Chapter 7 — Animal Behaviour

Behaviour is everything an animal does in response to a change inside its body or in its surroundings. When you touch a snail and it withdraws into its shell, when a cat arches its back at a strange dog, when a mother hen gathers her chicks under her wings at dusk, when a cow walks to the shed at milking time, all of these are behaviour. Behaviour includes movements, sounds, postures, colour changes, feeding, fighting, mating and nest building. Even standing still and freezing when a hawk passes overhead is behaviour.

Behaviour always has two parts. There is a stimulus, the change that the animal detects with its sense organs, and a response, the action the animal makes. Light, sound, smell, touch, temperature, hunger, thirst and the presence of another animal are all stimuli. The nervous system receives the stimulus, processes it and sends orders to muscles and glands, which produce the response.

The scientific study of animal behaviour is called ethology (from the Greek ethos, meaning character or habit). Ethologists prefer to watch animals in their natural surroundings rather than only in laboratory cages, because a caged animal often behaves very differently from a free one. The founders of modern ethology, Konrad Lorenz, Niko Tinbergen and Karl von Frisch, shared the Nobel Prize in 1973 for their discoveries about how animals organise their individual and social behaviour. Lorenz studied imprinting in geese, Tinbergen studied the instinctive pecking of herring gull chicks and the behaviour of the stickleback fish, and von Frisch decoded the dance language of honeybees.

Why should a student of biology study behaviour? First, behaviour is the animal's chief tool for survival: finding food, escaping enemies, finding a mate and raising young are all achieved through behaviour. Second, behaviour is inherited and evolves like any other character, so studying it tells us about evolution. Third, understanding behaviour helps us in practical ways: in rearing cattle, poultry and fish, in controlling pests, in conserving wildlife, and in training animals such as dogs and elephants. Finally, human beings are animals too, and much of what we learn from animals helps psychologists understand human learning and emotion.

Ethologists ask four questions about any behaviour, which Tinbergen formulated: What causes it right now (mechanism)? How does it develop during the animal's life? What is its survival value? How did it evolve? Throughout this chapter, keep these questions in mind whenever you meet a new example.

The first tool of the ethologist is patient observation. Jane Goodall sat quietly for months in the forests of Tanzania before chimpanzees allowed her near enough to see them using grass stems to fish termites out of mounds, the first record of tool use by a non-human animal. Salim Ali, the birdman of India, spent decades watching birds in the field and writing down what he saw. Good observation means recording exactly what the animal does, at what time, in what weather and in whose company, without adding what we imagine it feels.

Ethologists keep an ethogram, a catalogue of all the distinct actions an animal performs, such as feeding, grooming, threat display, submissive posture, calling and nest building. They note how often each action occurs and what happens just before and just after. From such records patterns emerge: for example, that a male fantail flycatcher sings most in the early morning during the breeding season.

The second tool is experiment. The ethologist changes one thing and sees whether behaviour changes. Tinbergen noticed that a herring gull chick pecks at the red spot on its parent's yellow beak to make the parent regurgitate food. He offered chicks cardboard models: a yellow beak with a red spot, a beak with no spot, a beak with a black spot, and even a plain red stick with white bands. The chicks pecked most at models with a red spot, and even more at the striped stick, showing that the chick responds to a simple sign stimulus (a red contrasting spot on a thin object) rather than to the whole parent. Such simple triggers of innate behaviour are called sign stimuli or releasers.

Tinbergen also studied the three-spined stickleback fish. In the breeding season the male develops a red belly and attacks any other red-bellied male entering its territory. Crude wooden models with a red underside were attacked fiercely, while a perfect model of a stickleback without red was ignored. Again a single feature, the red belly, was the releaser.

A third tool is comparison between species and between wild and captive animals, and a fourth is the study of development: does a chick reared alone in silence still sing its species' song? Such deprivation experiments separate what is inborn from what is learned.

Modern ethologists also use radio collars, camera traps, sound recorders and satellite tags. Tags on olive ridley turtles nesting on the Odisha coast have shown how far they travel across the Bay of Bengal. But every technique still rests on the old habit of watching carefully and asking why.

Behaviour that an animal shows correctly the very first time, without any practice or teaching, is called innate behaviour or inborn behaviour. It is written in the animal's genes and appears in every normal member of the species in the same form. A spider spins its web perfectly without ever seeing another spider do it. A cuckoo chick, hatched in a crow's nest and never meeting its parents, still pushes the crow's eggs out of the nest, grows up and later sings the cuckoo's call. A newborn human baby turns its head towards a touch on the cheek and sucks: the rooting and sucking reflexes.

Innate behaviour has four properties. It is inherited, passed from parents through genes. It is stereotyped, performed the same way each time by every individual. It is species specific, so every weaver bird builds the weaver-bird type of nest and no other. And it is present without learning, appearing complete at the first opportunity even in an animal reared in isolation.

