Animal Behaviour

I INTRODUCTION

Animal Behaviour, 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 the web, build a nest, sing a song, find a home, or capture food—at just the right time with little or no instruction. Such behaviour can be viewed from two quite different perspectives, discussed below: either animal learn everything they do (from “nurture”), or they know what to do instinctively (from “nature”). Neither extreme has proved to be correct.

II NURTURE: THE BEHAVIOURISTS

Until recently the dominant school in behavioural theory has been behaviourism, whose best-known figures are J. B. Watson and B. F. Skinner. Strict behaviourists hold that all behaviour, 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 behaviour is formed. Behaviourists 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. Behaviourists see salivation as a simple reflex behaviour—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 behaviour is shaped to suit the task. Behaviourists 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 behaviourist, then, animals must learn all the behavioural patterns that they need to know.

III NATURE: THE ETHOLOGISTS

In contrast, ethology—a discipline that developed in Europe—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, paralyse 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 behaviour is designed so that she can function in a single specialized way. Ethologists believe that this entire behavioural 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 ethnologists have even held that all novel behaviours 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 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).

IV SIGN STIMULI (RELEASERS)

Sign stimuli are crude, sketchy 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 downwards 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 behaviour. 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 odour of the bee attracts the wasp upwind; the sight of any small, dark object guides it to the attack; and, finally, the odour 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 and is known as display. 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 (see Courtship below).

V MOTOR PROGRAMS

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

The first motor program analysed 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 behaviour, but it is in fact 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 non-existent 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.

VI 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 behavioural 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 (separate forms), territorial defence, and courtship behaviour, 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 behaviour.

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 behaviour) 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 behaviour 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 behavioural boredom; repeated presentation of the same stimulus causes the normal response to wane. A chemical present on the tentacles of its arch-enemy, the starfish, triggers a sea slug’s frantic escape behaviour. 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 pre-wired to “learn” to ignore the normal background levels of stimuli and to focus instead on changes from the accustomed level.

VII PROGRAMMED LEARNING

The fourth contribution ethology has made to the study of animal behaviour 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.

A 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 pre-programmed 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 pre-wired 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.

B 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 odour 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 odour of the dangerous food, whereas quail recall only its colour. 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” behaviour.

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 behaviour has reduced the apparent role that simple copying and trial-and-error learning play in modifying behaviour.

VIII COMPLEX BEHAVIOUR PATTERNS

Evolution, working on the four general mechanisms described by ethology, has generated a nearly endless list of behavioural 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 kilometres 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 manoeuvre (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 behaviour. 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.

IX ANIMAL COURTSHIP

Animal courtship behaviour precedes and accompanies the sexual act, to which it is directly related. It often involves stereotyped displays, which can be elaborate, prolonged, and spectacular, and includes the exhibition of sign stimuli (releasers), dramatic body colours, plumage, or markings. It may also involve ritualized combat between rival males.

Its primary purpose is to bring both partners to a state of sexual receptiveness simultaneously. This is especially important in aquatic animals whose eggs are fertilized externally and may be dispersed by water currents before sperm can make contact with them. Copulation is often over quickly and an elaborate courtship ensures its success.

A male three-spined stickleback starts his courtship by building a nest inside a territory he defends. When a female approaches he performs a zig-zag dance towards and away from the nest until she turns towards him and raises her head. He then leads her to the nest and waits, head down, beside the entrance. If she enters the nest he nudges her tail. She lays eggs, and leaves by swimming through the nest. He follows, fertilizing the eggs. In this ritual, each response stimulates the next activity and an incorrect response causes the last step to be repeated.

Courtship rituals differ from one species to another and individuals are attracted only by the attentions of members of their own species. This greatly reduces the risk of unproductive hybrid matings, especially between species that look alike. Male Photinus fireflies attract mates by patterns of flashing light, but each species has its own distinct pattern.

Many female animals secrete odours, pheromones, when they are sexually receptive. The female silkworm moth uses her wings to disperse bombykol, a pheromone secreted by abdominal scent glands, which is detectable by males up to 10 km (6.2 mi) away. Female mammals, including the rhesus macaque and other primates, also use pheromones as sexual attractants.

In most species, however, males court females. Females invest much more time and energy in producing eggs and raising young than males invest in producing sperm, and males usually contribute little or nothing to the care of offspring. It is in the interests of females to choose mates that will give their young the best start in life and in the interests of males to mate with as many females as possible. A male, therefore, must persuade females of his suitability; he must court them.

