Sloth: slow-moving arboreal mammals of the Americas

General appearance and size

Today's tree-dwelling sloths reach a total length of 42 to 80 cm and weigh between 2 and 11 kg. The head is very short, the face rather round. The small eyes are wide apart, but look forward, the round nose is clearly flattened, the ears are small and hidden in the fur. The tail exists only as a stubby rudiment. Typically, the limbs are very long, the front ones longer than the rear ones, which is especially pronounced in the three-fingered sloths. The two present genera differ in the number of visible fingers. On the hind legs both groups have three toes each. The fingers and toes bear large, sickle-shaped curved claws and are about the same length.

The extinct sloths deviated in their appearance partly from the representatives living today and surpassed them in their body dimensions - apart from some very small representatives - partly considerably. Especially the forms from the Pleistocene reached extreme dimensions. Scelidotherium, for example, was 2.7 m long and weighed about 800 kg. Glossotherium was 3.25 m long and weighed a good 1.5 t. However, members of the largest known genera, Megatherium and Eremotherium, reached a total length of up to 6 m and weighed an estimated 4 to 6 t, making them the largest land-dwelling mammals in the Americas at that time, along with the proboscideans that had migrated from Eurasia. These mostly ground-dwelling sloths, like many of their phylogenetic predecessors, had short and strong limbs and also possessed a longer tail.

Skull and dentition characteristics

The skulls of modern two- and three-fingered sloths average 10.8 and 6.7 cm in length, respectively, and are generally distinctly rounded and short. The largest skulls within the sloths were possessed by Eremotherium and Megatherium, with measurements between 65 and 86 cm. In general, the sloths exhibit a rather diverse skull design. Forms with long narrow skulls like Nothrotherium and Scelidotherium or with short and broad snouts like Lestodon were found. General characteristics of the sloths are a strongly reduced middle jawbone and an elongated upper jaw. One of the most striking features of the skulls is the incompletely developed zygomatic arch, a characteristic that also occurs in the anteaters. In part, this feature, which has also been recorded in the unrelated pangolins, tempted one to conclude that the earliest representatives of the sloths had a rather insectivorous diet, but all fossil forms found to date have been identified as herbivores. Only some of the largest representatives of the sloths, such as Megatherium, Megalonyx and Paramylodon, secondarily possessed fully developed zygomatic arches again. Furthermore, a strongly downward elongated, laterally flattened bone outgrowth occurs at the anterior arch base on the zygomatic bone. This serves as an attachment point for the masseter muscle of the masticatory musculature and enables pronounced forward and backward chewing movements. A similar bone outgrowth is found in the Glyptodontidae, relatives of the armadillos, but there it has a different structure and also leads to a different attachment of the masseter muscles, so that both bone outgrowths probably developed independently. Also typical is the presence of a septomaxilla (Os nariale) in the anterior nasal cavity. This ossification otherwise occurs largely only in cloacal animals and older Mesozoic mammals.

Special features are also present in the dentition, which deviates from that of the higher mammals. Most sloths lack the front teeth, i.e. the incisors. As a rule, there are five teeth in the upper jaw and four in the lower jaw, making a total of 18 teeth. Only a few lineages, such as the evolved Nothrotheriidae, have reduced the front teeth. As a rule, two types of teeth can be distinguished in sloths: The posterior teeth are homodont, so that in the developed state no distinction can be made between premolars and molars; they are mostly called molariform. In the Bradypodidae, i.e. the three-fingered sloths, the anterior tooth is small and chisel-shaped (so-called anterior chisel-shaped tooth). The Megalonychidae, as well as the two-fingered sloths and the Mylodontidae, have transformed this into a canine tooth, which therefore bears the designation caniniform; it clearly exceeds the other teeth in height. In the closed dentition the upper and lower caniniform teeth are exactly reversed in comparison with those of the other Higher Mammals; that is, the lower rests behind the upper, and not, as is usually the case, the reverse. Therefore, it has been suggested that the anteriormost tooth in each case is not homologous to the canine proper. An exception is shown by the Megatheriidae, in which all teeth have the same shape. Studies of tooth formation in the embryonic stage provided information about the more precise dentition structure of the sloths. If the mineralization sequence of the teeth of sloths corresponds to that of other higher mammals, the rearmost tooth in each case forms the first molar, and the three teeth in front of it therefore correspond to the premolars. In the lower jaw, the premolars also include the anteriormost caniniform (choloepus) or chisel-shaped (bradypus) tooth. The anteriormost tooth in each case in the maxilla would be identical to the canine according to the sequence of tooth arrangement. In the embryonic state, incisors also form, but are subsequently reabsorbed very quickly. An additional characteristic of the teeth of sloths is the absence of enamel: the teeth consist mainly of different layers of dentin, including the particularly hard orthodentin. In addition, the occlusal surfaces of the posterior teeth do not have the pattern of different cusps and ridges characteristic of mammals - however, two transverse ridges (lophen) are formed, which enable the crushing of food. Another distinctive feature is that there is no change of teeth from deciduous to permanent dentition. Rather, teeth grow throughout life, with young teeth being round at first and later acquiring their more angular shape. It is possible, however, that a change of the upper caniniform tooth in two-fingered sloths occurs at a very early stage of individual development.

