Xenarthra (Sloths, Anteaters, and Armadillos)
XenarthraFamily: West Indian Sloths and Two-Toed Tree Sloths
Family: Three-Toed Tree Sloths
(Sloths, anteaters, and armadillos)
Number of families 4
Number of genera, species 13 genera; 30 species
Evolution and systematics
Evolved in South America, this diverse order first appears in the fossil record in the Paleocene, about 60 million years ago (mya). It has two main groups, the Pilosa and the Cingulata. The Pilosa contains sloths and anteaters, also known as the hairy xenarthrans, and the Cingulata includes the extinct glyptodonts and armadillos, the animals with bony carapaces. The group name "Xenarthra" refers to the additional articulations between the lumbar vertebrae, called xenarthrous processes. These extra articulations provide an unusually large amount of stability to the pelvic region, and have been hypothesized to confer advantages on sloths and arboreal anteaters in reaching out from one support to another with the body held horizontally, and to the armadillos for digging with great strength and speed. The hands in xenarthrans usually have two or three digits larger than the others, although the number of digits in tree sloth hands is reduced to two or three. All fingers have sharp claws, which are often long and laterally compressed. Other than the tree sloths, which have three toes on the hind foot, most xenarthrans have five toes, each bearing a sharp claw. Sloths and anteaters have a tendency toward supination of the forearm and hands. Freedom to rotate the wrist is useful for climbing in trees as well as allowing anteaters to turn the sharp claws upward and inward for protection while walking terrestrially. The hind foot in some extinct sloths was also capable of supination, and the animals walked on the outsides of the feet, again to protect long, sharp claws. The radius and ulna are separate, which also contributes to supination ability. The scapulae are large and have a second spinous process posterior to and parallel to the first. This characteristic is particularly important for increasing the surface area for attachment of the muscles used in retracting the forelimb as would be necessary in digging, and is most striking in the armadillos and anteaters although also present in the sloths, contributing to their ability to climb.
The earliest xenarthrans were small, resembling primitive armadillos more than sloths or anteaters. Presently, no direct ancestral fossil lineage leading to the xenarthran groups is known. By the time sloths, anteaters, armadillos, and glyptodonts occur in the fossil record, the distinctions among lineages as well as those within the lineages are clear. Past authors considered Paleanodonts to be xenarthran ancestors, but they are now recognized instead as ancestors to the Pholidota, a group that includes the living pangolins. Even though the Paleanodonts were not of the direct lineage leading to the Xenarthra, they probably were of a body form similar to xenarthran ancestors and may be the equivalent of a sister group. These animals were small, armadillo-like, but lacked the bony armor seen in the earliest xenarthrans. They had reduced dentitions, lacking enamel, as was probably a primitive characteristic of
the ancestral xenarthran. Both groups arose from Insectivores in the late Cretaceous, over 70 mya.
The earliest sloths appear in the fossil record in the Oligocene. These are the mylodonts, the family Mylodontidae, the group that retains more primitive characters than the other two, including dermal ossicles. The other two lineages, the family Megatheriidae and the family Megalonychidae, are first recorded in the fossil record in the Miocene. Although some extinct sloths were very large, all three early lineages were small to moderate and increased size through time, culminating in the giant Pleistocene megatheres and eremotheres. Although the largest sloths went extinct at the end of the Pleistocene, some may have persisted to less than 13,000 years ago. Speculations that extinct large sloths coexisted with humans and may have been driven to extinction by hunting activities are refuted by differences in the level of the strata in which sloth remains and human artifacts are found in caves. These differences indicate that the sloths used the caves at some time prior to their occupation by humans. To date, there is no conclusive evidence of human and sloth interaction.
The fossil record of the anteaters is the most fragmentary of the xenarthrans, and the earliest species do not significantly differ from the more recent ones. No fossil anteater had teeth, and even the earliest had elongate heads and, presumably, long tongues. Together with a body form reminiscent of the living anteaters, it is likely that by the time they appear in the fossil record, they were restricted to eating insects.
The Cingulata, or the armored xenarthrans, are closer in body form to the earliest of the xenarthrans. Of the three main groups, the chlamytheres or giant armadillos, the glyptodonts, and the armadillos, the first two are entirely extinct. The chlamytheres were similar to living armadillos in body form, although some were much larger. The cingulates that differed the most from the other lineages were the glyptodonts. As did the sloths, these animals increased in body size through time, and the largest survived into the Pleistocene. Not only did the glyptodonts have solid armor on their bodies, but their tails were encased in bony plates and some had solid bone club-like expansions at the ends. They probably were used very effectively for defense.
