Animal Relationships

Many animals eat only plant materials. The energy derived from plants goes into building the bodies of these animals, and is transferred to the bodies of animals that prey on them. There are some plants that capture and digest insects. Some animals—such as aphids and certain roundworms---live in or on living plant tissues.

In the food-making process, plants release free (uncombined) oxygen, without which animals could not live. In the process of respiration, animals in turn release carbon dioxide, which plants use in making food. When plants and animals die, the various chemical compounds of which their bodies are composed become part of the soil that nourishes new plants and, eventually, other animals. Many animals can live in otherwise unfavorable areas because of the moisture and shade provided by trees and other plants. Numerous insects breed under the bark of trees and shrubs.

Some animals help plants to reproduce. Certain insects, birds, and mammals (bees, hummingbirds, and some bats, for example) that visit plants for food or shelter carry pollen from one flower to another, bringing about fertilization. Seeds of plants are often widely distributed by being carried in or on the bodies of animals.

Animals not only depend upon other animals for food, but may also compete with them for food. For example, plant-eating insects such as locusts may compete with rabbits, rodents, and cattle. Lions, leopards, and cheetahs may compete with one another for the flesh of antelopes and zebras.

Animals of the same species compete with each other primarily for food, but may also compete for shelter or mates. Animals of the same species also often cooperate with each other. In some species, individuals assemble into groups for various purposes. Birds, for example, form groups when migrating. Some animals live permanently in groups, often as a protective measure.

When animals live in groups, there is usually a leader and a social order in which some individuals dominate others. Social insects—which include the termites and certain species of bees, ants, and wasps—have a highly developed social organization. Separate "castes" perform such tasks as laying eggs, caring for the young, and guarding the nest and the group. The members of each caste differ in body structure from members of the other castes, making the division of labor permanent. Among certain other animals with a social order the same individual does not necessarily perform the same task all its life.

Sometimes animals of different species, or an animal and a plant, live together in a special relationship. Symbiosis is the general term for relationships of this type. If both partners benefit from the association, it is called mutualism. An example is the relationship between the clown fish and the sea anemone. The bright colors of the clown fish attract prey to the stinging tentacles of the sea anemone. In turn, the clown fish, which is immune to the anemone's deadly sting, receives shelter from predators.

If one partner only benefits, it is called commensalism. Barnacles, for example, live on the skin of whales. The whales are not harmed, but the barnacles are benefited by being carried to fresh feeding grounds. If the relationship benefits one and harms the other, it is called parasitism. Tapeworms are parasites that live in the intestinal tract of humans and other animals.