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Strange Joints: Anteaters, Armadillos, and Sloths
By Susan Lumpkin

Sometime about 100 million years ago, when dinosaurs still dominated the Earth, a small animal, one of the first placental mammals, drifted away from its African relatives and founded a dynasty in South America. Freshly severed from Africa in the breakup of the Gondwana supercontinent, South America rested in "splendid isolation," as pioneering paleontologist George Gaylord Simpson put it in his fascinating 1980 book of the same name.

southern armadillo
This southern armadillo, which lives at the Small Mammal House at the Smithsonian's National Zoo, is a member of the group of animals called the Xenarthra. (Jessie Cohen/NZP)

That primitive creature diversified into the hundreds of strange and wonderful mammal species—anteaters, armadillos, sloths, and more—known collectively as the Xenarthra. When, just three million years ago, a link was forged between North and South America at the Isthmus of Panama, some of these wonders crossed that bridge to found northern xenarthran dynasties of their own.

Among the xenarthrans on both continents were many species of glyptodonts and pampatheres, some the size of rhinos. The beasts resembled armadillos but with better armor as protection from sabertooth predators. Some glyptodonts were truly huge—more than 16 feet long and five feet tall—and their heads, backs, and tails were covered with bony plates. The plates on their backs actually formed a rigid shell like a turtle's, but were ten feet long! One species wielded an armor-tipped tail like a mace to lash out at predators.

Ground sloths evolved in South America, too, and spread north, with one species reaching as far as Alaska. Based on fossils, scientists have identified four families of ground sloths (the classification level comparable to dogs versus cats) and nearly 90 genera. Some were small, and there were once even aquatic or semi-aquatic sloths, but many giant ground sloths lumbered across the land. The largest known of these plant-eating behemoths, Eremotherium eomigrans, was described in 2000 from fossils in Florida and likely weighed more than 10,000 pounds and reached 17 feet. This and similar giant sloths probably toppled trees for easier dining, much like elephants do. A member of a different family, Jefferson's ground sloth (Megalonyx jeffersonii)—the species that reached Alaska—was smaller, between eight and ten feet long, but still formidable.

Glyptodonts and ground sloths survived until as recently as about 10,000 years ago at the end of the Pleistocene epoch, when so many other of America's large mammals, from sabertooth cats to mammoths and giant short-faced bears, went extinct due to over-hunting by the first Americans, climate change, disease, or some combination of all three. But a few ground sloths hung on another 5,000 years on islands in the West Indies—until about the time the first people arrived on these islands.

Today, however, Xenarthra's surviving heirs are few. There are 21 species of armadillos, six species of sloths, and four of anteaters. And only one xenarthran, the nine-banded armadillo (Dasypus novemcinctus), lives in North America north of Mexico.

These small numbers, however, belie the remarkable diversity still remaining in the group. In the current classification scheme of mammals, which reflects what scientists know about how closely species and groups of species are related, the armadillos form their own order called the Cingulata—the same taxonomic level as, for example, Primates and Carnivora. The 21 species in nine genera come in a wide range of sizes. The largest, the giant armadillo (Priodontes maximus), weighs up to about 70 pounds, and the smallest, the pink fairy armadillo (Chlamyphorus truncatus), weighs a mere three ounces.

The living sloths—all arboreal leaf-eaters—and the anteaters, both arboreal and terrestrial, together form the order Pilosa. But the six sloths are divided into two families commonly called the two-toed sloths and the three-toed sloths. The 12 genera of the now-extinct ground sloths that survived past the end of the Pleistocene were all two-toed sloths.

The four anteaters, all toothless, are also divided into two families. The diminutive silky anteater (Cyclopes didactylus), about the size of a rat, forms its own family, while the giant anteater (Myrmecophaga tridactyla), which reaches 85 pounds, and the smaller, closely related northern and southern tamanduas (Tamandua mexicana and T. tetradactyla)form the other. Anteaters occupy both grasslands and forests from southern Mexico and Central America to South America as far as northern Argentina and Paraguay. Along with the xenarthran groups, they never extended into cooler northern climes, where ants are less abundant and only seasonally available.

These xenarthran oddballs, some of which lack teeth and all of which possess serious claws and have a tough time maintaining their body temperatures, hold what's left of the genetic heritage in one of the four major early dynasties of placental mammals. All four were founded and poised to diversify into many thousands of species when they inherited the world that dinosaurs abandoned 65 million years ago.