The dentition in xenarthrans is typically reduced in tooth types and numbers and all lack enamel. There is no milk dentition, and the teeth are ever-growing. No xenarthran has identifiable incisors. Sloths are the only xenarthrans with canine-shaped teeth, and in these animals they occlude upper in front of lower, opposite from the pattern in other mammals, making their relationships to true canine teeth uncertain. Therefore, in sloths those teeth are called "caniniform." Likewise, neither premolars nor molars can be distinguished in sloths, armadillos or glyptodonts, and the cheek teeth are all similar in appearance and all called "molariform." The anteaters are the only edentulous xenarthrans, although the
group was previously known as the Edentata. The teeth in sloths erupt as simple cones, and acquire the cusp pattern characteristic of each species through wear caused by movements of the masticatory muscles. The generation of tooth wear patterns in other xenarthrans has not been studied.
Xenarthrans living today range in size from the smallest, Chlamyphorus, the fairy armadillo, at about 5 in (12.5 cm) head and body length, to Myrmecophaga, the giant anteater, at about 47 in (120 cm) head and body length. Some extinct forms were larger; the extinct glyptodonts were over 6.5 ft (2 m) in head and body length, and the largest of the extinct sloths probably exceeded 10 ft (3 m) and were as heavy as modern elephants. It is probable that all early xenarthrans had some form of bony armor. Dermal ossicles occur in some extinct sloths, although living sloths and anteaters have entirely lost this tendency.
Tree sloths have pear-shaped bodies with large abdomens allowing a large cecum and long, slender limbs ending in elongated curved claws. Their fingers are bound together by skin, so the claws are often mistaken for their fingers or toes. All tree sloths have three claws on their hind feet, but the common names of "two-toed" and "three-toed" are based on the number of digits present on their front feet. Their outer pelage is long and coarse, and there is a short, soft, dense undercoat. Their heads are small and rounded. The eyes in Bradypus are small and dark; both sloths show only pinhole opening for the pupil and are not believed to see well. Tree sloths have external ears, but these are typically hidden in the elongate guard hair of the outer fur coat. Sloths are extremely unusual among mammals in having a variable number of cervical vertebrae (six in Choloepus and eight or nine in Bradypus). These additional vertebrae cause the neck to be longer in Bradypus and may contribute to the ability of this sloth to turn its head around to a greater distance than is typical for a mammal. The sloths also share the unusual xenarthran feature of having additional vertebral articular surfaces that allow them to stretch themselves horizontally from a vertical support while holding on only by the hind limbs.
Anteaters are recognized by their long, tapered snouts, remarkably long and thin tongue, and large, powerful foreclaws. The foreclaws are used both for defense and for the purpose of tearing open ant and termite mounds. The pelage is long and thick enough to temporarily protect the animals from invading insects. All but one species has a grasping prehensile tail.
Armadillos are the only living mammals with a protective bony skin armor. Unlike reptile shells, an armadillo's mail is interrupted by several folds of skin to assist with agility. The skin's surface is gray or brown, and quite soft. They are stocky, medium-sized mammals that walk low to the ground. Head and ear shape varies among species, and powerful limbs bear enlarged claws for digging burrows and gathering food.
Xenarthra is found strictly in the New World and evolved in the early Cenozoic of South America, with some species migrating to Central America, North America, and the Carribbean in the late Pliocene and in several different later waves as land bridges appeared. All living families now occur in Central and South America, with one species of armadillo ranging into the United States. Xenarthrans were more common in North America in the past, with extinct large sloths, giant armadillos, and glyptodonts being important elements of the fauna, and ranging as far north as Alaska, where some extinct sloth remains have been discovered.
Xenarthrans are found in most habitats of Central and South America. The living species of sloth are the most restricted, both genera living mainly in the lowland tropical rainforests and two-toed sloths inhabiting cloud forests to an altitude of about 6,560 ft (2,000 m). Anteaters and armadillos share these habitats, but anteaters, particularly the giant anteater, Myrmecophaga, also occupy savannas and pampas and more open areas. The armadillos are even more cosmopolitan, with some species occurring in the driest of South American desert habitats to savannas and tropical rainforests.
Xenarthrans are solitary, although mothers keep their young with them, in some cases for up to as long as a year. Sloth and anteater mothers carry their single babies with them until long after weaning, including when the young have achieved two-thirds the size of their mothers. Armadillos do not carry their babies but do stay with them, foraging together in a family group. Some armadillos may form loose social groups of adults, but more commonly are solitary.
