Animal Behavior - biology.

Animal Behavior - biology. I INTRODUCTION Animal Behavior, the way different kinds of animals behave, which has fascinated inquiring minds since at least the time of Plato and Aristotle. Particularly intriguing has been the ability of simple creatures to perform complicated tasks--weave a web, build a nest, sing a song, find a home, or capture food--at just the right time with little or no instruction. Such behavior can be viewed from two quite different perspectives, discussed below: Either animals learn everything they do (from "nurture"), or they know what to do instinctively (from "nature"). Neither extreme has proven to be correct. II NURTURE: THE BEHAVIORISTS Until recently the dominant United States school in behavioral theory has been behaviorism, whose best-known figures are J. B. Watson and B. F. Skinner. Strict behaviorists hold that all behavior, even breathing and the circulation of blood, according to Watson, is learned; they believe that animals are, in effect, born as blank slates upon which chance and experience are to write their messages. Through conditioning, they believe, an animal's behavior is formed. Behaviorists recognize two sorts of conditioning: classical and operant. In the late 19th century the Russian physiologist Ivan Pavlov discovered classical conditioning while studying digestion. He found that dogs automatically salivate at the sight of food--an unconditioned response to an unconditioned stimulus, to use his terminology. If Pavlov always rang a bell when he offered food, the dogs began slowly to associate this irrelevant (conditioned) stimulus with the food. Eventually the sound of the bell alone could elicit salivation. Hence, the dogs had learned to associate a certain cue with food. Behaviorists see salivation as a simple reflex behavior, something like the knee-jerk reflex doctors trigger when they tap a patient's knee with a hammer. The other category, operant conditioning, works on the principle of punishment or reward. In operant conditioning a rat, for example, is taught to press a bar for food by first being rewarded for facing the correct end of the cage, next being rewarded only when it stands next to the bar, then only when it touches the bar with its body, and so on, until the behavior is shaped to suit the task. Behaviorists believe that this sort of trial-and-error learning, combined with the associative learning of Pavlov, can serve to link any number of reflexes and simple responses into complex chains that depend on whatever cues nature provides. To an extreme behaviorist, then, animals must learn all the behavioral patterns that they need to know. III NATURE: THE ETHOLOGISTS In contrast, ethology--a discipline that developed in Europe but that now dominates United States studies as well--holds that much of what animals know is innate (instinctive). A particular species of digger wasp, for example, finds and captures only honey bees. With no previous experience a female wasp will excavate an elaborate burrow, find a bee, paralyze it with a careful and precise sting to the neck, navigate back to her inconspicuous home, and, when the larder has been stocked with the correct number of bees, lay an egg on one of them and seal the chamber. The female wasp's entire behavior is designed so that she can function in a single specialized way. Ethologists believe that this entire behavioral sequence has been programmed into the wasp by its genes at birth and that, in varying degrees, such patterns of innate guidance may be seen throughout the animal world. Extreme ethologists have even held that all novel behaviors result from maturation--flying in birds, for example, which requires no learning but is delayed until the chick is strong enough--or imprinting, a kind of automatic memorization discussed below. The three Nobel Prize-winning founders of ethology--Konrad Lorenz of Austria, Nikolaas Tinbergen of the Netherlands, and Karl von Frisch of West Germany (now part of the united Federal Republic of Germany)--uncovered four basic strategies by which genetic programming helps direct the lives of animals: sign stimuli (frequently called releasers), motor programs, drive, and programmed learning (including imprinting). A Sign Stimuli (Releasers) Sign stimuli are cues that enable animals to recognize important objects or individuals when they encounter them for the first time. Baby herring gulls, for example, must know from the outset to whom they should direct their begging calls and pecks in order to be fed. An adult returning to the nest with food holds its bill downward and swings it back and forth in front of the chicks. The baby gulls peck at the red spot on the tip of the bill, causing the parent to regurgitate a meal. The young chick's recognition of a parent is based entirely on the sign stimulus of the bill's vertical line and red spot moving horizontally. A wooden model of the bill works as well as the real parent; a knitting needle with a spot is more effective than either in getting the chicks to respond. Sign stimuli need not be visual. The begging call that a chick produces is a releaser for its parents' feeding behavior. The special scent, or pheromone, emitted by female moths is a sign stimulus that attracts males. Tactile (touch) and even electrical sign stimuli are also known. The most widespread uses of sign stimuli in the animal world are in communication, hunting, and predator avoidance. The young of most species of snake-hunting birds, for instance, innately recognize and avoid deadly coral snakes; young fowl and ducklings are born able to recognize and flee from the silhouette of hawks. Similar sign stimuli are often used in food gathering. The bee-hunting wasp recognizes honey bees by means of a series of releasers: The odor of the bee attracts the wasp upwind; the sight of any small, dark object guides it to the attack; and, finally, the odor of the object as the wasp prepares to sting determines whether the attack will be completed. This use of a series of releasers, one after the other, greatly increases the specificity of what are individually crude and schematic cues; it is a strategy frequently employed in communication. Most animal species are solitary except when courting and rearing young. To avoid confusion, the signals that identify the sex and species of an animal's potential mate must be clear and unambiguous. For example, a minnowlike fish called the stickleback uses a system of interlocking releasers to orchestrate its mating. When its breeding season arrives, the underside of each male turns bright red. This color attracts females but also provokes attacks by other males; red objects of almost any description will trigger male stickleback aggression. A female responds to the male's red signal with a curious approach posture that displays her swollen belly full of eggs. This incites the male to perform a zigzag dance that leads the female to the tunnel-like nest he has built. The female struggles into the nest, whereupon the male touches her tail with his nose and quivers. The resulting vibration causes the female to release her eggs for the male to fertilize. If the male fails to perform the last part of the ballet, the female will not lay her eggs; vibrating the female with a pencil, however, which she can plainly see is not a male stickleback, works perfectly well, although the male in this case, not having gone through the last stage of the ritual, refuses to fertilize the eggs and eats them instead. 