Strange Joints

Scientists have long recognized the Xenarthra as a group unto themselves and one of, if not the first, branch of placental mammals. The word xenarthra means "strange joints," a reference to the way their spinal vertebrae are linked, which is unique among mammals. At a higher level, the Xenarthra were once linked to other toothless oddities (some with teeth, but unusual ones) such as the ant-eating aardvark (Orycteropus afer) of Africa and scale-covered ant-eating pangolins (Manis spp.) of Africa and Asia. But in the past ten years or so, studies of molecular genetics have revealed that this seemingly sensible notion, based on physical similarities, is way, way off the mark. It turns out that the mammalian family tree is full of strange branches.

black and rufous giant elephant-shrew
Elephant-shrews belong to the Afrotheria, a group that may share a common ancestor with the Xenarthra. (Jessie Cohen/NZP)

Take the group that vies with the Xenarthra for the title of first branch in the tree: another bunch of strange bedfellows called the Afrotheria (theria means mammal). Just as that South American drifter spawned creatures as diverse as armadillos, anteaters, and now-extinct giant ground sloths weighing 8,000 pounds, its African sister's living legacy includes elephants and elephant-shrews along with manatees and a few others, including aardvarks, which are thus more closely related to elephants than they are to anteaters. Like the Xenarthra, the Afrotheria once boasted a much larger number of species than today's approximately 50 surviving species in seven different orders.

Whether it is entirely accurate to call the first xenarthrans and afrotherians "sisters" is the subject of much genetics research. Geneticists use various techniques, data sets, and analyses to create their trees, with sometimes conflicting results. Some trees do show the Xenarthra and the Afrotheria as sisters, meaning they share a common ancestor, and stepping back in time, that common ancestor is sister to the common ancestor of all other placental mammals. Other trees suggest that one or the other was first, meaning that the Afrotheria shared a single common ancestor with all of the other mammals, including the Xenarthra, which split off soon after, or vice versa.

There is scientific consensus, however, about the next groupings, or as scientists call them, clades. Whatever was its sister clade, the Boreoeutheria (for "northern true mammals") split next, soon followed by a division into the Laurasiatheria (Laurasia is the name of the northern hemisphere supercontinent formed of North America, Europe, and Asia) and the Euarchontoglires (a tongue-twister with a convoluted meaning), the last two of the major placental mammal lineages. These clades contain some surprises too. Humans and other primates, as well as rodents, rabbits, tree shrews, and flying lemurs, or colugos, belong to the Euarchontoglires. All the rest of the mammals, from bats to cats and horses to hedgehogs, are members of the Laurasiatheria. And ant-eating scaly pangolins, once thought to be akin to xenarthrans, instead share a common ancestor with Carnivora such as cats and dogs.

Worm Tongues

The family name of anteaters is Myrmecophagidae, which means, not surprisingly, "ant eater," although their diets may include termites as well. Another name proposed for these animals is vermilinguas, meaning "worm-tongued." All of the anteaters (and the pangolins, by the way) have extraordinarily long tongues, with very long faces, most extremely so in giant anteaters, to accommodate them. The giant anteater can extend its tongue two feet beyond the end of its snout and retract it again as often as 160 times a minute, covered in ants or sometimes termites that adhere to its sticky saliva. Such speed is crucial to these animals getting enough to eat because soldier ants and termites don't take kindly to invasions of their mounds. Using the long sharp claws on their powerful forelimbs, anteaters break into ant and termite mounds and begin slurping up the insects that stream forward. But almost immediately, soldiers begin rushing into the breach and start biting the predator's nose. As the amassing army gets bigger, the anteater extends its tongue farther and farther out of its mouth and away from the mound and its biting defenders. In less than a minute or so, however, the anteater surrenders, and moves on to sup briefly in another mound in its home range. Fortunately, ants and termites are abundant in the tropical forests and grasslands in which the giant anteater lives, and it visits many nests to get its fill of thousands—perhaps as many as 30,000—of ants per day.

However many an anteater eats in a day, ants and termites are not very nutritious food. As a result, anteaters are adapted to conserve energy. Giant anteaters tend to move slowly from meal to meal, and have a curious shuffle thanks to the fact that they walk on the knuckles of their front feet, much like gorillas and chimpanzees do. This protects their claws, which fold up behind the knuckles and wrist. Anteaters use their claws not only to break into mounds but to dig, to fight with other anteaters, and to defend against jaguars and pumas, their only predators, besides humans.         

giant anteater
Giant anteaters use their long, curved claws to dig for insect prey. (Mehgan Murphy/NZP)

Anteaters also conserve energy by not using as much as other mammals do to maintain a high body temperature. Most mammals maintain body temperatures between 95 and 100°F; anteaters' body temperatures, in contrast, range between 90 and 95°F. Their basal metabolic rate—the amount of energy a resting, warm animal uses to maintain its vital organs—is 29 to 33 percent that of similar-size mammals. Their thick, long fur keeps them from losing as much heat as other mammals, too. And to sleep—up to 14 hours a day—they use their claws to dig a shallow depression, lie down in it, and cover themselves with the long furry tail that makes up about half of their six-foot length.

