dr Seeman says that, though generally silent, a specimen in captivity uttered a shrill sound like a monkey wdien forcibly pulled away from the tree to which it was holding.

Genus Cholcepus.—Teeth 5/4 the most anterior in both jaws separated by an interval from the others, very large, caniniform, wearing to a sharp, bevelled edge against the opposing tooth, the upper shutting in front of the lower when the mouth is closed, unlike the true canines of heterodont mammals. Vertebrae: C 6 or 7, D 23-24, L 3, S 7-8, C 4-6. One species (C. didactylus) has the ordinary number of vertebrae in the neck ; but an otherwise closely allied form (C. hoffmanni) has but six. The tail is very rudimentary. The hand generally resembles that of Bradypus; but there are only two functional digits, with claws,—those answering to the second and third of the typical pentadactyle manus. The structure of the hind limb generally resembles that of Bradypus, the appellation "two-toed" referring only to the anterior limb, for in the foot the three middle toes are functionally developed and of nearly equal size. C. didactylus, which has been longest known, is commonly called by the native name of Unau. It inhabits the forests of Brazil. C. hoffmanni has a more northern geographical range, extending from Ecuador through Panama to Costa Rica. Its voice, which is seldom heard, is like the bleat of a sheep, and if the animal is seized it snorts violently. Both species are very variable in external coloration. (W. H. F.)






The above article was written by: Prof. W. H. Flower, LL.D., F.R.S., Director of the Natural History Department, British Museum.



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