The South American Forests—Discovery of the Sloth—How it derived its Name—Peculiarities of Dentition—Food—Fore Limbs and Fingers—Hind Limbs and Heel—Other Modifications of Structure—Kinds of Sloth—Waterton’s Captive Sloth—Habits of the Animal—Burchell’s Tame Sloths—Manner of Climbing Trees—Disposition—Activity among Trees—Naturalists’ Debate about Anatomy—Probable Conclusion regarding it—Skeleton—Vertebræ—the Rudimentary Tail—Most Distinctive Skeletal Characters—Arm, Wrist, Hand, Fingers, Claws—Mode of Walking—Great Utility of the Claws—Face of Sloth—Skull—Teeth—Classification—[TARDIGRADA]—[BRADIPODIDÆ]—[Genus BRADYPUS]—Characteristics—[Genus ARCTOPITHECUS]—Characteristics—[CHOLŒPODIDÆ]—[THE COLLARED SLOTH]—Description—Skull Bones—Habits—Circulation of the Blood—Rete Mirabile—[THE AI]—[THE UNAU]—Appearance—Skull and Teeth—Skeleton—Interesting Anatomical Features—Stomach—[HOFFMANN’S SLOTH]—Description—Habits
WHEN the dense forests of the northern parts of South America were first explored by Europeans, it was observed that active Spider Monkeys, Howlers, and their Quadrumanous allies, were not the only climbing animals which frequented the trees. For every now and then, hunters came in sight of creatures about the size of a large Monkey, but whose sluggish movements, long hair, short heads, small ears and tail, and very long claws, enabled them to be distinguished at once from their very lively companions. It was noticed that these new creatures, instead of climbing quickly and swinging from branch to branch and running along the boughs, moved very slowly, by hanging head and body downwards and grasping the branches with their long claws. During the daytime, these quiet animals were constantly found asleep, huddled up in the fork of a branch, and looking like great balls of tow, or else hanging by two legs, the rest of the body being curled up. Now and then, one was seen at the foot of a tree, and it appeared to run along the ground with great difficulty; for the arms were so long that it walked on the elbows, and the hind feet were turned in, so that it supported itself on the sides of its great hind claws. Naturally, the animal took its time in moving, and as it was never seen to be lively, it received the name of Sloth. Interesting from being so different in its habits from other arboreal animals, it became much more so, to naturalists, when its remarkable construction was ascertained; but still the hairy creature with a short face, small head, long neck, hardly any tail, and very long front limbs, retained its popular name.
A very slight examination of one of the Sloths showed that it had no front teeth, that is to say, neither incisors nor true canine teeth, and that the hinder teeth—the false and true molars—were not like those of any other mammal. The back teeth, few in number, have since been ascertained to be exceptionally simple in their structure, and evidently the masticating process is very simple. But when it was noticed that the Sloth fed upon leaves and young twigs, the absence of the necessity for more elaborate teeth was acknowledged. Then it was observed that they had very long arms, or rather fore limbs, for the fore-arm bones and the humerus are all unusually long, and also that they had great power of movement. Moreover, it was seen that the fingers were reduced to three in number in some kinds, and to two in others, and that they were furnished with long and strong claws, which did not interfere with a great amount of mobility in the wrist. The length of limb, the mobility of the wrist, and the great claws, enable the Sloth to bring the leaves to its mouth, to hang on, and to walk, as it were, beneath the branches. An examination of the hind limbs showed that they were shorter than the others, and always furnished, in all kinds of Sloths, with three great claws. But the ankle seemed to be turned in, as if there was a state of “club-foot.” This condition would enable the toes to clasp a bough without effort, but it would prevent the sole from being placed flat on the ground. As the knowledge of the anatomy of these constant tree-livers progressed, other modifications of structure, equally important in relation to the peculiar arboreal life and food, were gradually discovered. For instance, a remarkable flexibility of the neck, produced by the peculiar arrangement of the vertebræ; a rete mirabile, to a certain extent, in the limbs, resembling somewhat that in the Lemurs (Vol. I., pages 213, 245), and a complicated stomach suited for the digestion of leaves, and foreshadowing that of the Ruminants.