Ethologists recognise three main categories of innate behaviour. A reflex is a simple, quick, automatic response of a part of the body to a stimulus, such as blinking when something approaches the eye. A taxis (plural taxes) is a movement of the whole animal towards or away from a stimulus: moths flying towards a lamp show positive phototaxis, cockroaches running from light show negative phototaxis, and earthworms moving away from dry air show negative hydrotaxis. An instinct is a complex chain of actions, such as nest building, courtship or migration, that is inborn but takes time to complete and often depends on the animal's internal state, for example the breeding season.

Innate behaviour has great value for animals with short lives or no parental care. An insect that lives for a few weeks has no time to learn how to find a mate or lay eggs on the correct plant; it must know at birth. The disadvantage is rigidity. A sphex wasp drags a paralysed cricket to its burrow, leaves it at the entrance, goes in to inspect the burrow, comes out and pulls the cricket in. If an experimenter moves the cricket a few centimetres while the wasp is inside, the wasp brings it back to the entrance and inspects the burrow all over again, and will repeat this dozens of times. It cannot skip a step because the whole chain is fixed.

Innate behaviour is not completely unchangeable. It can be modified a little by experience, and it can be triggered only when the animal is in the right condition. A hungry animal responds strongly to food stimuli; a well-fed one ignores them. This internal readiness is called motivation or drive.

A reflex action is an immediate, involuntary and automatic response to a stimulus, usually involving only one part of the body, and usually protective. If your finger touches a hot pan you pull your hand away before you have even felt the pain. If a speck of dust flies at your eye, you blink. If food enters the windpipe you cough. If a doctor taps the tendon just below your kneecap, your lower leg kicks forward: the knee-jerk reflex. Sneezing, yawning, salivating at the smell of food, the narrowing of the pupil in bright light, and the withdrawal of a sea anemone's tentacles when touched are all reflexes.

Reflexes are fast because the message does not travel all the way to the brain for a decision. The pathway, called a reflex arc, runs from the receptor to the spinal cord and straight back to the muscle. In the hot-pan example: heat receptors in the skin of the finger are stimulated; a sensory neuron carries the impulse to the spinal cord; there it passes through a short relay (inter) neuron; a motor neuron carries the impulse out to the muscles of the arm; the biceps contracts and the hand jerks away. Only afterwards does a separate message reach the brain and you feel the pain and say "ouch". Because of this the brain is left free for more complicated work, and vital protective responses do not depend on thought.

Reflexes can be simple or complex. The knee jerk is the simplest, using only two neurons. The withdrawal reflex uses three. Coughing and swallowing involve a whole sequence of muscles coordinated by the medulla of the brain stem, but they are still reflexes because they are involuntary and stereotyped.

Reflexes are innate, but in mammals they can become linked with new stimuli by learning, as Pavlov showed with salivation in dogs. Such a learned reflex is a conditioned reflex, in contrast to the inborn unconditioned reflex. Reflexes are also affected by the animal's state: a frightened animal jumps at a small sound, and a sleeping one hardly responds.

Reflexes matter to doctors as well as biologists. Testing the knee jerk, the pupil reflex and the plantar reflex (curling of the toes when the sole is stroked) tells a doctor whether the spinal cord and nerves are working. In a newborn baby, the presence of the sucking, grasping and startle reflexes shows that the nervous system is developing normally, and their disappearance at the right age shows that the brain is taking over control.

An instinct is a complex, inborn pattern of behaviour made of many actions performed in a fixed sequence, usually to serve a major need of life: building a home, finding a mate, caring for young, or moving with the seasons. Unlike a reflex, which is a single quick act of one part of the body, an instinct may take hours or weeks, involves the whole animal, and is usually released only when the animal is in a suitable internal condition and meets the right sign stimulus.

The Baya weaver bird (Ploceus philippinus), common across Andhra Pradesh, is a fine example. At the start of the monsoon the male strips long blades from grass or palm leaves, flies to a branch overhanging water or a thorny tree, and weaves a hanging nest with a long entrance tube. He ties knots, weaves strips over and under, and shapes the chamber. A young male building for the first time produces the same design, though a little untidily; he does not learn it from a teacher. The female inspects the half-built nest and, if she accepts, the male completes it. Every step is inherited.

Other examples of instinct are the web spinning of spiders, the dam building of beavers, the comb building of honeybees (perfect hexagonal cells of wax), the tunnel digging of termites, and the elaborate courtship dances of peacocks and cranes. In mammals, a female rat builds a nest, licks her newborn, and retrieves any pup that strays, all correctly at her first litter.

