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.

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.

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