Two different kinds of Sloths were described in the first instance, and subsequently, several others. The first kinds known were the Ai, a Sloth with three claws on the fore limb, and the Two-toed Sloth, with two claws on the fore limb. The Ai was called Bradypus tridactylus, and the other the Unau, or Bradypus didactylus, names which have been changed somewhat, as will be seen further on.
Sloths are caught without much difficulty, and their habits, in captivity, have been observed in South America, and also after their removal to Europe. Waterton writes[56] on the subject:—
“Some years ago I kept a Sloth for several months. I often took him out of the house and placed him on the ground, in order to have an opportunity of observing his motions. If the ground were rough he would pull himself forward by means of his fore-legs, at a pretty good pace, and he invariably shaped his course towards the nearest tree; but if I put him upon a smooth and well-trodden part of the road, he appeared to be in trouble and distress. His favourite abode was the back of a chair, and often getting all his legs in a line upon the topmost part of it, he would hang there for hours together. The Sloth, in its wild state, spends its whole life upon trees, not upon the branches, but under them; he moves suspended from the branch, he rests suspended from it, and he sleeps suspended from it; hence his seemingly bungled conformation is at once accounted for. One day, crossing the Essequibo, I saw a large Two-toed Sloth on the ground upon the bank, and although the trees were not twenty yards from him, he could not make his way through the sand in time enough to make his escape before we landed. He threw himself on his back and defended himself with his fore-legs. I took a long stick and held it for him to hook on, and then conveyed him to a high and stately Mora. He ascended with wonderful rapidity, and in about a minute he was almost at the top of the tree. He now went off in a side direction, and caught hold of the branch of a neighbouring tree, and then proceeded towards the heart of the forest.”
At Santos, in Brazil, in 1826, Mr. Burchell kept a tame Sloth, a Bradypus tridactylus, which at the end of two months pined and died. It fed exclusively on the buds and leaves of a species of Cecropia, a tree having a slender stem of thirty or forty feet long, with horizontal branches, hollow internally, and naked, except at the extremities. It ate only the young shoots and terminal buds of the unexpanded leaves, rejecting the old leaves on the boughs, which were brought to it daily. It was always perfectly silent, and its countenance and manners were most expressive of melancholy. It fed by day, and slept much; being kept in a room, it sat upright upon its short tail, embracing the legs of a chair with its arms and legs.
When resident at Para, near the mouth of the Amazons, Mr. Burchell also kept two full-grown Sloths, and a young one of a three-toed species (not Bradypus tridactylus, but of nearly similar form and habits), in a garden enclosed with strong stockades. They were kept tied up to the pillars of a verandah, to prevent their escape. Against these pillars they always placed themselves in an erect position, embracing the pillar with all four legs; when not tied to the verandah, they got up into trees in the garden. They slept both by day and night, always fixing their arms round something or other. Their food, consisting of branches, was brought to them in the verandah. They appeared extremely stupid, and would never come to the food. They would eat no leaves but those of the Cecropia.
They did not mount very large trees, and they ascended with their breasts pressing the trunk of the tree, advancing the hind-leg beyond the fore-leg. On the ground, they could neither stand nor walk, but lay sprawling on their belly, and dragged, or rather warped, themselves along, laying hold of a bunch of grass or stone with their three claws, which operated like grappling-irons, or, rather, pincers. All these died in a month or two. In their wild state they are seldom seen, from their colour mingling with the grey foliage of the trees, and from their being so extremely quiet and slow. The tame Sloths never willingly remained on the ground, except to pass from one tree to another. All the movements of the animal are slow. It moves its claws slowly. In eating it chews slowly; it also climbs slowly. The moisture of the leaves it eats suffices it for drink, without descending to obtain water. None of those kept by Mr. Burchell were ever seen to drink. The full-grown animals were never heard to utter any sound, but the young one occasionally, though rarely, gave a short cry or whistling squeak, of a single note.
They showed no indication of fear, and seemed to give attention only with their eyes. They took no notice of the boy who often carried them across the garden to their place in the verandah, with their long arms sprawling; the only objects of their regard were trees. They fight on their backs, and grapple their enemy to strangulation. The use of the long wool that covers the body, and even the face, seems to be to guard them from the annoyance of insects. Possibly it may preserve them from the attacks of Snakes, which are, doubtless, their greatest enemies.