Skeletal features

Significant skeletal features exist mainly in the area of the spine. The neck of the three-fingered sloth includes eight to ten vertebrae, which gives the animals a very mobile head. Two-fingered sloths, on the other hand, have only five to seven cervical vertebrae, so their necks are much shorter. It is assumed that the one to three following the seven cervical vertebrae in the three-fingered sloths represent ribless thoracic vertebrae, but in the two-fingered sloths the foremost two thoracic vertebrae represent rib-bearing cervical vertebrae. In all fossil sloths from which the cervical spine is known, only seven vertebrae have been recorded to date. The number of thoracic vertebrae varies among the sloth genera, being 16 in the three-fingered sloths, 24 in the two-fingered sloths, and 22 in the extinct Hapalops. Very much reduced is the lumbar spine, which occupies three vertebrae in all known genera. Especially on the lumbar vertebrae and the posterior thoracic vertebrae additional articular surfaces occur, which are situated on the lateral articular processes, and are called xenarthric joints (secondary joints or xenarthrals). These connect the preceding articular processes with the succeeding articular processes and thus further stabilize the back. Today's sloths have only a short, stubby tail consisting of 4 to 5 vertebrae; extinct ones, however, sometimes had quite long tails with considerably more caudal vertebrae.

Individual peculiarities are also found in the locomotor system. As a special adaptation to their way of life, recent sloths have extremely long and slender limbs, with the front ones of the three-fingered sloths being about one and a half times as long as the rear ones; in the two-fingered sloths the ratio is much more balanced. Fossil sloths often exhibited shorter and more robust legs. Especially in the ground-dwelling sloths, the forelegs were usually shorter than the hind legs. Typical of all sloths is the appearance of a third trochanter as a muscle attachment point on the shaft of the femur, which is less pronounced than in the related armadillos and whose location varies greatly. The tibia and fibula are not fused, an exception being the large Megatheriidae. The three-fingered sloths have three (rays II to IV) on the front feet, the two-fingered sloths two clawed rays (rays II and III), which are usually of the same length. Three rays occur on each of the hind feet. The other toe rays are reduced in length and functionless. The number of claw-bearing rays in ground-living extinct sloths varies. Significant here, however, is the construction of the hind foot. Some representatives landed with the whole foot and thus had a plantigrade locomotion (sole walker). In others, the hind foot was twisted sideways, so that they walked with the fifth (outer) ray and the heel bone (pedolateral), a form of locomotion that evolved several times independently in the individual sloth lineages.

Fur

Two layers can be clearly distinguished in the coat of the three-fingered sloth: The short and very dense undercoat and the long and straw-like topcoat. The two-fingered sloths, on the other hand, have only the top coat. A special characteristic of the hair of sloths is the absence of the medullary canal (medulla). The hairline runs from the belly to the back and thus in the opposite direction to that of other mammals. This allows rainwater to run off better, which is an evolutionary adaptation to the predominantly hanging way of life in the branches of trees. Furthermore, the hairs of two-fingered sloths have 3 to 9 longitudinal ridges and grooves on the outer surface that run the entire length. This is unique among mammals. This does not occur in three-fingered sloths; their hairs have small air spaces under the cuticle. As protection, sloths have evolved an unusual camouflage: Algae settle in the hairs, which benefit the animals as a green-changing coloration between the leaves - an effect that is especially evident in the rainy season.

