CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

 

18. Evolutionary and Ecological Aspects of Behavior

 

18.3. Instinctive and Learned Behavior

 

There are two major kinds of behaviors: instinctive and learned. Instinctive behavior is behavior that is inherited, automatic, and inflexible. Learned behavior is behavior that changes as a result of experience. Both are involved in the behavior patterns of most organisms. Learning allows an organism to generate a completely new behavior or to slightly modify existing behavior patterns. Most animals (e.g., j ellyfish, clams, insects, worms, frogs, turtles) have a high proportion of instinctive behavior and very little learning. Others, such as many birds and mammals, have a great deal of learned behavior in addition to many instinctive behaviors.

 

The Nature of Instinctive Behavior

Instinctive behavior makes up most of the behavior in animals that have short life cycles, simple nervous systems, and little contact with their parents. These behaviors are performed correctly the first time without previous experience. Such behaviors are found in a wide range of organisms, from simple, one-celled protozoans to complex vertebrates.

 

Stimulus and Response

Instinctive behaviors occur as a result of a specific response to a particular stimulus. A stimulus is a change in the organism’s internal or external environment that an animal is able to detect. A response is an organism’s reaction to a stimulus.

In our example of the herring gull chick, the red spot on the parent’s bill serves as a stimulus for the chick. The chick responds to this spot in a genetically programmed way. The behavior is instinctive—it is done correctly the first time without prior experience. The chick’s pecking behavior, in turn, is the stimulus for the adult bird to regurgitate food. Obviously, these behaviors have adaptive value for the survival of the young, because they leave little to chance. The young will get food automatically from the adult. Instinctive behavior has great value to a species, because it allows correct, appropriate, and necessary behavior to occur without prior experience.

An organism can respond only to stimuli it can recognize. For example, it is difficult for humans to appreciate what the world is like to a bloodhound. The bloodhound is able to identify individuals by smell, whereas we have great difficulty detecting, let alone distinguishing among, many odors. Some animals, such as dogs, deer, and mice, can see only shades of gray. Others, such as honeybees, see ultraviolet light, which is invisible to us. Some birds and other animals are able to detect the magnetic field of the Earth. And some, such as rattlesnakes, are able to detect infrared radiation (heat) from distant objects.

 

Instinctive Behavior Cannot Be Modified

Instinctive behaviors can be very important for the survival of individuals of a species. This is especially true for fundamental, essential activities that do not require modification. The major drawback of instinctive behavior is that it cannot be modified when a new situation presents itself.

Over long periods of evolutionary time, genetically determined behaviors have been selected for and have been useful to most individuals of the species. However, instances of inappropriate behavior may be generated by unusual stimuli or unusual circumstances in which the stimulus is given. For example, many kinds of insects fly toward a source of light. Over the millions of years of insect evolution, this has been a valuable, useful behavior. It allows them easily to find their way to open space. However, the human species invented artificial lights and transparent windows, which cause the animals to engage in totally inappropriate behavior. We have all seen insects drawn to lights at night or insects inside houses, constantly flying against window panes through which sunlight is entering. This mindless, mechanical behavior seems incredibly stupid to us. Although individual animals die, the general behavior pattern of flying toward light is still valuable because most of the individuals of the species do not encounter artificial lights or get trapped inside houses, and they complete their life cycles normally.

Geese and other birds sit on nests. This behavior keeps the eggs warm and protects them from other dangers. During this incubation period, the eggs may roll out of the nest as the parents get on and off the nest. When this happens, certain species of geese roll the eggs back into the nest. If the developing young within the egg are exposed to extremes of heat or cold, they are killed; thus, the egg-rolling behavior has a significant adaptive value. If, however, the egg is taken from the goose when it is in the middle of egg-rolling behavior, the goose will continue its egg rolling until it gets back to the nest, even though there is no egg to roll (figure 18.3). This is typical of the inflexible nature of instinctive behaviors. It has also been found that many other, somewhat egg-shaped structures can stimulate egg-rolling behavior. For example, beer cans and baseballs trigger egg-rolling behavior. Thus, not only are the birds unable to stop the egg rolling in midstride, but also nonegg objects can generate inappropriate behavior because they resemble eggs.

