Biology 1B, Sec 112
5 May 2000
Xenarthra
Ecology
The animals of the order Xenarthra have narrow ecological niches that perpetuate their survival. The biotic and abiotic resources in the environment that they utilize exclude many potential competitors and predators, unlike the broad, general niches of the extinct xenarthrans. Many of their interactions and adaptations to the environment (autecology) have created a unique ecological niche guaranteeing their success. For instance, the anteaters and leaf-eating sloths maintain specialized, energy-rich diets, which is abundant because of the lack of competitors. Most edentates like sloths maintain low metabolisms that limit most species to a small home range. As an adaptation to the resulting slow movement from their low metabolism, these edentates have established a passive defense such as camouflage to ward off any predators (Dickson in MacDonald 1984: 770-771).
Armadillos inhabit a variety of environments such as woodlands, savannas, plains, and deserts. They live in South America, Central America, and the lower eastern part of the United States (Dickson in MacDonald 1984: 780). These dry areas allow armadillos to make labyrinthine burrow systems for shelter. The burrows are of significant use to the armadillos because they also function as emergency protection, food traps, nests, and foraging (Eisenberg 1981: 45). Some armadillos are active during the night (nocturnal), while most others function in the day (diurnal). Armadillos travel in various ways such as singly, in pairs, or sometimes in small bands.
The armadillo is involved in many predator, prey, and parasite-host relationships with other populations that live in their environment (community ecology). The armadillos use their claws to obtain food such as termites, other insects, vegetation, small animals, and some carrion (Dickson in MacDonald 1984: 780). Their claws also are utilized to threaten and attack predators which inhabit the same environment. Besides prey and predators, armadillos interact with a number of parasites including fleas, ticks, and mites.
Reproductive and parental behaviors are an important aspect of the armadillo's population ecology, the interaction amongst a population of armadillos. The common long-nosed armadillo in the United States mates in July and August which is the summer period of the Northern Hemisphere. In South America however, the mating period is from November to January, which is the summer period of the Southern Hemisphere (Dickson in MacDonald 1984: 780). After mating, female armadillos often times create a nest of straw or leaves in their burrow systems. While most armadillos give birth to one or two young, some species of armadillos exhibit polyembryony and delayed implantation. During polyembryony, female armadillos produce two to twelve young from a single fertilized ovum. Delayed implantation in the female armadillo is the process in which the attachment of the ovum to the uterine wall occurs after a delay period. This process is determined by the amount of food provided in the environment of the armadillos. For instance, in the Northern Hemisphere, breeding occurs from June to September as the implantation of the female armadillo's ovum is delayed until November. Thus, the pregnancy of the armadillo occurs between November and March so that the armadillo gives birth in spring when food is most abundant (Anderson et al. (ed.) 1984: 223).
In most armadillos like the six-banded armadillo, the young armadillos are born with closed eyes. In other armadillos like the nine-banded armadillo, the young armadillos are born with open eyes. After a few hours, young armadillos can walk. As the male abandons the young armadillos, the female armadillo continues to play a key role in her young's lives. The female armadillo works to groom, brood, and suckle the young for about 60 days. Furthermore, the parental care of the female is accented by how she always retrieves any young far from the nest (Eisenberg 1981: 49). These reproductive characteristics distinguish the armadillos from many other organisms in the Order Xenarthra.
Anteaters inhabit the forests and savannas of tropical Central and South America. In this environment, the home ranges of the anteaters measure at about 6,200 acres. The abundance of anteaters varies with the habitat. For example, larger numbers of anteaters inhabit mainly the tropical rain forests and grasslands than in the mixed deciduous forests. Some anteaters are adapted to live in trees (Dickson in MacDonald 1984: 772). Similar to armadillos, most anteaters are nocturnal, while some remain diurnal. Anteaters travel singly and rarely in pairs.
The community ecology of anteaters is demonstrated in their predator-prey relationship where the anteaters, as predators, use their claws to open ant nests and prey upon the ants, termites, and other insects found inside. Anteaters are also the home to parasites like ticks and fleas (Dickson in MacDonald 1984: 772).