Instincts often work as a chain in which each step provides the stimulus for the next. In the courtship of the stickleback, the male's zigzag dance releases the female's approach; her approach releases his leading her to the nest; her entry releases his nudging her tail; nudging releases her egg laying; the eggs release his fertilising them. Break the chain at any point and the sequence stops.

Instinct has both strengths and weaknesses. Its strength is reliability: no time or teacher is needed, and the behaviour is ready when the need arises. Its weakness is inflexibility: the animal cannot adjust when the situation is unusual. A hen that hears her chick peeping under a glass jar runs to rescue it, but if the jar is soundproof she ignores the same chick struggling in full view, because the sign stimulus is the sound, not the sight.

In animals with larger brains, instinct is increasingly mixed with learning. A young songbird inherits a rough template of its song but must hear adults to perfect it. Human beings have very few pure instincts; almost everything we do is shaped by learning on a foundation of a few inborn drives such as hunger, fear and the urge to communicate.

Learned behaviour is behaviour that changes as a result of the animal's experience. It is not present at birth in a complete form; it appears, improves or changes with practice, and it differs from one individual to another according to what each has experienced. A puppy is not born knowing its name, that the sound of a car means its owner is home, or that the kitchen is where food appears, but a few weeks of living in a house teach it all these things.

Learning requires a nervous system that can store information, in other words memory. The more developed the brain, especially the cerebrum, the greater the animal's capacity to learn. Earthworms and planarians can learn only very simple things; insects such as honeybees can learn the position of flowers and the colour of the hive entrance; birds and mammals learn a great deal; primates, dolphins, elephants and above all human beings learn most of what they do.

Learned behaviour has properties opposite to those of innate behaviour. It is acquired, not inherited, though the capacity to learn is inherited. It is variable, differing between individuals and changing over time. It is adaptable: the animal can adjust its behaviour to new situations, which is the great advantage over rigid instinct. And it takes time and practice, which is its cost: a young animal that must learn everything is helpless for a long period and needs parental protection, as human infants do.

Ethologists and psychologists recognise several types of learning, which we shall study one by one. Habituation is learning to ignore a repeated stimulus that brings neither reward nor harm. Imprinting is the rapid learning by a very young animal of what its parent (or its species) looks like, during a short sensitive period. Conditioning is the learning of an association between a stimulus and a response (classical conditioning) or between an action and its consequence (operant conditioning). Trial-and-error learning is learning by trying different actions and repeating the ones that succeed. Insight learning is solving a new problem by understanding the situation, without trial and error. Imitation is learning by copying another animal.

In real life innate and learned behaviour are woven together. A kitten is born with the instinct to pounce, but it learns by playing with its mother's tail and with its litter mates exactly how to pounce on a mouse. A honeyguide bird instinctively leads a human or a honey badger to a bee's nest, but it learns which people follow it. The dividing line is a matter of degree. What every student should remember is that learning gives an animal a second inheritance, the inheritance of experience, on top of the inheritance of genes.

Habituation is the simplest form of learning: an animal gradually stops responding to a stimulus that is repeated many times and turns out to be harmless and unrewarding. When you tap the water near a snail it withdraws into its shell; tap again and again, and after several taps it stops withdrawing. It has learned that the tap means nothing. Birds in a field are startled by a scarecrow on the first day, but within a week they perch on its hat, because the scarecrow never moves or harms them. People who move to a house beside a railway line stop noticing the trains. Habituation saves the animal from wasting energy and attention on meaningless stimuli, leaving it alert to the ones that matter. It is different from fatigue, because a new stimulus, or the same stimulus after a rest, brings the response back.

Imprinting is a special, rapid and lasting form of learning that occurs only during a short sensitive period early in life. The Austrian ethologist Konrad Lorenz studied it in greylag geese. Goslings normally follow their mother within hours of hatching. Lorenz divided a clutch of eggs; half hatched under the mother goose and followed her, and half hatched in an incubator and saw Lorenz first. These goslings followed Lorenz everywhere, walking behind him, swimming after him, and even ignoring their real mother when placed with her. When the two groups were mixed and then scattered, each group ran to its own "parent". The goslings had imprinted on the first large moving object they saw during the first day or two of life.

Imprinting has three special features. It happens only during the sensitive period (in ducks and geese roughly 13 to 16 hours after hatching, and never after about two days). It is very rapid, needing only a brief exposure. And it is practically irreversible: a bird imprinted on a human continues to prefer humans, and when adult may even try to court people rather than its own kind. Lorenz's goslings imprinted on his boots, and a famous photograph shows him striding across a meadow with a line of goslings behind him.

Imprinting has two survival values. Filial imprinting keeps a chick close to its mother, who protects it and leads it to food; a chick that followed any goose would be lost in a crowded colony. Sexual imprinting, in which the young bird learns what its own species looks like, ensures that it chooses a mate of the correct species when it grows up.

