Mammal

V

Diet of Mammals

Mammals use food to keep themselves warm as well as to power their bodies and grow. As a result, they have to eat more frequently than cold-blooded animals, or ectotherms, which do not maintain a constant warm temperature. This is particularly true of the smallest mammals because, despite having fur, their small body size means that they lose heat at a rapid rate. To facilitate eating, specialized teeth help them to collect their food, and also to process it before it is swallowed. Mammals' teeth occlude, which means that they fit together in a precise way when the mouth is closed, allowing them to nibble, gnaw, slice, or chew.

The smallest predatory mammals feed on animals such as earthworms and insects, and often find their prey by touch. Bigger insect-eaters, such as pangolins and anteaters, usually locate their food by sight or smell. They gather food in much bigger quantities using their sticky tongues, which can sweep up thousands of ants or termites in a matter of minutes.

The majority of bats also feed on insects, but they have a very different technique for catching their flying prey. Using a system called echolocation, a bat sends out bursts of high-pitched sound toward objects and interprets the returning echoes as images that guide a bat toward its prey so that it can hunt even in total darkness. Dolphins and sperm whales use a similar system to locate food underwater.

True carnivores, which include dogs, cats, and their relatives, often spend a considerable time tracking down their prey. For most of these flesh-eating animals, the senses of smell and hearing are at least as important as vision. Carnivores normally hunt alone, but some species—notably wolves and lions—hunt in organized groups. By doing this they can tackle prey larger than themselves. Carnivores have pointed front teeth, called canines, which help them hold their prey. Most also have specialized rear teeth, called carnassials, which work like shears to slice through their food, and to crush bones.

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The most successful of the plant-eating mammals are the ruminants—hoofed mammals that include cattle, sheep, goats, and their relatives. Ruminants have a highly specialized three- or four-part stomach that helps these animals digest the cellulose in plant cell walls, which no mammal can digest on its own. After eating their food, they regurgitate it and chew it a second time, before swallowing it once again. Microorganisms that reside in the stomach then break down the chewed-up mass, releasing nutrients that the mammal absorbs.

In water, few mammals apart from manatees and dugongs live purely on plants. Instead, most aquatic mammals eat animal food, but they catch it in two quite different ways. Pursuit hunters, such as otters, seals, and toothed whales, chase individual prey through the water, much like carnivores chase their prey on land. However, the largest whales feed on much smaller animals, scooping them up in vast quantities. These whales do not have teeth, and instead strain their food with a screen of fibrous plates called baleen. Using this system of filter feeding, a blue whale can consume over 4 metric tons of food in a day.

VI

Reproduction

Mammals produce relatively few young each time they breed, but they provide their young with the best chances of survival. In all mammals except monotremes, the young start their development inside their mothers, where they are safe from most of their enemies. The parents continue to protect their young after they are born, fending off predators and providing the offspring with food.

Some mammals breed throughout the year, but most have distinct breeding seasons that allow birth to coincide with times when food is plentiful. In some species—for example, baboons—changes in skin color show when a female is ready to mate. However, in most mammals, males detect fertile females by sensing airborne chemicals called pheromones. The males of some mammals, including cats and horses, sense these chemicals with a receptor on the roof of the mouth called the Jacobson’s organ. As the males sniff, they curl their upper lip and seal their nostrils to draw air over the scent organ.

Young mammals are normally conceived immediately after mating. The gestation period—the time between conception and birth—varies from as little as 12 days in the case of some marsupials, to over 20 months in the African elephant. A mammal's size and its gestation period are not directly related. The tiny mouse lemur, for example, weighs about 57 g (about 2 oz), but has a gestation period of nearly 9 weeks. In a few mammal species, including rodents, bats, and kangaroos, development can be brought to a temporary halt at a very early stage. This system, called delayed implantation or embryonic diapause, allows the time of birth to be finely adjusted. In rodents, it helps to make sure that successive litters are spaced apart. In kangaroos, it allows the birth of developing young to be kept 'on hold' until rain produces an abundance of food.

Mammals usually give birth on their own, but in some species—for example, dolphins and elephants—nonbreeding females sometimes gather around to help during the birthing process. Many placental mammals lick the embryonic membranes off the newborn young, and often nudge them toward the mother’s mammary glands for their first meal of milk. By contrast, female marsupials seem to pay little attention to their young, leaving them to find their own way into the pouch unaided.

Milk produced in the mother’s mammary glands provides newborn mammals with water and important nutrients. Mammals are born with a strong sucking instinct that helps them feed immediately. In monotremes, milk simply oozes out onto a patch of skin, but in marsupials and placentals, milk is released by a set of nipples or teats. Young placental mammals leave their mother's nipples between feedings, but newborn marsupials remain firmly fastened to their mother's nipples by their mouths and cannot be dislodged.

The period before weaning forms a crucial part of a young mammal's development. During this time, it plays with its siblings and learns social and survival skills from its parents. During weaning, young carnivorous mammals start to share food their parents have caught, while young plant-eating mammals begin to feed for themselves. Independence comes only when the adult teeth are fully formed, enabling the young animal to switch to an adult diet.

