Placental Mammals

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figure 43-12

The North American pika (a) is a lagomorph, related to rabbits and hares. The North American porcupine (b), a rodent, ranges from Canada to northern Mexico.

figure 43-12

The North American pika (a) is a lagomorph, related to rabbits and hares. The North American porcupine (b), a rodent, ranges from Canada to northern Mexico.

(a) North American pika, Ochotona princeps
(b) North American porcupine, Erethizon dorsatum

Nearly 95 percent of all mammalian species are placental mammals, making up the infraclass Eutheria of the subclass Theria. They are classified into about 20 orders.

Order Xenarthra

The order Xenarthra (zuh-NAHR-thruh) includes about 30 living species of anteaters, armadillos, and sloths living in southern North America, Central America, and South America. Biologists think that, based on fossil and molecular evidence, Xenarthra evolved as a unique lineage in what is now South America. The name xenarthra means "strange joints" and refers to the unique structure of the lumbar vertebrae of members of this order.

This order was once named Edentata (EE-den-TAH-duh), meaning "toothless," because many members of this order do not have prominent teeth. Anteaters completely lack teeth. Armadillos and sloths have peglike teeth that lack enamel. Most edentates feed on insects, which they capture with a long, sticky tongue. With their powerful front paws and large, sharp claws, they rip open anthills and termite nests. Armadillos supplement their insect diet with small reptiles, frogs, mollusks, and scavenged meat. Sloths, on the other hand, are herbivores; their continuously growing teeth are adapted to grinding plants.

Order Lagomorpha

The order Lagomorpha (LAG-uh-MAWR-fuh), members of which are called lagomorphs, includes about 70 species of rabbits, hares, and pikas. A pika (PIE-kuh) is shown in Figure 43-12a. Lagomorphs are native to many continents. They differ from rodents in that lago-morphs have a double row of upper incisors, with two large front teeth backed by two smaller ones. The teeth of lagomorphs continue to grow throughout their lifetime. Such teeth are an adaptation to a herbivorous diet.

Order Rodentia

Related to Lagomorpha is the order Rodentia (roh-DEN-chuh), members of which are called rodents. Rodentia is the largest mammalian order, which includes more than 1,800 species, or about 40 percent of all placental mammals. Rodents flourish on every continent except Antarctica and are adapted to a wide range of habitats. They tend to produce many young in each litter. Squirrels, marmots, chipmunks, gophers, muskrats, mice, and rats are rodents. The porcupine in Figure 43-12b is also a rodent.

A rodent's teeth consist of a few molars or premolars and two pairs of incisors that continue to grow as long as the rodent lives. The sharp incisors are an adaptation to gnaw on seeds, twigs, roots, and bark. As a rodent gnaws, the back surface of the tooth wears away faster than the front surface, maintaining the tooth's edge.

Order Primates

The order Primates is made up of 235 living species, including lemurs, tarsiers, lorises, monkeys, gibbons, apes, and humans. Most primates are omnivores and have teeth suited for a varied diet. Primates have brains that have a relatively large cortex, which make possible the complex behaviors characteristic of this group.

A wide range of body sizes and adaptations allow primates to live in a variety of terrains. The smallest known primate, the pygmy mouse lemur, weighs only about 30 g and was discovered in 2000 in Madagascar, where it lives mostly in trees. In contrast, the largest primate, the mountain gorilla, can weigh 140-180 kg (300-400 lb) and lives on the ground in dense African mountain forests.

Most primates have forward-facing eyes, a feature that enables depth perception. Many primates are active at night and have large eyes adapted for night vision. All primates have grasping hands and, with the exception of humans, grasping feet. Some primates also have a grasping tail. Many primates live in trees, where grasping feet, hands, and tails are essential adaptations. In humans, grasping hands serve many purposes.

Order Chiroptera

The only mammals that truly fly, the bats, make up the order Chiroptera (kie-RAHP-tuh-ruh). There are more than 900 species of bats, and they live throughout the world, except in polar environments. A bat's wing is a modified front limb with a membrane of skin that stretches between extremely long finger bones to the hind limb, as shown in Figure 43-13a. A bat's wingspan can measure up to 1.5 m (4.5 ft). The bat's clawed thumb sticks out from the top edge of the wing. Bats use their thumbs for walking, climbing, and grasping.

Most bats have small eyes and large ears and navigate by echolo-cation. Most bats are active at night and feed on insects. However, some tropical bats are active in the day and feed on fruit or flower nectar. These bats locate food by using their large eyes and keen sense of smell. A few species of bats feed on meat or blood.

Order Insectivora

The order Insectivora (in-sek-TIV-uh-ruh) includes about 390 species of shrews, hedgehogs, and moles living in North America, Africa, and Europe. Figure 43-13b shows a shrew. Most members of this order are insectivores, which means "animals that eat insects." However, not all insectivores are members of the order Insectivora. Furthermore, some Insectivora eat meat.

Scientists disagree about whether to include certain families of mammals in this order. For example, a family of mammals called colugos, which are commonly called flying lemurs, was once placed in Insectivora. This family is now usually classified as another order, Dermoptera. The Insectivora are usually small animals with a high metabolic rate. Most have long, pointed noses that enable them to probe in the soil for insects, worms, and other invertebrates. Their sharp teeth are adapted for grasping and piercing prey.

figure 43-13

(a) Bats, in the order Chiroptera, are the only mammals that truly fly. The physics of the bat's wing in flight gives the bat more lift in relation to its body weight than most birds have. Thus, bats can remain airborne at slower speeds than birds can. (b) Shrews, in the order Insectivora, must eat more than twice their own body weight daily to fuel their high metabolic rate.