Imprinting is important in conservation. Whooping cranes and other endangered birds reared by humans have imprinted on their keepers and refused to breed with cranes; now keepers wear crane-shaped puppets and hide their faces while feeding chicks. Salmon imprint on the smell of the stream in which they hatched and return to it years later to spawn, an example of olfactory imprinting.

Conditioning is learning by association. In classical conditioning an animal learns to give an inborn response to a new stimulus that was previously meaningless, because that stimulus has repeatedly occurred together with the natural stimulus. The Russian physiologist Ivan Pavlov discovered it around 1900 while studying digestion in dogs.

Pavlov fitted a small tube to a dog's cheek so that the saliva flowing from its salivary gland could be measured. When food (meat powder) was placed in the dog's mouth, saliva flowed: this is the natural, inborn reflex. Food is the unconditioned stimulus and salivation is the unconditioned response. Pavlov then rang a bell just before giving the food, and repeated this pairing many times. At first the bell alone produced no saliva. But after a number of pairings the dog salivated on hearing the bell, even when no food followed. The bell had become a conditioned stimulus, and salivation to the bell is the conditioned response or conditioned reflex. The dog had learned that the bell predicts food.

Pavlov went on to discover the rules of conditioning. Extinction: if the bell is rung many times without food, the conditioned response slowly fades. Spontaneous recovery: after a rest, the response to the bell partly returns. Generalisation: the dog salivates to a bell of a slightly different tone as well. Discrimination: if only one tone is followed by food and another is not, the dog learns to salivate only to the first. The timing also matters: the conditioned stimulus must come just before the unconditioned stimulus, not after it.

Classical conditioning is everywhere in daily life. Fish in a pond come to the surface when they hear the footsteps of the person who feeds them. Cattle walk to the shed when they hear the milking pail. A child who has had a painful injection cries on seeing a white coat. The sight of a lemon being cut makes your mouth water. Advertisers pair a product with pleasant music so that the product itself later gives a pleasant feeling.

Conditioning is the basis of much animal training. A dog is trained to sit by pairing the word "sit" with a gentle push and a reward; an elephant learns the mahout's commands; circus animals learn to respond to whistles. It also explains fears: a person bitten by a dog may fear all dogs for years, a conditioned fear that can be reduced by extinction, that is, by gradually meeting friendly dogs without harm.

Pavlov received the Nobel Prize in 1904 for his work on digestion, but it is his discovery of the conditioned reflex that made him famous, because it showed that learning could be studied by measurement, like any other process in physiology.

In trial-and-error learning an animal facing a problem tries many different actions more or less at random; actions that fail are dropped, actions that succeed and bring a reward are repeated, and with practice the successful action is performed faster and more smoothly. The American psychologist Edward Thorndike studied this with cats placed in a "puzzle box" that could be opened by pulling a loop of string. A hungry cat, seeing food outside, scratched, pushed and bit at random until by chance it pulled the loop and escaped. On each repeated trial the cat took less time, until finally it pulled the string at once. Thorndike called this the law of effect: an action followed by a satisfying result is more likely to be repeated, and an action followed by discomfort is less likely.

B. F. Skinner refined this into operant conditioning using the Skinner box, in which a rat or pigeon receives a food pellet whenever it presses a lever or pecks a disc. The animal first presses the lever by accident, gets food, and soon presses it deliberately. The food is a reinforcement; a reward that strengthens behaviour is positive reinforcement, and the removal of something unpleasant (switching off a mild shock) is negative reinforcement. Punishment, an unpleasant result, weakens behaviour. The difference from Pavlov's classical conditioning is that in operant conditioning the animal's own action (the operant) produces the result, whereas in classical conditioning the response is a reflex triggered by a stimulus and the animal's action does not change what happens.

Trial-and-error learning is seen throughout the animal kingdom. A young toad snaps at a bee, is stung, and thereafter avoids anything with black and yellow stripes. A crow trying to open a walnut drops it from different heights until it finds the height that cracks the shell. A kitten learns which way to bat a ball to make it roll. A rat learns the path through a maze to reach food, making fewer wrong turns each time. A child learns to ride a bicycle by many falls, gradually keeping the actions that keep the cycle upright.

The speed of trial-and-error learning depends on the animal's brain, its motivation (a hungry rat learns a maze faster than a fed one), and how quickly the reward follows the action. A reward that comes immediately after the action is far more effective than one that comes minutes later, which is why dog trainers give a treat the instant the dog obeys.

Operant conditioning is the foundation of animal training in circuses, in guide dogs for the blind, in sniffer dogs at airports and in dolphin shows. In human life it explains why children repeat behaviour that earns praise and why habits are so hard to break: the habit was reinforced hundreds of times. Teachers use it, knowingly or not, every time they praise a correct answer.