Although xenarthrans do not typically associate with others of their own species, they do maintain territories, which include feeding locations and favorite resting places. Sloths eat the leaves of about 60 species of trees and vines, although they have preferences for a more limited number of food trees that they learned by accepting partially digested leaves from the mouths of their mothers as early as a few weeks after birth. Anteater young learn the locations of the nests their mothers frequent, although after weaning, they may move to a different or partially different territory. Armadillo young disperse into areas adjacent to those occupied by their mothers.
Tree sloths are well known for the slowness of their movements, and their propensity to sleep for many hours a day. Common names include "peresozo," which translates to "lazy
man," and the name "sloth" symbolizes one of the seven deadly vices. However, the slowness with which the animals move is determined by their low metabolic rate, necessitated by a leaf diet that provides poor quality nutrition. Another correlate of their low metabolic rate is that sloths are poor at regulating their body temperature, and become even more inactive in cool weather. The diet of one extinct sloth of the genus Nothrotheriops has been determined from the analysis of dung balls preserved in a cave in Arizona. These animals ate a variety of plant materials that include some species present in the area today. With a similar quality of nutrition to that of living sloths, it is reasonable to assume that these extinct sloths were also slow moving. Anteaters also have a low metabolic rate although they do not sleep as much or move as slowly as do sloths, and armadillos are as active as most other mammals.
Feeding ecology and diet
Although they are descended from insectivorous ancestors, the sloths have become herbivores, feeding primarily on leaves. It is commonly believed that they subsist entirely on the leaves of Cecropia trees, although they actually consume leaves, buds, and flowers from more than 60 species of trees and lianas. The myth about sloths spending their entire lives in a single tree probably arose because they move slowly, sleep for approximately 20 hours a day and are most easily seen in Cecropia trees, which have a single slender trunk and a compact canopy, making sloths more visible when in them. They prefer new, young growth, and so move frequently to take advantage of plants in their home ranges that are putting out new growth. All sloths probably ingest insects upon occasion, and Choloepus will actively seek bird eggs and will take nestlings if it can capture them. Sloths are cecal, or hindgut, fermenters, and process their leafy diet slowly, eliminating solid and a small amount of liquid waste once every five or so days.
Anteaters feed almost entirely on ants and termites, and in short bouts. They typically dig a small hole in the nest with the largest claw on one forefoot and lick up the insects that come to investigate the problem and to repair the nest. Within a short time soldier ants or termites become alerted to the breach and swarm out to defend their home. Anteaters therefore feed in short bouts, and can often be seen brushing at their eyes and ears after leaving the nest. Although they have thick, tough skin, evidently ant and termite soldier mandibles are able to pinch hard enough to annoy them. In a single day an anteater will visit many nests, feeding for only a minute or two at each. The three living anteater genera differ greatly in size and partition the insect resources similarly. The smallest genus, Cyclopes, feeds on the smallest insects, medium-sized Tamandua feeds on medium-sized prey, and the giant anteater takes the largest ants and termites.
In contrast to herbivorous sloths and insectivorous anteaters, the armadillos are omnivores, feeding on insects, small invertebrates, and plant materials as they occur in their habitat. They may take different foods at different times of the year and according to seasonal availability. This is probably the ancestral diet for the group. The extinct giant armadillos probably had food habits similar to those of living armadillos. It has been assumed, based primarily on the dentition that emphasizes the ability to grind plant foods, that the glyptodonts were herbivores and may have specialized on grasses.
Few courtship behaviors have been observed in sloths, and males and females remain together only for the length of time required to mate several times. Females have a simplex uterus and give birth to a single young, born fully furred, and with eyes open or soon open, that can hang onto it's mother's fur shortly after birth. The mother carries the baby continuously for six months, nursing it but also allowing it to feed on leaves by which it learns the mother's feeding preferences prior to weaning. During their time together, the baby infrequently moves far enough from the mother to lose physical contact. Even when exploring or feeding, the young sloth maintains contact with the mother by using at least one foot. Sloth babies as young as two weeks have been observed to feed on some of the leaves the mother is eating, and it has been reported that mother sloths may regurgitate partially digested leaves for the baby to lick from her lips. Whether this is true or not, young sloths frequently are seen licking at the lips of their mothers, and this is evidently how they learn her food preferences. At weaning, the mother leaves the territory they occupied together and moves to a different part of her range. She will remain away from the part of her territory where she left her offspring for anywhere from two weeks to a few months.