Through fossilized findings of soft tissue in some caves of North and South America, fur covering has also been demonstrated in extinct forms, such as Mylodon, Megalonyx, and Nothrotheriops. The surviving hairs are usually long and shaggy, and also lighter or darker brown in color. It is likely that numerous fossil sloths thus possessed fur. It is discussed whether the extremely large representatives, such as Megatherium and Eremotherium, which were widespread in tropical regions, were also hairy. Here, the huge body sizes and the associated and necessary thermoregulation speak for the fact that, as in other large, modern herbivores, such as elephants, rhinoceroses and hippos, there was no visible fur cover, but the skin was more or less naked. Some of the large ground sloths, such as Mylodon or Glossotherium, also had discus- or lenticular bone platelets (osteoderms) formed in the skin. These were small, 0.5 to 3 cm in diameter, and in contrast to similar formations in armadillos and glyptodonts, had a simple, compact structure. Since they are usually found in isolation, they were most likely formed randomly in the skin and not firmly fused together as in the carapaces of modern armadillos.

Internal organs

Unlike other mammals that specialize in leafy foods, sloths digest food in the anterior gastrointestinal tract. The stomach consists of three partially separated chambers connected to the stomach gate. When completely full, it can reach up to one-third of an animal's total body weight. A peculiarity is presented by the lungs, which in both genera do not show a distinct separation into separate wings, possibly connected with the construction of the thorax. The spleen is irregular-tubular in shape in the three-fingered sloths, but flat-triangular in the two-fingered sloths. The kidneys lie very deep in the abdominal cavity, and the adjacent urinary bladder reaches 12 cm in diameter and stores up to 1 l of urine. In young animals it occupies almost half of the abdominal cavity. Only the two-fingered sloths possess a gall bladder - but in these it is very small, three-fingered sloths have not developed one. In contrast to numerous non-primate mammals, the uterus has a simple shape, but thus differs markedly from some representatives of the armadillos as close relatives within the secondary articulated animals.

Sense organs

In general, sloths orient themselves through the sense of smell and touch. The sense of sight of today's sloths is underdeveloped due to the lack of the ciliary muscle, which makes the animals short-sighted. The strongly convex curved cornea and the very thick eye lens lead to a low resolution of the field of vision. However, the visual ability of juveniles is better than that of adults. Since the lens of two-fingered sloths is much narrower in diameter than that of three-fingered sloths, it is also less powerful.

The hearing is also less well developed. However, the ear of today's sloths is designed for a rather low frequency range between 0.3 and 30 kHz, which is mainly due to the structure of the cochlea. Adult animals are mainly activated by sounds of 2 to 8 kHz, which is also used by young animals when they are separated from their mother. The calling sounds of young two-fingered sloths are on average lower than those of young three-fingered sloths. However, since the acoustic sense apparently plays only a minor role, only a few calling sounds are known in sloths. By comparing the inner ear of recent species with extinct forms, a similar frequency range could be determined for Glossotherium and Lestodon, among others. Thus, the optimal hearing frequency was between 1.7 and 2.4 kHz, while the upper limit was reached at 15.3 to 16.5 kHz.

Sloths are distributed mainly in central and northern South America and parts of Central America, as well as on some islands in the Caribbean. They inhabit tropical rainforests of the lowlands and mountain forests, rarely using more open landscapes. Higher altitudes above 2400 m are visited only occasionally. Both present genera do not tolerate cooler temperatures. Due to the lack of underfur, two-fingered sloths are more sensitive to cold and have a higher thermoneutrality (24 °C) than three-fingered sloths (18 °C). However, the fur of the two-fingered sloth is much denser at higher altitudes.

The origin of sloths is most likely to be found in South America. Central and North America were reached in several waves of dispersal. Sloths were first recorded in the Caribbean Islands in the early Oligocene, about 34 million years ago, and in the Upper Miocene, about 8 million years ago, they colonized North America. Immigration to both regions probably occurred via small islands. With the closure of the Isthmus of Panama and the formation of a land bridge between South and North America in the Pliocene about 4 million years ago, and the Great American Faunal Exchange that began with it, there was further colonization of Central and North America by sloths. In the Pleistocene they reached their greatest expansion over both continents. Thus, Mylodon occurred almost to the southern tip of South America, while Megalonyx is recorded in the far north of North America in Alaska. Sloths used a variety of habitats throughout their tribal history. Thus, as today, they were found in dense forests, but also in savannahs and steppes to deserts, as well as in high mountainous areas and on the seashore.