The anteater's reproductive and parental behaviors characterize the population ecology of the anteaters. Anteaters group into pairs only during parental care and mating. The courtship of the Giant anteater and Silky anteater are unknown today; however, it is likely that the breeding individuals coupled before copulation and parted immediately after mating as observed in today's anteaters. In their environment, anteaters like the Giant anteater breed in the fall from March to May (Dickson in MacDonald 1984: 773). During birth, the female stands upon her hind legs and tail. The young anteater then crawls out and climbs onto the back of the female. Bearing only one young, the female anteater suckles her young for about six months. Sometimes, the parents of the Silky anteater feed their young semi-digested insects that they regurgitated. Furthermore, young anteaters remain with their mothers for about a year after their birth. The two-toed young anteater rides upon the tail of its mother, while the young giant anteater remains on the back of its mother. The parent Silky anteaters carry their young for a certain period, yet occasionally leave the young alone in a leaf nest within the hollow of a tree (Dickson in MacDonald 1984: 775). The young giant anteaters can cavort at one month after birth. When they are fully grown after two years, they eventually leave their mothers.
Sloths inhabit dense tropical rain forests from Brazil to Honduras. Examples of their interactions with their habitat, the autecology, can all be found in the realms of the trees - their fixed home. The three-toed sloths and two-toed tree sloths can occupy the same tree up to several days. The curved claws of sloths allow them to climb and hang in the trees. In their arboreal environment, sloths are herbivorous and feed upon leaves, fruits, and buds (Dickson in MacDonald 1984: 776).
One form of community ecology is the commensual relationship that the sloths embrace with the mites, beetles, and small moths. These insects live in the hairy coats of the sloths and feed on the algae growing on the fur. Therefore, the presence of these insects do not affect the sloths' activity or health. A second form of community ecology is the camouflage provided by the algae on the sloths' fur. As the three-toed and two-toed tree sloths inhabit the trees, the algae make it difficult for any potential predator to spot them against their green background (Dickson in MacDonald 1984: 776).
Population ecology is exemplified by the reproductive behavior and parental care system of the sloths. Similar to the armadillos and anteaters, these loners live alone until it is time for the annual mating period when they find a mate. Most sloths breed throughout the year. However, in Guyana, the Pale-throated three-toed sloth gives birth after the rainy season between July and September (Dickson in MacDonald 1984: 778). The gestation period for female sloths vary and can be as long as about 263 days in the two-toed sloth. This long gestation period occurs due to delayed implantation (Eisenberg 1981: 46). When it gives birth to its young, the female two-toed sloth hangs upside down. The young two-toed sloth appears head first and face upward. Eventually, it grasps the female's fur and pulls itself towards the female's chest while the female assists in some of the pulling as well. Tree sloths usually bear one young, which weigh about 300 to 400 grams (Dickson in MacDonald 1984: 778). Even after birth, the young sloth's eyes remain closed and it suckles its mother for about five months. The young is carried by the mother and reaches for leaves to eat from her back. Six to nine months after its birth, the young sloth becomes too heavy as it approaches a weight that is 15 percent of its mother's weight (Eisenberg 1981: 47). To discourage clinging of the young sloth, the mother sloth brashly leaves the young sloth alone. The young sloth develops to an adult at 2.5 to 3 years of age.
Morphology
Edentata means 'without teeth,' but only anteaters are strictly toothless. Sloths and armadillos have premolars and molars that lack an enamel, but they have a single 'open root' which allows continuous growth. True incisors and canine teeth are absent in all Edentata, but sloths have enlarged canine like-premolars.
The Edentata have radiated to fill a variety of feeding niches that played the selection tool for the characters that are present today. The armored body of the armadillos reflects their anti-predator behavior. The specialization for tree life by the sloths and anteaters have imposed modifications in the structure of their bodies and subsequent specialization in their behavior patterns. All are heavily clawed, with one claw per toe that is very long and strong. This prevents smooth locomotion, they must walk on their knuckles which results in an awkward gait (Britannica).
The pleisomorphic traits of edentates include the possession of five toes on the hind feet, a simple uterus, tendency to develop dermal bone under an overlying epidermal scale (such as the elaborate armor of armadillo), and a small, uncomplicated brain. Their specialized traits include reduced dentition, a long sticky tongue, and powerful, clawed forefeet linked to their insect diets.
Sloths have internal organs (liver, stomach, spleen, and pancreas) that take different positions from that of other mammals which makes them highly adapted for climbing. Thus, they can spend almost all their life hanging and walking upside down from tree branches. For example, they eat, sleep (average of 15 hours/day), and give birth all in the tree. Their long legs are designed for suspension of the body rather than for "column-like' support. An apomorphy is that they have extremely large multi-compartmental stomachs, which contain cellulose-digesting bacteria (University of Michigan).