Insight learning is the highest form of learning. The animal solves a new problem suddenly, by grasping the relationship between the parts of the situation, without going through random trial and error. It seems to think the problem out, and once the solution is found it is remembered and used again at once.

The classic experiments were done by the German psychologist Wolfgang Kohler with chimpanzees between 1913 and 1917. A chimpanzee named Sultan was kept in a cage with a bunch of bananas hung from the roof, too high to reach, and a few wooden boxes lying about. After some jumping and failed attempts Sultan sat and looked, then suddenly dragged a box under the bananas, climbed on it and took them. Later, with the fruit higher still, he stacked two and then three boxes. In another test bananas were placed outside the cage beyond arm's reach, with two hollow bamboo sticks inside, each too short. After a long pause Sultan pushed the thinner stick into the end of the thicker one, making a long stick, and raked in the fruit. The solution came all at once, after a period of apparent thought, not after hundreds of random tries.

Insight learning needs a large, well-developed cerebrum, and it is found mainly in primates, elephants, dolphins and some birds such as crows and parrots. A New Caledonian crow bends a piece of wire into a hook to lift a bucket of food from a tube. Sea otters use stones as anvils to crack shellfish. Human beings show insight constantly, whenever we say "Aha!" and see how to solve a puzzle. Insight uses past experience: Sultan could join the sticks because he had earlier played with sticks and boxes; but he combined those experiences in a new way for a new problem.

Imitation (observational learning) is learning by watching and copying another animal, usually a parent or an older member of the group. Young chimpanzees learn termite fishing and nut cracking by watching adults for years. In 1953 on Koshima Island in Japan a young macaque named Imo began washing sand off sweet potatoes in a stream; within a few years most of the troop had copied her, and the habit is still passed on today. Blue tits in England learned from each other to peck open the foil caps of milk bottles left on doorsteps. Song birds learn their song by imitating adult males. Human children learn language, gestures and skills largely by imitation, and the spread of a learned habit through a group by imitation is the beginning of culture.

These higher forms of learning give animals a great advantage: a new solution found by one individual can spread to the whole group without waiting for evolution to build it into the genes.

Many animals live not alone but in groups: a herd of deer, a flock of parakeets, a school of sardines, a troop of langurs, a pack of wild dogs, a colony of honeybees, a termite mound holding a million individuals. Social behaviour is the behaviour of animals towards other members of their own species, and includes cooperation, communication, division of labour, dominance and submission, mutual defence and care of the young.

Group living has clear advantages. Protection: many eyes spot a predator sooner; a herd of chital feeding with langurs overhead benefits from the langurs' alarm calls. A flock of starlings wheeling together confuses a hawk. Musk oxen form a ring with horns outward around their calves. Finding food: pelicans drive fish into shallow water together; wild dogs and lions hunt cooperatively and can bring down prey far larger than themselves. Division of labour: in bee, ant and termite colonies different individuals do different jobs. Care of young: in elephant herds all the females guard and help every calf. Warmth: emperor penguins huddle through the Antarctic winter, taking turns on the cold outer edge. Group living also has costs: more competition for food and mates, and faster spread of disease and parasites.

Social groups are usually organised by a dominance hierarchy or pecking order. In a flock of hens, hen A can peck every other hen, hen B can peck all but A, and so on down to the lowest hen, which is pecked by all. Once the order is settled by a few early fights, each hen knows her place, fighting stops, and the group feeds peacefully. Wolves, baboons and langurs have similar ranks, with a dominant (alpha) male or female who eats first and mates most. Rank is shown by posture: a dominant wolf stands tall with raised tail, a submissive one crouches, lowers its tail and rolls over.

The most highly organised societies are those of social insects. A honeybee colony has one queen, the only fertile female, who lays up to 2,000 eggs a day; a few hundred drones, males whose only work is to mate with a new queen; and 20,000 to 60,000 workers, sterile females who do every other task. A worker's duties change with age: cleaning cells, feeding larvae with royal jelly, building comb, guarding the entrance, and finally foraging for nectar and pollen. A termite colony adds a king and soldiers with huge jaws. In these colonies individuals are so dependent on one another that the colony behaves like a single organism.

Social behaviour is also seen in the altruism of some animals, where one individual takes a risk for others: a ground squirrel gives an alarm call that may attract the predator to itself; a worker bee dies when it stings a raider. Such behaviour spreads because the individuals helped are close relatives who share the same genes.