The song with which many male birds proclaim their presence also serves to attract females, but once they arrive the male must impress them, often by displaying extravagant or brightly coloured plumage. The plumage of the mallard drake is brighter in the breeding season, and the peacock impresses the peahen by displaying his huge tail. Males of some species, such as the sage grouse, gather in large numbers at a particular place, called a lek, where they all display while passing females make their choices. In other species, males present gifts to the female, and bowerbirds try to attract females into elaborate display grounds that they have constructed and decorated with brightly coloured objects.

Such extravagant plumage and behaviour evolved (see Evolution) by “runaway” sexual selection as opposed to natural selection. Males adorned with ornamentation that restricted their mobility, or who spent time obtaining gifts or constructing decorated bowers, showed their strength and competence. Females chose the most spectacular, so with each generation the displays became more extreme until they reached limits beyond which the males would be too encumbered to survive. Darwin’s view of male ornamentation was that it appealed, quite arbitrarily, to female “whim”, but other theorists believe that extravagant male ornamentation advertises genuine male qualities to otherwise sceptical females.

X SOCIAL ORGANIZATION

Animal social organization, the sum of all the relationships among members of a group of animals all of the same species, varies considerably. It ranges from the cooperation between a male and female during courtship and mating, to the most complex societies, in which only one female at a time produces young, all other females collaborating in the care of offspring and maintenance of the colony. Some animal societies are hierarchical, with dominant and subordinate members; others are loose arrangements of fairly independent family groups.

These relationships have evolved in response to the circumstances under which the species live. Many birds establish a territory during the breeding season. This ensures a supply of food for the young, but by excluding all other individuals the pair must mate only with one another. Because of this most birds are monogamous, even if they are not territorial. Some aquatic species mate for life. Only if one mate dies will an albatross, kittiwake, or swan seek a new partner. Monogamy is much rarer among mammals, although prairie voles mate for life, as do gibbons and some lemurs.

Where the food supply is dispersed, animals tend to be solitary. Bears, most cats, and European hedgehogs live solitary lives, the female accompanied by young which leave as soon as they are capable of living alone. Adult males and females meet only to mate.

Social groups based on mating and the rearing of young are often seasonal, members dispersing once the young leave. Other groupings, for protection or hunting, are permanent. In open country, many mammals live in large groups. Herds of antelope, deer, zebras, and horses, and troops of baboons are familiar examples. They benefit from the safety of numbers.

Often, these groups comprise females and their young with one adult male. This is a harem. The male mates with all the adult females and spends much of his time trying to prevent them deserting. Adult males without harems form all-male herds, but individuals constantly try to acquire harems by abducting females. The male of the harem is not necessarily the leader of the group. A herd of horses, for example, is led by the senior female, to whom all others defer. Other groups may have more than one male. A wildebeest herd comprises about 150 females and young and up to three bulls, which patrol outside the herd, keeping it together and guarding it from predators.

Elephants form extended-family herds of females and their young, which often include an old male relative among Asian elephants. Other adult males live outside the herd and male African elephants form their own groups.

Lions are the only cats that live in social groups and collaborate in hunting, a pride consisting of up to 3 males and about 15 females and young. Dogs are much more social. Hunting dogs live in packs of up to 90 individuals. They collaborate in hunting and share food amicably, allowing the young to feed first and disgorging food for latecomers. Wolves mate for life and packs consist of one or more family groups, sometimes with outsiders that have been accepted. A strict social hierarchy is maintained by ritualized postures and gestures.

Species that live in colonies exhibit extreme social relationships and are said to be “eusocial”. They include social insects, such as termites, ants, wasps, and some bees, and one species of mammal, the naked mole rat of eastern Africa. The organization of social insects is based on the roles of certain groups within the colony. There is usually one reproductive female, the queen, who may lay thousands of eggs in her lifetime. Most of the other insects in the colony are involved in the construction, maintenance, and defence of the colony. Certain groups of insects may have specific physical features which relate to their roles. The queen bee is usually much longer and has an enlarged abdomen for egg-laying, the worker bees are equipped with stings to defend the colony and pollen baskets for collecting pollen, while the drones are stingless and do not have pollen baskets as their only role is to mate with the queen before they die. Much of the insect colony behaviour is determined either by instinct or by pheromones released by the queen.

A 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 labour 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 favour 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 favours 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 favours without paying them back.

A second kind of altruism is exemplified by the behaviour of male sage grouse, which congregate into displaying groups—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 favoured few in the centre. Natural selection has not gone wrong here, however; males move further inward every year, through this celibate and demanding apprenticeship, until they reach the centre 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-quarters related to one another—more related than they would be to their own offspring. Genes that favour 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. In their study of sociobiology, 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 behaviour may be connected more with animal behaviour than was hitherto imagined.

Reviewed By:
Michael Allaby

Credited Image: Yesstyle

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