VII

Locomotion

The distant ancestors of today's mammals had five-toed feet and small, roughly equal limbs. Many small mammals—particularly insectivores and rodents—still follow this pattern, and move about by running on all four limbs on the ground, or by climbing tree trunks and along branches. But in many other mammals, the shape of the limbs has completely changed, allowing very different methods of movement.

Most primates are good climbers. Some primitive primates climb like early mammals, using claws to maintain their grip, but advanced primates, such as monkeys and apes, climb in another way. These animals have long legs, grasping toes, and flattened nails instead of claws. Instead of moving along the top of branches, they can also hang below them. Some species of primates—particularly gibbons—use a method of movement called brachiation. Hooking their hands over branches, they swing through the treetops. At top speed, a brachiating gibbon can move through the forest canopy as fast as a human can run.

On the ground, most heavily built mammals, such as badgers and bears, walk on the soles on their feet. This gait works well to maintain a grip, but it is not good for rapid movement. Hunting mammals, such as dogs and cats, have legs that are long and more slender. Instead of walking on their soles, these animals walk on the pads of their toes, with their heels staying high off the ground. This adaptation reduces friction and increases leverage, permitting much more rapid movement. Hoofed animals take this trend even further. These animals have very long legs, and they stand on the very tips of their toes. The combination of small, hard hooves and a long stride enables them to run quickly for long periods of time.

A small but varied assortment of mammals, including jerboas and kangaroos, have given up running on four legs, and instead hop on two. Hopping works best in open habitats, and is a surprisingly energy-efficient way of moving about. Every time a hopping mammal makes contact with the ground, tendons in its legs stretch like elastic. The energy from the stretched tendon helps to power the next jump.

Many forest mammals have evolved flaps of skin that act like wings enabling them to glide from tree to tree. Bats, however, are the only mammals capable of powered flight. A bat's wings are highly modified front legs, with four extremely long fingers that work like the spokes of an umbrella, opening up to support the wing. The remaining finger, which is much shorter, has a hooked claw and is often used for grooming the fur.

In the place of legs, marine mammals have evolved sturdy flippers. Seals have all four limbs and they use their hind legs to swim. In whales, however, the hind limbs have disappeared, leaving only tiny vestiges of bones hidden away inside the body. To power themselves, whales use tail flukes, horizontal flaps that are stiffened by tough fibers instead of bones. As a whale bends its backbone up and down, its flukes speed it through the water.

Given their speed and stamina, large mammals are able to exploit different habitats by migrating (see Animal Migration). In water, most toothed whales occupy the same range throughout the year, and it is unclear how far they migrate. Baleen whales, on the other hand, are well-known migrants. Most species give birth in tropical waters, and then migrate to their feeding grounds in the colder waters much closer to the poles.

On land, most migratory mammals are plant-eating species. They include tropical animals such as wildebeests and zebras, which migrate to take advantage of rain, and also Arctic species such as the caribou, which migrates between tundra and coniferous forest. Some herds travel up to 48 km (30 mi) a day, covering over 1600 km (1000 mi) in the course of a year.

VIII

Survival

Like all other animals, mammals have to overcome a wide range of hazards in order to survive. Some of the problems they face, such as extreme cold or heat, are constant and predictable features of life. They deal with these by physical adaptations such as good insulation, and by special forms of behavior such as hibernation and migration. Attacks by predators present quite different problems, because they are more random and therefore much more dangerous. For most mammals, the key to surviving them lies in keen senses and rapid responses.

In open ground, most plant-eating mammals use special danger signals that warn if an attack is imminent. Rabbits, for example, stamp their hind feet on the ground if they see or hear danger, while many grazing mammals, such as antelopes, give a barklike alarm call if they sense an unfamiliar scent in the air. These sounds bring feeding to an instant halt, as the members of a group ready themselves to run. In some primates, more elaborate warning systems have evolved. African vervet monkeys have specific alarm calls that are used for three kinds of predators—eagles, leopards, and snakes. The eagle call makes the monkeys run for dense cover, while the leopard call sends them running up trees to the highest branches. The snake call simply increases general wariness as the monkeys continue to feed.

In other species, evolution has transformed specific features of the body into effective weapons. For example, many mammals communicate by scent, and mark their territories with strong-smelling secretions that are produced by special glands. In skunks, these secretions have taken on a completely different role, and are used as chemical weapons. The skunk's conspicuous black-and-white markings warn that it can be dangerous if attacked.

More widespread defense systems are based on armaments made of keratin—the same substance that forms hair—and also on structures made of bone. In hairs, keratin is thin enough to be flexible, but when it is laid down in much thicker layers, the same substance can form tough scales. Pangolins are covered with sharp-edged scales, and they roll up if threatened, leaving only their scales exposed. Many armadillos use a similar defensive technique, although instead of having scales, they are protected by interlocking plates made of bone.

