Animal Behavior

B

Motor Programs

A second major discovery by ethologists is that many complex behaviors come prepackaged as motor programs—self-contained circuits able to direct the coordinated movements of many different muscles to accomplish a task. The dancing of sticklebacks, the stinging action of wasps, and the pecking of gull chicks are all motor programs.

The first motor program analyzed in much detail was the egg-rolling response of geese. When a goose sees an egg outside its nest, it stares at the egg, stretches its neck until its bill is just on the other side of the egg, and then gently rolls the egg back into the nest. At first glance this seems a thoughtful and intelligent piece of behavior, but it is a mechanical motor program; almost any smooth, rounded object (the sign stimulus) will release the response. Furthermore, removal of the egg once the program has begun does not stop the goose from finishing its neck extension and delicately rolling the nonexistent object into the nest. Such a response is one of a special group of motor programs known as fixed-action patterns. Programs of this class are wholly innate, although they are frequently wired so that some of the movements are adjusted automatically to compensate for unpredictable contingencies, such as the roughness and slope of the ground the goose must nudge the egg across. Apparently, the possible complexity of such programs is almost unlimited; birds’ nests and the familiar beautiful webs of orb-weaving spiders are examples.

Another class of motor programs is learned. In the human species, walking, swimming, bicycle riding, and shoe tying, for example, begin as laborious efforts requiring full, conscious attention. After a time, however, these activities become so automatic that, like innate motor programs, they can be performed unconsciously and without normal feedback. This need for feedback in only the early stages of learning is widespread. Both songbirds and humans, for example, must hear themselves as they begin to vocalize, but once song or speech is mastered, deafness has little effect. The necessary motor programs have been wired into the system.

C

Drive

The third general principle of ethology is drive. Animals know when to migrate, when (and how) to court one another, when to feed their young, and so on. In most animals these abilities are behavioral units that are switched on or off as appropriate. Geese, for example, will only roll eggs from about a week before egg laying until a week after the young have hatched. At other times eggs have no meaning to them. The switching on and off of these programs often involves complex inborn releasers and timers. In birds, preparations for spring migration, as well as the development of sexual dimorphisms, territorial defense, and courtship behavior, are all triggered by the lengthening period of daylight. This alters hormone levels in the blood, thereby triggering each of these dramatic but essential changes in behavior.

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In general, however, no good explanation exists for the way in which motivation is continually modulated over short periods in an animal’s life. A cat will stalk small animals or toys even though it is well supplied with food; deprived of all stimuli, its threshold (the quality of stimulus required to elicit a behavior) will drop sufficiently so that thoroughly bored cats will stalk, chase, capture, and disembowel entirely imaginary targets. This unaccountable release of what appears to be pent-up motivation is known as vacuum activity—a behavior that will occur even in the absence of a proper stimulus.

One simple mechanism by which animals alter their levels of responsiveness (and which may ultimately help explain motivation) is known as habituation. Habituation is essentially a central behavioral boredom; repeated presentation of the same stimulus causes the normal response to wane. A chemical present on the tentacles of its archenemy, the starfish, triggers a sea slug’s frantic escape behavior. After several encounters in rapid succession, however, the threshold for the escape response begins to rise and the sea slug refuses to flee the overworked threat. Simple muscle fatigue is not involved, and stimulation of some other form—a flash of light, for instance—instantly restores the normal threshold (a phenomenon known as sensitization). Hence, nervous systems are prewired to “learn” to ignore the normal background levels of stimuli and to focus instead on changes from the accustomed level.

D

Programmed Learning

The fourth contribution ethology has made to the study of animal behavior is the concept of programmed learning. Ethologists have shown that many animals are wired to learn particular things in specific ways at preordained times in their lives.

D

1

Imprinting

One famous example of programmed learning is imprinting. The young of certain species—ducks, for example—must be able to follow their parents almost from birth. Each young animal, even if it is preprogrammed to recognize its own species, must quickly learn to distinguish its own particular parents from all other adults. Evolution has accomplished this essential bit of memorization in ducks by wiring ducklings to follow the first moving object they see that produces the species-specific exodus call. The call acts as an acoustic sign stimulus that directs the response of following. It is the physical act of following, however, that triggers the learning process; chicks passively transported behind a calling parent do not imprint at all. (In fact, presenting obstacles so that a chick has to work harder to follow its parent actually speeds the imprinting process.) As long as the substitute parent makes the right sounds and moves, ducklings can be imprinted on a motley collection of objects, including rubber balls, shoe boxes, and human beings.