Anteaters, which are probably polygynous, generally produce only a single offspring; twins are quite rare. Offspring nurse from a single pair of mammae. Because the mother is unable to grasp the newborn due to her enlarged foreclaws, a young anteater must climb up the fur to the mammae. In most species, the young are still carried on the mother's back for a great deal of time after weaning.
For most armadillo species, mating occurs in the summer and is probably polygynous. A majority of species have a litter of one to three young per year, although Dasypus females exhibit obligate polyembryony, giving birth to two to 12 genetically identical young. Gestation is 140 days, and newborns are blind and naked. Their soft, leathery skin hardens into armor within a few days. The young are nursed for 2–2.5 months, begin to walk around after a week, and open their eyes after 3–4 weeks.
Of the 30 total Xenarthra species, three are ranked as Endangered, five as Vulnerable, two as Lower Risk, and six as Data Deficient—12 of these are armadillo species. Primary threats are human encroachment in the form of habitat destruction
and exploitation for food. Accurate census data on anteaters, however, is difficult to obtain due to their solitary behavior and large ranges. It is also unclear how well they adjust to changing habitat. The giant anteater (Vulnerable) is the only member of the family Myrmecophagidae recognized by the IUCN—more detailed study is necessary to determine the status of the remaining anteater species.
Sloths are a CITES I endangered species, primarily as a result of habitat destruction. In areas where the rainforest survives and the traditional degree of diversity is maintained, sloths still can do well. For example, on Barro Colorado Island in Panama, a protected habitat, there were up to 8.5 three-toed sloths per 2.5 acres (hectare). This high density is supported because sloths learn tree species preferences from their mothers and inherit part of her territory at social weaning. Neighboring sloths prefer different tree species combinations, and so pass on different preferences to their young, thereby reducing the competition for the same trees in any area.
Significance to humans
Sloths are infrequently used by humans as a food source because they have a small muscle mass and are reputed to be tough chewing. They are renowned as the namesake of the vice "slothfulness" because of their slow-moving habits, although the general public is not very familiar with their appearance or the details of their habits. Choloepus does well in zoological parks, and can be tamed and kept as a pet, but the more docile Bradypus, although hardly in need of taming, does not survive in captivity away from its home rainforests, probably because of specialized dietary preferences. Three-toed sloths are referred to as "ai's," named after one of the soft calls they make. The common name for two-toed sloths is "unau," although both genera may be referred to as "peresozo," or "lazy man," because of the slowness of their movement.
Anteaters are hunted as a food source as well as for their skin—some species are killed for the thick tendons in their tails, which can be used to make rope. Native Amazonian tribes have also been known to use anteaters to rid their homes of ants and termites. Armadillos are known to be valuable for medical research on leprosy, typhus, trichinosis, and birth defects. They are also used as a food source and as a material for crafts such as musical instruments, decorations, and charms. Pichi armadillos (Zaedyus pichiy) are occasionally taken in as pets.
Carroll, R. L. Vertebrate Paleontology and Evolution. New York: W. H. Freeman and Co., 1998.
McKenna, M. C., and S. K. Bell. Classification of Mammals above the Species Level. New York: Columbia University Press, 1997.
McNab, B. K. "Energetics, population biology and distribution of xenarthrans, living and extinct." In The Evolution and Ecology of the Armadillos, Sloths and Vermilinguas, edited by G. G. Montgomery, 219–232. Washington, DC: Smithsonian Institution Press, 1985.
Nowak, R. M. Walker's Mammals of the World. Baltimore and London: The John Hopkins University Press, 1999.
Webb, S. D. "Late Cenozoic mammal dispersal between the Americas: The great American biotic interchange." In Topics in Geobiology 4, edited by F. G. Stehli and S. D. Webb. New York: Plenum Press, 1985.
Hanson, R. M. "Shasta ground sloth food habits, Ramparts Cave, southwestern Arizona." Paleobiology 4 (1978): 302–19.
McNab, B. K. "Food habits, energetics and the population biology of mammals." American Naturalist 116 (1980): 106–124.
Naples, V. L. "Cranial osteology and function in the tree sloths Choloepus and Bradypus." American Museum Novitates 2739 (1982): 1–41
——. 1990 "Morphological changes in the facial region and a model of dental growth and wear pattern development in Nothrotheriops shastensis." Journal of Vertebrate Paleontology 10, no. 3 (1990): 372–389.
Virginia L. Naples, PhD