There are two families of sloths: two-toed and three-toed. The difference is that one has two toes on its forelimb and the other has three. They are both clumsy, awkward walkers that move in a deliberate fashion and climb trees very slowly, although the two-towed are much faster than the three-toed sloths. However, they are suprisingly good swimmers as proven during periods of flood (Britannica).
Two-toed sloths are medium-sized animals; they have bodies that are around half a meter in length and weights up to around 9 kg. They have long, usually pale gray-brown fur with green-blue algal growth during rainy seasons, which provide excellent camouflage from predators. Under the coarse outer fur near the skin, the hair grades into a layer of finer, shorter under-fur. External ears are much reduced in size. Their forelimbs and hind limbs are very long, with the forelimbs being slightly longer than the hind limbs (though the difference is less exaggerated than the three-toed sloths). The forelimbs have two large, curved claws - these are attached to digits that are connected by a web of skin (syndactylous). The hind limbs have three claws and are also syndactylous. These claws are used to slash a predator for self-defense.
The skulls are relatively short. They have no incisors or canines, but the anterior cheek teeth are enlarged, triangular in cross section, and canine-like. The sloths have an overbite - it causes the wearing away and continual sharpening of the edges of their teeth. Their tail is small or absent.
Three-toed sloth's bodies are around 0.5m length and weigh between 3 and 5 kg. Similar to the two-toed sloths, they are covered with a long, shaggy mass of hair beneath which under-fur of finer texture can be found to grow. Some have unusually long neck hairs that take the form of a mane. Their hair brushes in a direction opposite of most mammals, flowing from stomach to the back. Thus, when hanging beneath a branch, their hair point towards the ground, perhaps helping it to shed rain. They are tan or yellow-brown in color with a greenish cast like the two-toed sloths, due to the presence of algal cells in the grooves of individual hairs.
The forearms of three-toed sloths are longer than their hind limbs. Both fore and hind feet have three very large, hook-like claws. Similar to the two-toed sloths, their claws are separate but the digits to which they are attached are syndactylous. The skull is short and rounded but is flattened dorso-ventrally. Incisors and canines are absent. Each tooth is a simple, ever-growing peg with a thin layer of dentine. Instead of enamel, the teeth are coated with cementum. The teeth are smaller near the front of the mouth, and the upper and lower teeth do not meet.
Anteaters range from 250 grams (Cyclopes) to over 30 kg (Myrmecophaga) and are between 170 and 210 cm long. Their body fur can either be coarse and long (Myrmecophaga) or short and silky (Cyclopes), with diagonal black and white shoulder strips. They have small eyes, and small, rounded ears with long, elongated snouts. Their tails are long and prehensile (has the ability to seize and hold).
Anteaters have remarkable forelimbs - each of the 5 digits has long and sharp claws where the third claw is especially well developed. The hind feet are less specialized, having 4 or 5 toes but quite normal claws. An apomorphy is their excellent detection of smell that can smell out favorite ant nests. They have long, narrow tongues with minute, backward-pointing spines. It can be pushed out 60cm up to 150 times/min (Giant anteater) to obtain their 'prey' (ants). The sheath containing the tongue and tongue-retracting muscles is anchored on the breastbone. The salivary glands secrete enormous quantities of viscous saliva onto the tongue, and trapped ants are drawn back and masticated first by horny papillae on the roof of the mouth and sides of the cheeks, then by the muscular stomach (Dickson in MacDonald 1984: 775).
Armadillos very in size from 120 grams (fairy armadillo) to 60 kg (giant armadillo), and body length ranging from 125 mm to around 1 m. Their snouts are short and triangular in some species, long and tubular in others. Some species have large external ears, and others do not. The eyes are generally small (University of Michigan).
All armadillos have powerful forelimbs, with 3-5 digits (depending on the species) tipped with heavy, curved claws. Their bodies are mostly gray or brown, although pink fairy armadillos have a pinkish shell and pure white, dense fur on their sides and belly.
The skulls are flattened with a long lower jaw. They have long extendable tongues, and no canines or incisors. Most species have 14-18 teeth in each jaw, but the Gian armadillo has a total of 80-100 small and vestigial teeth, more than any other mammal.
The defining feature of the armadillos is their "shell" - an apomorphy. The armor develops from the skin and is composed of strong bony plates, or scutes, overlaid by horn. Broad and rigid shields persist over the shoulders and hips, whereas there are varying numbers of bands over the middle of the back where the scutes are connected to the flexible underlying skin. The limbs have irregular horny plates covering at least parts of their surfaces; they may also be hairy. The top of the head is always covered by a shield of keratin-covered scutes, and the tail is covered by bony rings (Britannica).