Communication is the passing of information from one animal to another by signals that change the receiver's behaviour. Animals communicate by sound (bird song, the alarm bark of a langur, the roar of a tiger, the croak of frogs in the rains, the chirp of crickets), by sight (the peacock's tail, the firefly's flashes, the raised hackles of a dog, the colour of a chameleon), by smell (a dog marking a lamp post with urine, a female moth releasing a scent that a male can detect kilometres away, the trail an ant lays to food), by touch (grooming in monkeys, the nuzzling of a cow and calf) and even by electricity in some fishes. Chemical signals that pass between members of a species are called pheromones; the queen bee's pheromone keeps the workers sterile and united, and the alarm pheromone released by a stinging bee brings others to attack.

The most remarkable communication known in insects is the dance language of the honeybee, decoded by the Austrian scientist Karl von Frisch in the 1940s. When a forager bee finds a rich source of nectar she returns to the hive and dances on the vertical face of the comb, surrounded by other workers who touch her with their antennae and taste the nectar she carries.

If the food is near the hive, within about 50 to 100 metres, she performs the round dance: she runs in a small circle, turns and runs in a circle the other way, again and again. This says only "there is food close by" and the flower's scent on her body tells which flower to seek.

If the food is farther away, she performs the waggle dance, shaped like a figure of eight. She runs a short straight line while waggling her abdomen from side to side, circles back to the start, runs the straight line again, and circles back on the other side. This dance carries three pieces of information. Distance: the farther the food, the slower the dance and the longer the waggle run; a rate of about ten straight runs in 15 seconds means roughly 100 metres, and two runs means about 6 kilometres. Direction: the angle between the straight waggle run and the vertical on the comb equals the angle between the sun and the food source as seen from the hive. A waggle run straight up means "fly towards the sun"; straight down means "fly away from the sun"; 60 degrees to the right of vertical means "fly 60 degrees to the right of the sun". Richness: the more vigorous and longer the dancing, the better the food. Bees also correct the direction as the sun moves across the sky.

Von Frisch proved all this by placing dishes of sugar water at known distances and directions, marking the foragers with paint, and watching the dances through a glass-sided hive. He also showed that bees see colours (but not red) and ultraviolet, and can use polarised light to find the sun's position on cloudy days. For decoding this "language" he shared the 1973 Nobel Prize.

Courtship is the special behaviour that animals show before mating to attract and win a mate of the correct species, to signal readiness, and to reduce aggression between the partners. It is mostly innate, and each species has its own display, so that it also prevents mating between different species.

The peacock is India's classic example. In the breeding season, with the first rains, the male spreads his enormous train of eye-spotted feathers into a fan, quivers it with a rattling sound and struts before the peahen, turning to show every angle. The peahen chooses the male with the largest and most perfect train; studies show that peahens prefer males with more eye-spots, which indicate good health. The sarus crane, seen in the fields of Andhra Pradesh and north India, performs a dance in which the pair bow, leap into the air with wings spread, and call together with necks raised; the pair mate for life and dance every season. Male frogs and toads croak in chorus after the rains, each species with its own call, and females move to the loudest callers. Fireflies flash in a species-specific pattern. Male fiddler crabs wave one huge claw. Sticklebacks perform the zigzag dance, and bower birds of Australia build decorated arbours of twigs and blue objects that serve only to impress females. In many species males fight or display against each other, as stags lock antlers and blackbucks spar with their spiral horns, and the winner mates.

Courtship serves several purposes: it brings male and female of the same species together at the right time, it synchronises their readiness so that eggs and sperm are released together, it allows the female to choose the healthiest partner, and it calms the natural fear or aggression between two animals coming close.

Territorial behaviour is the defence of an area, the territory, against other members of the same species. The territory may hold the animal's food supply, its nest site, or its mates. A male robin sings from a perch to announce that a patch of garden is his; if another male enters, the owner sings louder, displays his red breast, and finally attacks. A tiger marks the boundary of a territory of 20 to 100 square kilometres by spraying urine on trees and scratching bark, and other tigers read these marks and keep away. Male lizards do push-ups and show their coloured throat fans; dogs mark lamp posts; a pair of Indian robins drives other robins from the yard where they nest. Territories are usually defended by signals such as song, scent and display rather than by actual fighting, which saves energy and injury. Territorial behaviour spaces animals out so that each has enough food, and it reduces the crowding that spreads disease.

Both courtship and territory are strongly controlled by hormones. In birds the growth of the testes in spring, triggered by longer days, raises testosterone, and this starts singing, display and territorial fighting; when the season ends the hormones fall and the behaviour stops.

Migration is the regular, seasonal, long-distance movement of animals from one region to another and back, usually to escape harsh weather or scarce food and to reach good breeding grounds. It is largely innate: many young birds migrate correctly on their first journey without adults. It is triggered by changes in day length that alter hormone levels and drive the animal to eat heavily, store fat and grow restless.