 

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FIGURE 18.3. Inflexible Instinctive Behavior

Geese use a specific set of head movements to roll any reasonably round object back to the nest. There are several components of this instinctive behavior, including recognition of the object and head-tucking movements. If the egg is removed during the head-tucking movements, the behavior continues as if the egg were still there. This demonstrates the inflexible nature of instinctive behavior.

 

Instinctive Behavior Can Be Complex

Some activities are so complex that it seems impossible for an organism to be born with such abilities. For example, a spider web is not just a careless jumble of silk threads; it is so precisely made that scientists can recognize what species of spider made it by the pattern of threads in the web. However, web spinning is not a learned ability. A spider has no opportunity to learn how to spin a web, because it never observes others doing it. Furthermore, spiders do not practice several times before they get a proper, workable web. It is as if a “program” for making a particular web were in the spider’s “computer” (figure 18.4).

 

 

FIGURE 18.4. Complex Instinctive Behavior

(a) The kind of web constructed by a spider is determined by instinctive behavior patterns. (b) Instinctive behavior by this masked weaver is responsible for its construction of this complex nest.

 

Might these behavior patterns be the result of natural selection? Many kinds of behaviors are controlled by genes. The “computer” in our example is really the spider’s DNA, and the “program” is a specific package of genes. Through the millions of years that spiders have been in existence, natural selection has modified the web-making program to refine the process. Certain genes of the program have undergone mutation, resulting in changes in behavior. Imagine various ancestral spiders, each with a slightly different program. The inherited program that gives individuals the best chance of living long enough to produce a new generation is the program selected for, and the most likely to be passed on to the next generation.

 

The Nature of Learned Behavior

The alternative to preprogrammed, instinctive behavior is learned behavior. Learning is a change in behavior as a result of experience.

Learning is very significant in long-lived animals that care for their young. Animals that live many years are highly likely to benefit from an ability to recognize previously encountered situations and to modify their behavior accordingly. Furthermore, because the young spend time with their parents, they can imitate their parents and develop behaviors that are appropriate to local conditions. These behaviors take time to develop but have the advantage of adaptability. For learning to become a dominant feature of an animal’s life, the animal must also have a memory, which requires a relatively large brain in which to store new information. This is probably why learning is a major part of the lives of only a few kinds of animals, such as the vertebrates. Nearly all human behavior is learned. Even such important behaviors as walking, communicating, feeding oneself and sexual intercourse must be learned. Table 18.1 compares instinctive and learned behaviors.

 

TABLE 18.1. Comparison of Instinct and Learning

 

Instinct

Learning

The animal is born with the behavior.

The animal is not born with the behavior.

Instinctive behavior is genetically determined.

Learned behavior is not genetically determined, but the way in which learning occurs is at least partly hereditary.

No experience is required; the behavior is done correctly the first time.

Performance improves with experience; learning the behavior requires practice.

The behavior cannot be changed.

The behavior can be changed.

Memory is not important.

Memory is important.

New instinctive behavior can evolve as gene frequencies change and new species evolve.

The ability to learn can evolve as gene frequencies change and new species evolve. However, changes in the actual behaviors learned are not the result of genetic changes.

Instinct is more common in simple animals with short lives and little contact with their parents.

Learning is typical of more complex animals with long lives and extensive contact with parents.

Instinctive behaviors can be passed from parents to offspring only by genetic means.

Learning allows acquired behaviors to be passed from parents to offspring by cultural means.

 

18.3. CONCEPT REVIEW

5. List two ways in which learned and instinctive behaviors differ.

6. Briefly describe an example of unlearned behavior in a particular animal. Explain why you know it is unlearned.

7. Briefly describe an example of learned behavior in a particular animal. Explain why you know it is learned.