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Instinct is behaviour that is inherited rather than learned. We might say that a person who often fights has an aggressive instinct. But the person may not have been born with a desire to fight--and, with a different home or school background might never have developed that trait. Scientists use the terms instinct and instinctive behaviour only for activity that involves neither experience nor learning. To be truly instinctive, a behaviour trait must be typical of almost all members of one or both sexes of a species of animal.
The mating behaviour of a fish called the three-spined stickleback includes many examples of instinct. The male stickleback chooses a mating area and drives other fish from it. He then collects plants and forms them into a small mound. He wriggles through the completed mound, creating a tunnel. This tunnel, which is slightly shorter than the fish, becomes his nest. Meanwhile, his normally dull-coloured body has changed colour. His belly becomes bright red and his back bluish-white. He then starts to court females. Whenever a female, her abdomen swollen with eggs, enters his mating area, he swims toward her and performs a zigzagging "dance." He continues this dance until a female follows him to his nest, where she lays her eggs. She then swims away, and the male fertilizes the eggs. He stays near the nest to protect the eggs and, later, the young.
The stickleback does not learn his complicated mating ceremony. Yet all male sticklebacks perform basically the same actions. They are born with the pattern built into their nervous system.
In some cases, individual animals of the same species vary the details of an instinctive behaviour pattern. Individual goldfinches may use different proportions of various materials to build their nests. They may also carry the materials in different ways. But most instinctive behaviour is relatively rigid. Members of any species do not usually differ greatly in carrying out the built-in pattern.
How instinctive behaviour works
The releasing stimulus. Most instinctive behaviour is released (brought about) by a stimulus, something that makes the animal act as it does. For example, scientists have learned that a male stickleback begins to court a female when he sees her swollen abdomen. In laboratory experiments, male sticklebacks also court cardboard models--but only if the models have swollen abdomens.
The female stickleback is attracted by the male's bright red belly. Females also follow crude models with red bellies more often than they follow exact copies of males without red bellies. The swollen abdomen of the female and bright red belly of the male are both examples of releasing stimuli.
A reflex, such as blinking at a bright light, is also a type of unlearned behaviour released by a stimulus. But reflex actions are less complicated than instinctive behaviour.
Hormone changes. In many cases, a releasing stimulus acts on one or more glands in an animal's body. For example, seasonal changes in the amount of daylight affect the glands of some kinds of birds. The glands secrete fluids called hormones. A change in the amount of hormones secreted stimulates the birds to migrate. If a gland does not secrete a certain hormone properly, an animal may not be able to carry out the instinctive behaviour associated with that hormone.
Some glands produce hormones only at a certain stage in an animal's life. The sex glands, for instance, do not function completely in young animals.
Instinct and learning
All animals perform both instinctive actions and learned actions. Instinct almost completely determines the behaviour of insects, spiders, and crustaceans (such animals as crabs and lobsters). These animals can learn only a little, and so their survival depends on built-in behaviour patterns. Higher animals, including fish, amphibians, reptiles, birds, and mammals, can learn more. They also modify their instinctive behaviour by learning. Young chicks, for example, crouch motionless when any moving object appears above them--even if it is only a falling leaf. Older birds have learned that leaves will not harm them, and so they do not react as they once did. But older birds do crouch motionless at the approach of a hawk.
The higher the animal, the more it can learn and the less it depends on instinct. Fish behave more by instinct than do birds, and birds perform more instinctive actions than do mammals. Among human beings, infants smile and suck instinctively. But as human beings grow older, most of their acts are learned.
Imprinting. One type of behaviour pattern is called imprinting. It occurs when an animal learns to recognize a stimulus that will later release instinctive behaviour. For example, a gosling follows the first moving object it sees after hatching. The young goose "recognizes" the moving object as its parent. It later "recognizes" similar objects as members of its own species. The gosling's behaviour works out well if the first moving object is an adult female goose. But young geese hatched in an incubator may see human beings first--and become imprinted on them. That is, the goslings act toward human beings as they should act toward other geese. After they mature, they even court human beings. In many cases, animals that become imprinted toward animals of another species never learn to recognize members of their own species.
Combining instinct and learning. Survival for animals higher than insects depends on a combination of instinct, learning, and body changes. For an animal to develop normally, it must come in contact with the normal releasing stimuli for members of its species.
Scientists have reared monkeys separately from all other monkeys. Such monkeys do not display normal social or sexual behaviour if, after reaching adulthood, they are placed with other monkeys. Normal adult monkey behaviour results from the experiences of growing up as a member of a group of monkeys. These experiences enable a monkey to perfect its instinctive behaviour through learning.
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