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. 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. D1 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. D2 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. IV COMPLEX BEHAVIOR PATTERNS Evolution, working on the four general mechanisms described by ethology, has generated a nearly endless list of behavioral wonders by which animals seem almost perfectly adapted to their world. Prime examples are the honey bee's systems of navigation, communication, and social organization. Bees rely primarily on the sun as a reference point for navigation, keeping track of their flight direction with respect to the sun and factoring out the effects of the winds that may be blowing them off course. The sun is a difficult landmark for navigation because of its apparent motion from east to west, but bees are born knowing how to compensate for that. When a cloud obscures the sun, bees use the patterns of ultraviolet polarized light in the sky to determine the sun's location. When an overcast obscures both sun and sky, bees automatically switch to a third navigational system based on their mental map of the landmarks in their home range. Study of the honey bee's navigational system has revealed much about the mechanisms used by higher animals. Homing pigeons, for instance, are now known to use the sun as their compass; they compensate for its apparent movement, see both ultraviolet and polarized light, and employ a backup compass for cloudy days. The secondary compass for pigeons is magnetic. Pigeons surpass bees in having a map sense as well as a compass as part of their navigational system. A pigeon taken hundreds of kilometers from its loft in total darkness will nevertheless depart almost directly for home when it is released. The nature of this map sense remains one of ethology's most intriguing mysteries. Honey bees also exhibit excellent communication abilities. A foraging bee returning from a good source of food will perform a "waggle dance" on the vertical sheets of honeycomb. The dance specifies to other bees the distance and direction of the food. The dance takes the form of a flattened figure 8; during the crucial part of the maneuver (the two parts of the figure 8 that cross) the forager vibrates her body. The angle of this part of the run specifies the direction of the food: If this part of the dance points up, the source is in the direction of the sun, whereas if it is aimed, for example, 70° left of vertical, the food is 70° left of the sun. The number of waggling motions specifies the distance to the food. The complexity of this dance language has paved the way for studies of higher animals. Some species are now known to have a variety of signals to smooth the operations of social living. Vervet monkeys, for example, have the usual set of gestures and sounds to express emotional states and social needs, but they also have a four-word predator vocabulary: A specific call alerts the troop to airborne predators, one to four-legged predators such as leopards, another to snakes, and one to other primates. Each type of alarm elicits a different behavior. Leopard alarms send the vervets into trees and to the top branches, whereas the airborne predator call causes them to drop like stones into the interior of the tree. The calls and general categories they represent seem innate, but the young learn by observation which species of each predator class is dangerous. An infant vervet may deliver an aerial alarm to a vulture, a stork, or even a falling leaf, but eventually comes to ignore everything airborne except the martial eagle. V THE QUESTION OF ALTRUISM One fascinating aspect of some animal societies is the selfless way one animal seems to render its services to others. In the beehive, for instance, workers labor unceasingly in the hive for three weeks after they emerge and then forage outside for food until they wear out two or three weeks later. Yet the workers leave no offspring. How could natural selection favor such self-sacrifice? This question presents itself in almost every social species. The apparent altruism is sometimes actually part of a mutual-aid system in which favors are given because they will almost certainly be repaid. One chimpanzee will groom another, removing parasites from areas the receiver could not reach, because later the roles will be exchanged. Such a system, however, requires that animals be able to recognize one another as individuals, and hence be able to reject those who would accept favors without paying them back. A second kind of altruism is exemplified by the behavior of male sage grouse, which congregate into displaying groups known as leks. Females come to these assemblies to mate, but only a handful of males in the central spots actually sire the next generation. The dozens of other males advertise their virtues vigorously but succeed only in attracting additional females to the favored few in the center. Natural selection has not gone wrong here, however; males move further inward every year, through this celibate and demanding apprenticeship, until they reach the center of the lek. The altruism of honey bees has an entirely genetic explanation. Through a quirk of hymenopteran genetics, males have only one set of chromosomes. Animals normally have two sets, passing on only one when they mate; hence, they share half their genes with any offspring and the offspring have half their genes in common with one another. Because male Hymenoptera have a single set of chromosomes, however, all the daughters have those genes in common. Added to the genes they happen to share that came from their mother, the queen, most workers are three-fourths related to one another--more related than they would be to their own offspring. Genes that favor a "selfless" sterility that assists in rearing the next generation of sisters, then, should spread faster in the population than those programming the more conventional every-female-for-herself strategy. This system, known as kin selection, is widespread. All it requires is that an animal perform services of little cost to itself but of great benefit to relations. Bees are the ultimate example of altruism because of the extra genetic benefit that their system confers, but kin selection works almost as well in a variety of genetically conventional animals. The male lions that cooperate in taking over another male's pride, for example, are usually brothers, whereas the females in a pride that hunt as a group and share food are a complex collection of sisters, daughters, and aunts. Even human societies may not be immune to the programming of kin selection. Anthropologists consistently report that simple cultures are organized along lines of kinship. Such observations, combined with the recent discovery that human language learning is in part a kind of imprinting--that consonants are innately recognized sign stimuli, for instance--suggest that human behavior may be more of a piece with animal behavior than was hitherto imagined. See Sociobiology. Contributed By: Carol Grant Gould James L. Gould Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.