This parental-imprinting phase is generally early and brief, often ending 36 hours after birth. Another round of imprinting usually takes place later; it serves to define the species image the animal will use to select an appropriate mate when it matures. Ethologists suspect that genetic programming cannot specify much visual detail; otherwise, selective advantage would probably require chicks to come prewired with a mental picture of their own species. As the world has become increasingly crowded with species, the role of sign stimuli in some animals has shifted from that of identifying each animal’s species uniquely to that of simply directing the learning necessary to distinguish an animal’s own kind from many similar creatures. This strategy works because, at the early age involved, most animals’ ranges of contact are so limited that a mistake in identifying what to imprint on is highly unlikely.

D

2

Characteristics of Programmed Learning

Imprinting, therefore, has four basic qualities that distinguish it from ordinary learning: (1) A specific time, or critical period, exists when the learning must take place; (2) a specific context exists, usually defined by the presence of a sign stimulus; (3) the learning is often constrained in such a way that an animal remembers only a specific cue such as odor and ignores other conspicuous characteristics; and (4) no reward is necessary to ensure that the animal remembers. These qualities are now becoming evident in many kinds of learning, and the value of such innately directed learning is beginning to be understood: In a world full of stimuli, it enables an animal to know what to learn and what to ignore. As though for the sake of economy, animals need pick up only the least amount of information that will suffice in a situation. For example, ducklings of one species seem able to learn the voices of their parents, whereas those of another recall only what their parents look like. When poisoned, rats remember only the taste and odor of the dangerous food, whereas quail recall only its color. This phenomenon, known as rapid food-avoidance conditioning, is so strongly wired into many species that a single exposure to a toxic substance is usually sufficient to train an animal for life. The same sorts of biases are observed in nearly every species. Pigeons, for instance, readily learn to peck when food is the reward, but not to hop on a treadle for a meal; on the other hand, it is virtually impossible to teach a bird to peck to avoid danger, but they learn treadle hopping in dangerous situations easily. Such biases make sense in the context of an animal’s natural history; pigeons, for example, normally obtain food with the beak rather than the feet, and react to danger with their feet (and wings).

Perhaps the example of complex programmed learning understood in most complete detail is song learning in birds. Some species, such as doves, are born wired to produce their species-specific coos, and no amount of exposure to the songs of other species or the absence of their own has any effect. The same is true for the repertoire of 20 or so simple calls that virtually all birds use to communicate messages such as hunger or danger. The elaborate songs of songbirds, however, are often heavily influenced by learning. A bird reared in isolation, for example, sings a very simple outline of the sort of song that develops naturally in the wild. Yet song learning shows all the characteristics of imprinting. Usually a critical period exists during which the birds learn while they are young. Exactly what is learned—what a songbird chooses to copy from the world of sound around it—is restricted to the songs of its own species. Hence, a white-crowned sparrow, when subjected to a medley of songs of various species, will unerringly pick out its own and commit it to memory. The recognition of the specific song is based on acoustic sign stimuli.

Despite its obvious constraints, song learning permits considerable latitude: Any song will do as long as it has a few essential features. Because the memorization is not quite perfect and admits some flexibility, the songs of many birds have developed regional dialects and serve as vehicles for a kind of “cultural” behavior.

A far more dramatic example of programmed cultural learning in birds is seen in the transmission of knowledge about predators. Most birds are subject to two sorts of danger: They may be attacked directly by birds of prey, or their helpless young may be eaten by nest predators. When they see birds of prey, birds regularly give a specific, whistlelike alarm call that signals the need to hide. A staccato mobbing call, on the other hand, is given for nest predators and serves as a call to arms, inciting all the nesting birds in the vicinity to harass the potential predator and drive it away. Both calls are sign stimuli. Birds are born knowing little about which species are safe and which are dangerous; they learn this by observing the objects of the calls they hear. So totally automatic is the formation of this list of enemies that caged birds can even be tricked into mobbing milk bottles (and will pass the practice on from generation to generation) if they hear a mobbing call while being shown a bottle. This variation on imprinting appears to be the mechanism by which many mammals (primates included) gain and pass on critical cultural information about both food and danger. The fairly recent realization of the power of programmed learning in animal behavior has reduced the apparent role that simple copying and trial-and-error learning play in modifying behavior.

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