The Sloth spends nearly the whole of its life in the trees, and travels along the branches body downwards. It rarely comes to the ground, on which it walks with difficulty, and it occasionally takes to the water and swims. It looks slothful enough when asleep, for it then resembles a bunch of rough hair, and a jumble of limbs close together, hanging to a branch; but when awake, it is industrious in its search for nice twigs and leaves, and moves along the under side of the branches of the trees with some activity. It seizes the ends of adjoining branches, clinging to the leafy mass, and moves from tree to tree quickly enough, when it is requisite, and it has a very singular power of moving the head and neck backwards in seeking food. When the atmosphere is still, the Sloth keeps to its tree, feeding on the leaves and twigs, but when there is wind, and the branches of neighbouring trees come in contact, the opportunity is seized, and the animal moves along the forest, under the shady cover of the boughs. The Indians have a saying that “when the wind blows the Sloths begin to crawl,” and the reason is thus evident enough—the animal cannot jump, but it can hang, swing, and crawl suspended. Mr. Waterton states, however, that “the Sloth travels at a good round pace, and were you to see him passing from tree to tree you would never think of calling him a Sloth. Being born up in a tree, living amongst the branches, feeding on leaves, and finally dying amidst the foliage, and enjoying life as much as any other animal, its structure and conformation are, of course, admirably suited for this arboreal existence. Its power of grasp is great, and is assisted by the great bent claws as it hangs by its feet when asleep, and also often when it is dead. One which was much frightened by being taken from the forest had a pole placed near it at a little distance from the ground, on two supports. It clung directly to the pole and hung on. A Dog was then made to attack the Sloth, which seized it in its long claws, and did not let go until the enemy died.”
Leading thus a very unusual kind of life, up amongst the dense foliage, and having some very unusual peculiarities of construction, much debate occurred many years since regarding the general conformation as well as the special anatomy of the Sloths. One school of anatomists considered the Sloths incomplete and abnormal animals, moving with “pain” on the ground, and another regarded their unusual and peculiar anatomy as singularly beneficent.
But whilst it is perfectly evident that the long limbs and their joints, and the peculiar turning in of the ankles, and the structure of the clawed hands and feet, are all admirably adapted for the peculiar life which the animal leads, it appears to be consistent with anatomical reasoning to believe that the Sloth is an instance of retrograde development; that, in fact, the peculiar formation of the skull, neck, wrists, and ankles, is the result of the laws of disuse and adaptation operating on ancestral animals, which once had their anatomy more consistent with a perfect mammalian type.
SKELETON OF THE SLOTH.
When the Sloths were first carefully watched and studied, their length of neck and their ability to turn the head, so as to look at a person standing directly behind or beneath them, without swerving the body, struck Mr. Burchell especially. This curious peculiarity led to a careful examination of the skeleton of the different kinds, and much discussion followed, for it was found that in the Sloth examined (the Three-clawed Ai) there were more neck bones (vertebræ of the cervical region) than in other Mammalia. Instead of the common number of seven neck bones, there were nine. This elongation of the neck of course permitted a greater amount of twisting than could occur in an animal with fewer neck bones. But there are other reasons why the head can be so much twisted round, for the spines on the neck bones are small, and the joint between the skull and the first vertebra is so fashioned that this remarkable motion is possible. There was a great deal of discussion about the extra neck bones, and as the last two had rib-like projections from their sides, some anatomists considered them to belong to the true rib-bearing vertebræ, or those of the back (the dorsal). But when the other Sloths were examined it was found that the number of the bones of the neck in all the two-fingered kinds was not as great as in other animals. There are only six neck vertebra in one well-known species (Cholœpus Hoffmanni, for instance), whilst there are seven in another two-toed Sloth.
Eating largely and of bulky substances, the Sloths require a large digestive cavity, and the ribs are numerous, and the body is long and broad. There is much variation, however, in the number of the back bones in the dorsal and lumbar regions. Thus in the Ai there are sixteen dorsal and three lumbar vertebræ, whilst in the Two-fingered Sloth there may be twenty-three or twenty-four dorsal bones, and two, three, or four lumbar vertebræ. The ribs are close together and are broad. As the hind limbs require strong muscles, for the animal hangs on by them whilst it is feeding itself with the fore hands, the pelvis is large and is strengthened by having the hip and haunch bones (ilia and ischia) united to the conjoined sacral vertebræ, which may be six, seven, or eight in number. Moreover, all the strength of the pelvis is behind, the fore part or pubic bones being slender and united in front.
Some small tail bones exist, for that organ is rudimentary in all the Sloths, there being a stump in the Ai, and eleven very small bones; but in the Two-fingered Sloths the tail is not visible, and there are four little ossicles. There are no long and very prominent spines to any of the back bones, and the whole bony column of the spine is readily curved and bent. The animals so constantly bring the hands and feet close together, when hanging, that a ready bending of the spine is absolutely necessary. Moreover, in sleeping they often rest in the fork of a tree, or on a branch, and place the head between the hind legs, rolling the body up as it were in a ball, and this is facilitated by the peculiar construction of the long chain of back bones with small spines.