Keratin also forms horns, spines, and quills. Horns grow around a stubby core of bone, which locks the horn firmly in place. Unlike antlers found in deer, which are shed and regrown annually, horns keep growing throughout life. In some large grazers, such as African and Asian buffalo, they can span over 1.5 m (5 ft) from tip to tip, making them formidable weapons. Spines and quills are only anchored in the skin, but they grow over large parts of the body. Animals such as echidnas and hedgehogs use their spines as a passive form of defense, but porcupines use their quills in a much more active way, charging backwards to leave them embedded in an enemy's mouth or skin.

For other mammals, hunting weapons such as teeth, tusks, and claws double as emergency defenses in the event of an attack. The teeth of a hippopotamus are large enough to slice through a crocodile menacing its young, while the tusks of a walrus, which can be over 61 cm (2 ft) long, can sometimes fend off attacking orcas, or killer whales. Cats' claws are normally retracted in the paws, which keeps them permanently sharp, making them particularly effective weapons for attack and defense.

Many small mammals, such as rodents, are vulnerable to attack from birds or other predators when they are feeding out in the open. To defend themselves, these animals employ camouflage, in which they use their natural coloring to blend in with their surroundings. For instance, voles forage for food in areas littered with dead leaves that are the same color as the voles’ fur, making them virtually invisible from the watchful eyes of a bird flying above. Certain predators, such as the tiger, employ camouflage to remain unseen until they are ready to pounce on their prey.

IX

Origin of Mammals

Mammals evolved from reptiles called therapsids, and first appeared over 200 million years ago. The transition from reptile to mammal was a gradual one and involved a slow accumulation of mammalian features over a long period of time. One of these new mammalian features, found in all modern mammals, is a space behind the eye socket that allows extra room for muscles powering the jaws. Another is a hard bony palate that separates air moving through the nose from food moving through the mouth. Mammal-like reptiles also developed a new kind of jaw hinge, and increasingly complex teeth. The number of bones in the lower jaw progressively decreased from seven, until only one was left. Instead of being completely lost, the extra bones changed shape and took on a completely new role, helping to conduct sound waves from the eardrum to the inner ear. At some point in this long transition, fur, mammary glands, and a warm-blooded lifestyle also evolved.

One of the earliest mammals known, a species called Morganucodon, looked like a shrew, and was only about 10 cm (about 4 in) long, while a related species, called Megazostrodon, was also about the size of a shrew. These early mammals were almost certainly active at night and are thought to have lived on insects or small vertebrates.

By the Jurassic period, when dinosaurs dominated life on land, mammals had already been in existence for millions of years. During the long reign of the dinosaurs, several different groups of mammals evolved. Most of these groups have been named for the structure of their teeth. Morganucodon and Megazostrodon, for example, belonged to a group called the triconodonts—so called because their teeth had three cusps, or conical points, roughly in a line. Symmetrodonts had molars with three cusps arranged in a triangle, while multituberculates had rodent-like incisors, and large molars adapted for grinding up plant food. Teeth are made from hardy material that fossilizes well, and in many cases they provide scientists with the only clues that show how these early mammals lived.

Scientists once assumed that all mammals dating from the Mesozoic Era resembled shrews or small rodents, and that they mainly ate insects. New finds of nearly complete fossil skeletons have revealed much more variety. The Jurassic Castorocauda, which was recently discovered in Inner Mongolia, was about the size of a platypus. It had a beaverlike body and tail, and swam to catch fish like an otter. Repenomamus from the Cretaceous Period, which was found in present-day China, reached the size of a badger and was a fierce predator—one specimen was found with a baby dinosaur in its stomach.

When dinosaurs disappeared at the end of the Cretaceous Period 65 million years ago, mammals found themselves in a world of unprecedented opportunities, without the ruling reptiles that had held them in check for so long. Some mammal groups, such as the multituberculates, eventually became extinct themselves, but others flourished and diversified as they filled the ecological roles that reptiles had left vacant. In particular, small placental animals developed a wide range of new diets, and an even bigger range of body forms. By about 30 million years ago, some of them had already taken to the seas, while on land their descendants included saber-toothed cats, the forerunners of today's pigs, and also giant browsers (leaf eaters) such as Indricotherium. Standing up to 5.5 m (18 ft) at the shoulder and weighing about 20 metric tons, this hornless relative of the rhinoceros was probably the largest mammal that has ever lived on land.

Despite their huge variation in size, these animals all shared the basic mammalian body plan that they had inherited from their distant ancestors. Most mammals still have four limbs, hair, and relatively large brains, yet their shared ancestry also explains similarities that are much less obvious. All mammals have four-chambered hearts, a muscular diaphragm separating their heart and lungs from their abdominal cavities, a lower jaw made of a single bone, and the same arrangement of tiny bones in the inner ears. Almost all mammals, including whales, mice, and giraffes, have seven vertebrae in their necks. The arms of humans, the flippers of seals, and the wings of bats all have the same number and arrangement of bones.