Birds are the greatest migrants. Every winter the wetlands of Andhra Pradesh, such as Kolleru and Pulicat lakes, fill with ducks, teals, pintails, flamingos, pelicans and waders that have flown from Siberia, Central Asia and the Himalaya to escape the northern winter, returning north in March to breed. The Siberian crane once flew to Bharatpur in Rajasthan. The Arctic tern holds the record, flying from the Arctic to the Antarctic and back every year, about 35,000 kilometres. The bar-headed goose crosses the Himalaya at over 7,000 metres on its way to Indian lakes. Amur falcons pass through Nagaland each autumn on their way from Siberia to Africa.

Mammals also migrate: wildebeest cross the Serengeti in vast herds following the rains; humpback whales swim from polar feeding waters to tropical calving grounds; fruit bats move with the fruiting of trees. Among fishes, hilsa swim up the rivers of Bengal and Andhra to spawn, salmon return from the sea to the stream of their birth, and the European eel crosses the Atlantic to breed. The olive ridley turtle comes ashore in tens of thousands on the Odisha coast in a mass nesting called arribada. Insects migrate too: monarch butterflies travel 4,000 kilometres from Canada to Mexico, and swarms of desert locusts cross the Arabian Sea into Rajasthan and Gujarat.

How do migrants find their way? Experiments with caged birds and with birds carried far from home show that they use several compasses: the position of the sun by day, corrected for time with an internal clock; the pattern of stars around the pole star at night; the Earth's magnetic field, sensed by tiny magnetic particles or by special molecules in the eye; and, near home, familiar landmarks such as coastlines and rivers, smells, and low-frequency sounds. Homing pigeons released hundreds of kilometres away return to their loft using the same abilities.

Other seasonal behaviours are hibernation, the deep winter sleep of bears, hedgehogs and many frogs and snakes, in which body temperature, heartbeat and breathing fall very low and the animal lives on stored fat; and aestivation, the summer dormancy of the Indian lungfish, land snails and some frogs, which bury themselves in mud or seal their shells to survive heat and drought until the rains return. Both are innate responses to unfavourable seasons, triggered by temperature and day length, and both conserve energy when food is unavailable.

Parental care is any behaviour of a parent that increases the survival of its offspring after the eggs are laid or the young are born. It ranges from none at all to years of devoted attention, and the amount of care is matched to the number of young: animals that give no care produce enormous numbers of eggs, while those that give much care produce few.

Most fishes, frogs and insects lay hundreds or thousands of eggs and leave them. A female cod lays millions of eggs of which perhaps two survive. Yet some show remarkable care. The male stickleback builds a nest, fans the eggs with his fins to keep them oxygenated and guards the fry. The male seahorse carries the eggs in a pouch. The mouth-brooding tilapia keeps its eggs and young in its mouth. The female scorpion carries her young on her back. Some frogs carry tadpoles on their backs or in their vocal sacs, and the male midwife toad wraps the egg strings round his legs. Crocodiles guard the nest, help the hatchlings out of the eggs and carry them to water in their jaws.

Birds show elaborate breeding behaviour. The sequence in a typical Indian garden bird such as the red-vented bulbul is: establishing a territory, courtship, choosing a nest site, building the nest, laying, incubating the eggs (keeping them warm with the brood patch of bare skin), feeding the nestlings, removing their droppings, defending them from crows and cats, and teaching the fledglings to fly and feed. Both parents usually share the work. Nests vary from the weaver's hanging basket and the tailor bird's leaf sewn with cobweb, to the hornbill's tree hole in which the female is sealed with mud by the male, who feeds her through a slit for weeks. Ground-nesting birds like the lapwing pretend to have a broken wing to lure a predator away from the chicks. Brood parasites such as the koel and cuckoo lay eggs in the nests of crows and babblers and let the hosts do all the work.

Mammals give the most care because the young are born helpless and depend on milk. The mother suckles, cleans, warms and protects them, and in social species the whole group helps: elephant calves are guarded by all the cows of the herd, and lion cubs are nursed by any lactating female. Care continues long after weaning as the young learn hunting or foraging by watching adults. Chimpanzee and elephant young stay with their mothers for years, and human children longest of all.

Parental care is innate in its basics, released by hormones such as prolactin (which causes milk secretion and broodiness) and oxytocin, and by sign stimuli from the young: the gaping red mouth of a nestling releases feeding, the cry of a lamb releases the ewe's search. Yet in mammals experience matters too; a first-time mother often handles her young more clumsily than an experienced one.

Behaviour does not happen by magic. Every act, from a reflex blink to a peacock's display, is produced by the body's nervous system and endocrine system working together, and it is shaped by the environment in which the animal lives.

