Animal

III

Animal Habitats

Few parts of Earth’s surface are entirely devoid of animal life. Animals cannot survive in places where water is unavailable or permanently frozen, or where temperatures regularly exceed 55° C (130° F). However, in all habitats that lie between these extremes, animal life abounds.

A

Aquatic Habitats

Animal life first arose in water. Millions of years later, marine and freshwater habitats continue to support a large proportion of the animal life on earth. Aquatic habitats—particularly in the seas and oceans–rarely experience abrupt changes in conditions, which is a major advantage for living things.

In the seas and oceans, the greatest diversity of animal life is found in habitats close to shores. The richest of all these habitats are coral reefs, underwater ridges that form in clear water where the minimum temperature is 20° C (68° F) or above. Coral reefs are composed of an accumulation of the remains of coral—invertebrates with stony skeletons—calcareous red algae, and mollusks. One of the reasons for the great diversity of animal life in reefs is that living coral creates a complex three-dimensional landscape, with many different microhabitats. The smallest crevices provide hiding places for scavengers such as crabs and shrimps, while larger ones conceal predators such as octopuses and moray eels. Over half the world’s fish species live in coral reefs, many hiding away by day and emerging after dark to feed.

On reefs and rocky shores, many animals are sessile, meaning that they spend their entire adult lives fixed in one place. These species, which include sponges, barnacles, and mollusks, as well as reef-building corals themselves, typically spend the early part of their lives as drifting larvae, before settling on a solid surface and changing shape. Sessile animals are common in aquatic habitats because it is relatively easy for them to collect food, which typically is pushed in the animal’s direction by water currents. By contrast, very few sessile animals have evolved on land.

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In open water, depth has a marked influence on animal lifestyles. The surface layers of the open sea teem with small and submicroscopic animals, which feed either on algae and other plantlike organisms or on each other. These animals form part of the plankton, a complex community of living things that drifts passively with the currents. Many planktonic animals can adjust the depth at which they float, but larger animals such as fish, squid, and marine mammals, are strong enough to commute between the surface and the depths far below.

Even in the clearest water, light quickly fades with increasing depth. Deeper than about 150 m (500 ft), not enough light penetrates for photosynthesis to occur, so algae are unable to survive. With increasing depth, water pressure rises and temperature falls, ultimately coming close to the freezing point on the ocean floor. Despite these extreme conditions, animal life is found in the ocean’s greatest depths, fueled by the constant rain of organic debris that drifts down from far above. In a habitat where prey is widely scattered, many deep-sea fish can swallow animals larger than themselves, an adaptation that allows them to go weeks or months between meals.

B

Land Habitats

On land, animal habitats are strongly influenced by climate, the combination of precipitation and temperature conditions experienced in a region. At or near the equator, year-round moisture and warmth generates a constant supply of food. Further north or south, seasonal changes become much more pronounced, shaping the type of animals that live in different habitats and their strategies for survival (see Animal Distribution).

Tropical and subtropical forests are home to by far the largest number of animal species on land. These animals include the majority of the world’s insects, most of its primates, and a large proportion of its birds. Tropical forests have existed longer than any other forests on earth and their plants and animals have evolved an elaborate web of interrelationships.

Much of the animal life of tropical forests is still poorly known, and new species are constantly being discovered. The majority of these newly identified animals are invertebrates, but larger animals have also come to light during the 20th century. Major discoveries have included three large but secretive plant-eating mammals: the okapi, discovered in Central Africa in 1900; the kouprey, discovered in the forests of Cambodia in 1937; and the sao la, which was identified in forests bordering Laos and Vietnam in 1993.

Unlike tropical forests, temperate forests provide animals with an abundance of food during spring and summer, but a dearth during the winter. In this habitat, animals have evolved several different strategies for avoiding starvation during the winter months. Food hoarders, such as squirrels and jay birds, bury surplus food during the fall, and dig it up again when other food supplies run out. Other forest animals, such as the common dormouse, avoid food shortages by hibernation, a period of inactivity when body temperature is lowered. A third group of animals—composed chiefly of birds, but also including some bats and insects–migrates to warmer regions before the winter begins and returns again in spring. In boreal forests, which are found in the far north, the seasonal swings are more extreme. Here only a few species stay and remain active during the winter months.

For land animals, the most testing habitats are ones that experience intense drought or extreme cold. Desert animals cope with heat and water shortage by behavioral adaptations, such as remaining below ground by day, and also by physiological adaptations. North American kangaroo rats, for example, can live entirely on dry seeds without ever drinking liquid water. They do this by losing very little moisture from their bodies and using all the “metabolic water” that is formed when food is broken down to release energy.

In tundra and on polar ice, winter air temperatures can fall to below -40° C (-40° F), which is far colder than the temperature of the surrounding seas. The smallest inhabitants of tundra, which include vast numbers of mosquitoes and other biting flies, spend winter in a state of suspended animation and are kept alive by chemical antifreeze within their tissues. The few animals that do remain active on land or ice during winter, such as seals and male emperor penguins, rely on a thick layer of insulating fat to prevent their body heat leaking away. Without this fat, they would die within a matter of minutes.

IV

Feeding

Animals all feed on organic matter, but their diets and way of obtaining food vary enormously. Some animals are omnivores, meaning that they are capable of surviving on a very wide range of foods. Many other animals, from giant pandas to fleas, have extremely precise requirements and cannot deviate from their highly specialized diet.

A

Herbivores and Carnivores

In general, animals eat plants, other animals, or the remains of living things. Plant-eaters, or herbivores, often do not have to search far to find things to eat, and in some cases—for example wood-boring insects—they are entirely surrounded by their food. The disadvantage of a plant-based diet is that it can be difficult to digest and is often low in nutrients.

To overcome the first of these problems, most herbivores have tough mouthparts for chewing and grinding their food. Many plant-eating animals, from termites to cattle, have complex digestive systems containing microorganisms that break down cellulose and other indigestible plant substances, turning them into nutrients that the animals can absorb. The second problem—lack of nutrients–is harder to sidestep, particularly in a diet made up largely of leaves. As a result, leaf-eaters often have to feed for many hours each day to obtain the nutrients that they need.

Carnivores live on flesh from other animals that is often nutrient-rich and easy to digest but difficult to obtain. Finding and capturing this kind of food calls for keen senses. But even though a hunter has acute vision or a highly developed sense of smell, a large proportion of a hunter’s victims manage to escape. If this happens too often, a predator quickly starves.

Some mammalian predators, such as the lion and wolf, increase their chances of success by hunting in groups. While this strategy enables them to tackle larger prey, a successful kill has to be shared among members of the group. But in the animal world as a whole, many other predators adopt a less energy-intensive approach to catching their food. Instead of actively searching out their prey, they position themselves in a suitable location and wait for their prey to come within striking distance.

In this method of hunting, camouflage and other forms of deception play a prominent role. Most animals that use a lie-and-wait strategy blend in with their surroundings, but a few use lures to entice their prey within range. A typical example is the alligator snapping turtle of North America, which waves a ribbon of pink flesh on its tongue that resembles a worm. Any fish venturing toward it is swallowed whole.

In predatory animals, teeth or other mouthparts often play a part in catching and subduing food as well as in preparing it for digestion. These mouthparts include canine teeth in carnivorous mammals, venomous fangs in snakes, and poisonous “harpoons” in some marine mollusks. These harpoons can impale and kill small fish. Each harpoon is used just once, and afterwards it is expelled and another is formed in its place.

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