Systematics
Xenarthra comprises a suborder of Edentata, which is part of Class Mammalia. The other suborder of Edentata, Palaenodonta, consists of two extinct families, namely Metacheiromyidae and Epoicotheriidae (Britannica). Xenarthra represents one of the earliest eutherian, or placental, mammals to evolve. The earliest known fossil xenarthrans date back to the Paleocene and early Eocene and were found in South America. Pholidota, a mammalian order comprised of pangolins, is referred to as the "sister group" of Edentata because it shares many of the conservative mammalian features present in edentates (Eisenberg 1981: 57). The common ancestor of both orders radiated during the early Cretaceous period in Africa and South America. Their lineages diverged, however, during the latter part of the Cretaceous when the edentates became endemic to South America and the pholidotes became isolated in Africa (Eisenberg 1981: 42).
The three infraorders of Xenarthra evolved as each of them adapted to a specific feeding pattern. The Infraorder Vermilingua is comprised of modern-day anteaters that became adapted to eating ants and termites. Another lineage, adapted to browsing, resulted in the infraorder Pilosa, the ancestor to modern-day sloths. The third clade, Loricata, became specialized to a terrestrial grazing herbivorous lifestyle and is represented by armadillos (Eisenberg 1981: 43).
The exact degree of relatedness among these three groups of xenarthrans, however, is currently a matter of debate. Since anteaters resemble the most primitive xenarthrans, evolutionists question their relationship to the sloths. Some even argue that the family of anteaters, Myrmecophagidae, and the family of three-toed sloths, Bradypodidae, are not closely related and should be placed in separate suborders. Another matter of debate is the origin of the two extant genera of tree sloths. Current evidence shows that two separate extinct families of ground sloths, the Megalonychidae and the Megatheriidae, may have given rise to the two modern genera, Choloepus and Bradypus, respectively (Britannica). Thus, tree sloths are a polyphyletic group, while both anteaters and armadillos comprise monophyletic clades.
Edentata and Pholidota represent two orders of Eutherians. Prototheria and Theria are the two subclasses of Mammalia, and Eutheria evolved from the latter. Eutheria is most closely related to Metatheria, which gave rise to modern marsupials and also evolved from Theria (Eisenberg 1981: 5). Evidence of the close phylogenetic relationship between marsupials and xenarthrans is the primitive divided womb of the latter, which is only a step removed from the double womb of marsupials. Among the Eutherians, three separate lineages emerged, two of them being Edentata and Pholidota. The third, which branches into Ernotheria and Preptotheria, contains the ancestor to all other modern placental mammals (Eisenberg 1981: 42). Originally, pangolins of the order Pholidota and South African aardvarks of the order Tubulidentata were classified as edentates. This was because members of these two orders share adaptations for an insectivorous diet with anteaters and armadillos. These traits, however, were later shown to be analogous and thus not indicative of a phylogenetic relationship between the groups (Britannica).
There are several distinguishing apomorphic features of xenarthrans. The name "Xenarthra" comes from xenarthrous posterior trunk vertebrae which provide lumbar reinforcement for digging. All xenarthrans have developed certain projections of the scapula bone called the acromion and coracoid processes. They lack true canines, incisors, and premolars, but have a long, sticky tongue and powerful, clawed forefeet that adapt them to an insectivorous diet. In order to conserve energy, xenarthrans have a low metabolic rate and maintain a low body temperature of 30-34 degrees Celsius (Britannica). They also have only one duct for both urinary and genital purposes (Gunderson 1976: 106). Males have internal or only partly descended testes and a small penis with no glands (Eisenberg 1981: 43). Some primitive traits of the group include a simple uterus, a small brain, and the possession of five toes on the hindfeet.
Sources
Gunderson, Harvey L. 1976 Mammalogy. New York: McGraw-Hill Book Company
Eisenberg, John F. 1981 The Mammalian Radiations: An Analysis of Trends in Evolution, Adaptation, and Behavior Chicago: The University of Chicago Press
Dickson, Christopher R. "Edentates." p770-783 in Encyclopedia of Mammals David MacDonald (ed.) 1984 New York: Facts on File, Inc
Anderson, Sydney and Jones, J. Knox Jr. (eds.) 1984. Orders and Families of Recent Mammals of the World New York: Wiley
(Internet Resources)
Britannica Online Encyclopedia 23 April 2000
The University of Michigan, Museum of Zoology, Animal Diversity Web 3 May 2000