The nervous system gives the fast responses. Sense organs detect stimuli; sensory nerves carry impulses to the spinal cord and brain; the brain compares them with stored memories and the animal's present needs and sends impulses along motor nerves to muscles. The more complex the brain, especially the cerebral cortex, the more flexible and learned the behaviour. A cockroach with a tiny brain behaves mostly by reflex and taxis; a dog with a large cerebrum can learn dozens of commands; a chimpanzee can solve problems by insight.

The hormones give the slower, longer-lasting readiness or drive. Testosterone in male birds and mammals in the breeding season brings on song, display, territorial fighting and mating; when the season ends and the hormone falls, the same birds live peacefully in mixed flocks. Oestrogen and progesterone prepare the female for mating and for nest building, and prolactin makes her broody. Adrenaline, released in danger, prepares the body to fight or flee. Thyroid hormone controls the level of activity and the moult of feathers. Melatonin, released in darkness, sets the daily and seasonal clocks and so triggers migration and hibernation. Pheromones, secreted outside the body, act as hormones between individuals.

Behaviour follows biological rhythms. Most animals have a daily (circadian) rhythm of about 24 hours: diurnal animals like squirrels and most birds are active by day, nocturnal ones like owls, bats and moths by night, crepuscular ones like rabbits and nightjars at dawn and dusk. The rhythm is set by an internal clock in the brain and adjusted daily by sunlight. There are also lunar rhythms (the arribada of turtles and the spawning of corals follow the moon), tidal rhythms in shore animals, and annual rhythms of breeding, moult and migration.

The environment shapes behaviour in two ways. Over the animal's lifetime it provides the experiences from which learning builds; a rat reared in a rich cage with toys develops a heavier cerebral cortex and learns faster than one reared in a bare cage. Over generations it selects the innate behaviours that survive best: desert animals are nocturnal to avoid heat, arctic animals hibernate, island birds that never met predators lose their fear and are easily wiped out when cats arrive. Behaviour is therefore always the product of heredity (genes) and environment (experience) together, never one alone.

Finally, behaviour is affected by internal state. A hungry animal searches for food and ignores a mate; a satisfied one does the reverse. Illness, fatigue, age and stress all change behaviour, which is why a good farmer or veterinarian watches an animal's behaviour as the first sign of disease.

Human beings share the basic machinery of behaviour with other animals. We have reflexes: the knee jerk, the blink, the sneeze, the sucking and grasping of a newborn. We have a few inborn drives: hunger, thirst, fear of sudden loud sounds and of falling, the urge to sleep, and the need for company. We show habituation when we stop hearing the ceiling fan, classical conditioning when our mouth waters at the sight of tamarind, operant conditioning when a child repeats what earns praise, and imitation when a toddler copies the way a parent speaks. Our daily rhythm of sleep and waking is a circadian rhythm like that of any other animal, and jet lag is what happens when it is disturbed.

But human behaviour differs from that of every other animal in degree so great that it becomes a difference in kind. The human cerebral cortex is far larger in proportion to the body, and so learning, memory, insight and reasoning dominate. We have language, a system of arbitrary symbols that can describe things absent, past or imaginary, which no honeybee dance or chimpanzee gesture can do. We pass knowledge across generations by teaching, writing and now electronic media, so that culture accumulates: a child today inherits the discoveries of thousands of years. We can plan far ahead, foresee consequences and choose to act against our immediate drives, which is the basis of self-control and moral behaviour. Even our few instincts are heavily modified by upbringing: what people fear, eat, and find attractive varies enormously between cultures.

Human behaviour is shaped by heredity and environment together, exactly as in animals. Studies of twins show that temperament has a genetic component, while the family, school, friends and society provide the experiences that build personality. Hormones affect us too: adrenaline in fear, the changes of adolescence, and the daily cycle of melatonin.

Why study animal behaviour at all? It has many practical uses. In agriculture and animal husbandry, knowing the feeding, breeding and social habits of cattle, poultry and fish improves their health and yield: hens lay more when kept in stable groups, and fish are bred by imitating the water conditions that trigger spawning. In pest control, the pheromone traps used in cotton and mango orchards attract male insects and stop them breeding, avoiding poisonous sprays. In wildlife conservation, knowledge of migration routes protects the wetlands and corridors that elephants, turtles and cranes depend on, and knowledge of imprinting lets us rear endangered birds without spoiling them. In medicine and psychology, conditioning is used to treat phobias and addiction, and animal studies of learning underlie modern teaching methods. In animal welfare, understanding that animals feel fear and need company has changed how zoos, farms and laboratories are run. And in everyday life, knowing how a dog reads our posture or why a cow kicks makes us safer and kinder.

The message of this chapter is that behaviour, like structure, is a product of evolution, that innate and learned behaviour together let animals survive in a changing world, and that we ourselves are part of that story.