Anteaters not only move slowly, they live slowly. Born after a relatively long gestation of up to 190 days, a newborn giant anteater—litter size is one in all anteaters—weighs about three pounds and is fully furred, with adult markings. Immediately, the baby has the motor skills to climb onto and cling to its mother's back. Mother and baby are virtually inseparable for months. A baby nurses for about six months, but may ride around on its mother for nine months or a year, and doesn't leave its mother for a solitary life until it's about two. If it escapes predators, fires, and other threats, it may live a long time. Although life spans in the wild are not known, the record longevity of giant anteaters in zoos is about 30 years.

A Not-so-Giant Future?

Giant anteaters are found over a large geographic area, from parts of Central America through South America east of the Andes as far south as northern Argentina and Uruguay. Always uncommon, they appear to be declining and may be extinct in some places, including much of Central America and Uruguay. In Brazil, they are confined to national parks and reserves; anteaters studied in one park, which boasts one of the largest populations, are highly inbred. Because so little is known about these animals, however, the World Conservation Union (IUCN) lists them as "near threatened" on the Red List of Threatened Species, which means that the species is close to becoming or likely to be labeled as vulnerable or endangered in the near future. The litany of threats to giant anteaters is familiar: Ongoing loss of habitat to crop and pasture land, human settlements, and uncontrolled grassland fires set to clear land for agriculture. Believing them to be dangerous, farmers kill them as pests, and the slow-moving animals are hit by vehicles on roads and attacked by domestic dogs. Some indigenous groups in Brazil and elsewhere hunt giant anteaters for meat, preferring their taste to that of other wild species—in some areas, unsustainable hunting is a more serious threat than habitat loss.

The Smithsonian's National Zoo and several other North American zoo members of the Association of Zoos and Aquariums are breeding giant anteaters, hoping to build a healthy zoo population in case the species slips into further trouble in the wild. The first birth of a giant anteater at the Zoo, a female named Aurora, represents an important step toward achieving that goal.

The outlook for the rest of the xenarthrans is mixed. A few are doing fine and a few more are known to be in serious trouble; most, however, are so little studied that their status is determined by educated guesses, not rigorous research.    

Tamanduas and the silky anteater are considered of "least concern" by the IUCN, although there are ominous questions about trends in their numbers. Similarly, all but two of the sloth species are of least concern, including the Linnaeus' two-toed sloth (Choloepus didactylus)in the Amazonia exhibit at the Zoo. But again, population trends are unknown. One, the maned sloth (Bradypus torquatus) is found only in the disappearing coastal forest of Brazil and is endangered. Another, the pygmy three-toed sloth (B. pygmaeus), is critically endangered; this species lives only in mangrove forests on a single island off the coast of Panama, and these forests cover only about half a square mile of the island.

two-toed sloth
Today's sloths are mostly arboreal, but large terrestrial and aquatic sloths lived in the Americas more than 10,000 years ago. (Mehgan Murphy/NZP)

Nearly half of the 21 armadillo species are of least concern, with trends in their numbers mostly unknown. So little is known about two of the species that no judgment about their status is possible. Ten species are near threatened or vulnerable. Habitat loss threatens most of the armadillos, but subsistence hunting for food is also a significant threat to most. Many people find armadillos very good eating. If that seems strange, consider that the white meat of nine-banded armadillos, which weigh from about seven to 14 pounds, tided Texans over during the Depression, and you can still find recipes for armadillo online.

Hunting hasn't had a big impact on the versatile nine-banded armadillo, which has the widest range of all the xenarthrans and, for a member of this group, a large litter size of four. In fact, this species has been steadily expanding its range and growing in numbers in the United States since it first crossed the Rio Grande about 150 years ago, probably assisted by people looking for meals as the wide river itself represented a significant barrier. People also helped nine-banded armadillos cross the Mississippi, taking them to Florida in the early part of the last century. From these two fronts, they've naturally colonized Louisiana, Arkansas, Oklahoma, Alabama, Mississippi, and Georgia, and scientists predict they will continue to march northward, limited only by extreme winter temperatures and the need for water. They would do fine in Washington, D.C., and New York City, for instance. One armadillo species already makes its home in Washington: five southern three-banded armadillos (Tolypeutes matacus) live in the Small Mammal House at the Zoo.

The remaining xenarthrans hold a significant portion of the genetic diversity of the mammals. It would be a shame to lose any more species of a group with such a long history, whose very strangeness makes learning more about its members all the more important to our understanding of mammalian evolution.

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ZooGoer 36(6) 2007. Copyright 2007 Friends of the National Zoo.
All rights reserved.

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