(a) Peter's epauletted fruit bat, Epomophorus crypturus
(b) Least shrew, Cryptotis barua

Word Roots and Origins ungulate from the Latin ungula, meaning "hoof"

figure 43-14

This caribou (a) is an artiodactyl. Artiodactyls are native to all continents except Australia and Antarctica. Although this tapir (b) looks like a pig (Order Artiodactyla), it is a perissodactyl. The similarity to a pig is an example of convergent evolution.

figure 43-14

This caribou (a) is an artiodactyl. Artiodactyls are native to all continents except Australia and Antarctica. Although this tapir (b) looks like a pig (Order Artiodactyla), it is a perissodactyl. The similarity to a pig is an example of convergent evolution.

(a) Caribou, Rangifer tarandus
Costa Rica
(b) Baird's tapir, Tapirus bairdi

Order Carnivora

The 274 living species of the order Carnivora (kahr-NIV-uh-ruh) are distributed worldwide. Most members of this order are called carnivores, which means "animals that eat meat." Dogs, cats, raccoons, bears, hyenas, otters, seals, and sea lions are some well-known carnivores. Most are skilled hunters with strong senses of sight and smell. Other adaptations of carnivores include strong jaws, long canine teeth, and clawed toes to seize and hold prey. Many terrestrial carnivores have skeletal adaptations, such as long limbs, to run quickly.

Aquatic carnivores, known as pinnipeds, include the sea lions, seals, and walruses. They are efficient at swimming, with streamlined bodies and four limbs adapted as flippers. Although pinnipeds spend much of their time in the sea feeding, they return to land to sleep and to give birth. They are generally larger than land carnivores, and their large size helps them maintain body temperature. Most pinnipeds can dive to depths of 400 m (1,313 ft) and remain underwater for up to five minutes, but some can remain submerged for as long as one hour. Some scientists once placed pinnipeds in their own order—Pinnipedia.

Order Artiodactyla

Mammals with hoofs are ungulates (UHNG-yoo-lits). Two main groups of ungulates are characterized by their foot structure and by the presence of either a rumen or a cecum.

Ungulates with an even number of toes are artiodactyls, in the order Artiodactyla (AHRD-ee-oh-DAK-tuh-luh). This order includes about 210 species of deer, cattle, giraffes, pigs, and camels. Artiodactyls are native to every continent except Antarctica and Australia. Figure 43-14a shows a common artiodactyl, a caribou. Most artiodactyls can run quickly to escape predators.

Most artiodactyls are herbivores, although pigs are omnivores. Their molars are usually large and flat, for grinding plant material. Most artiodactyls are ruminants, or animals that have a rumen.

Order Perissodactyla

Ungulates with an odd number of toes are perissodactyls, in the order Perissodactyla (PUH-ris-oh-DAK-tuh-luh). This order includes about 17 living species, such as horses, zebras, rhinoceroses, and tapirs. Most species are native to Africa and Asia. However, some species of tapirs, such as the one in Figure 43-14b, live in Central and South America. Perissodactyls have a cecum.

Order Cetacea

Closely related to Artiodactyla is the order Cetacea (see-TAY-shuh), members of which are called cetaceans. Cetaceans include about 90 species of whales, dolphins, and porpoises worldwide. The orca in Figure 43-15a is an example. Cetaceans have fish-shaped bodies with forelimbs modified as flippers. They lack hind limbs and have broad, flat tails that help propel them through the water.

figure 43-15

(b) Manatee, Trichechus manatus figure 43-15

(b) Manatee, Trichechus manatus

Cetaceans are totally aquatic but evolved from land-dwelling mammals. They breathe through modified nostrils called blowholes. Adult cetaceans lack hair except for a few bristles on the snout. A thick layer of blubber below the skin provides insulation. Cetaceans use echolocation to navigate, communicate, and find prey.

Two subgroups of cetaceans are the toothed whales and baleen whales. Toothed whales include sperm whales, narwhals, dolphins, porpoises, and orcas. Toothed whales can have up to 100 teeth. They prey on fish, squid, seals, and other whales. Baleen whales, such as blue whales, lack teeth and filter food from the water with the baleen attached to the roof of the mouth.

Order Sirenia

Four species of manatees and dugongs (DOO-gawngz) make up the order Sirenia (sie-REE-nee-uh), commonly called the sirenians. These large torpedo-shaped herbivores live in tropical seas, estuaries, and rivers. Their front limbs are flippers modified for swimming. Like whales (order Cetacea), sirenians lack hind limbs and have a flattened tail for propulsion. Although manatees and dugongs look like whales, they are more closely related to elephants. The similarities between whales and sirenians came about through convergent evolution. Figure 43-15b shows the only sirenian found in North America, the manatee.

This orca (a), a cetacean, looks very different from most artiodactyls, but scientists think cetaceans are closely related to artiodactyls. These manatees, or sea cows (b), belong to one of four species of Sirenians. Sirenia may share recent ancestry with Proboscidea.

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