Category Archive Animal Behaviour

Crypsis (Deception In Predator-Prey Interaction)

In nature predator and prey have evolved together for millions of years. Prey must deceive the predator in order to escape getting killed, while the predator must also use deception to catch the prey unawares. Crypsis is also a form of mimicry but the former has a wider meaning that includes mimicking even non-living objects such as stones, rocks, twigs and even the background.

Protective colouration: Majority of the animals match the background in colour to escape the attention of the predator. For example, hares and rabbits are earth-coloured, grasshoppers are green and beach crabs have the same colour as pebbles.

Counter-shading: This is also called natural shading, in which the protectively coloured animals have darker shade on the dorsal side and lighter on the ventral side of body. This is to neutralise the sunlight falling from above, which lightens the upper side of the body, while the shade below darkens the colour. Some animals press their bodies against the ground and remain motionless when in danger in order to eliminate the shadow. Flatfish, cuttlefish and chameleon can even change the body colour to neutralise the effect of light and shade. Squid (Abraliopsis) possesses light-producing organs on the ventral side, which can be switched on when there is sunlight from above and they become invisible from below.

Disruptive coloration: Another method to enhance the effect of natural shading is obliteration of the outline of body by having spots, patches or stripes, which break the continuity of the outline and thus help the animal to camouflage much more effectively. For example, salamanders, deers, leopards, tigers, fishes, rays all have patches on the body.

Protective resemblance: Some animals resemble their habitat not only in colour but also in structure, so that the camouflage is complete and they become completely inconspicuous in their environment. Caterpillars of the family Geometridae resemble the twig of the plant on which they feed in colour as well as shape. Similarly, sea horse possesses thread-like processes emanating from body, which make it invisible among seaweeds where it lives. Stick insects of the order Phasmida are found on grasses and have thin and elongated body and appendages, which make them look like strands of grasses.

Indian Leaf insect which feeds on broad-leaved plants in eastern Himalayas, has the entire body flattened and wings and legs shaped like leaves. The green, leaf-like insect is impossible to detect on green plants even by trained entomologists.

Aggressive resemblance: Predators also use crypsis so as to approach the prey undetected to mount a sudden attack. Many predators camouflage to ambush an unsuspecting prey that may wander near them. Tigers have stripes that help them to hide in tall grasses. Slow moving predators like praying mantis and ambush bug have the same colour and shape as the plant on which they sit and attack by surprise. Crab spiders are known to resemble flowers on which they wait for the prey, not only in colour but also in appearance.

The American hawk (Buteo albonotata) resembles vultures in general appearance. Since vultures are scavengers, birds are not scared of them and venture close to them, when the hawk in the garb of vultures attacks them.

Cryptic actions: Feigning death or injury is sometimes adopted as a measure to distract the attention of predator. Many beetles curl up like pebbles and remain motionless to escape attention of the predator. South American opossum also feigns death when alarmed. Echidnas, hedgehogs and porcupines also curl up when alarmed and feign death. Their body spines become erect as defence structures. Some birds, which make their nests on the ground, e.g. ostriches, lapwings and prancticoles feign injury on the wing or leg and struggle to run or fly to distract the attention of the predator from its nests.

Dymantism: Some butterflies, namely, pansies, colites and Melanitis, expose bright colours of wings when they suddenly fly, which temporarily frightens the predator and gives them time to escape. Some birds also expose flashes of bright plumage when they take to flight. Emperor moth is nocturnal and rests in shady places in daytime. Its wings possess large eyespots, which give a false impression of a hiding cat, which is enough for birds to avoid them. The butterfly, Nymphalis, has two pairs of eyespots on wings, which are revealed by lowering its wings when it is attacked by a predator.

Aposematism: Majority of protected species sport bright colours on the body to advertise their presence, so that the predators are warned not to attack them. The bright colours help in quick learning by the predators. For example, wasps that are protected by a sting are brightly coloured. Unpalatable butterflies, such as Danaus, are brightly coloured and are carefully avoided by the birds.

H.A. Ford demonstrated aposematism by conducting an experiment in which he used red and blue artificial caterpillars. Birds avoided the red ones.

Cryptic organs to misdirect attack: Swallowtail butterflies have long brightly coloured tail on the hind wing while the rest of the body is dull coloured. When the predators attack, they tend to catch the brightly coloured tail while the vital parts of the body escape damage. The small lantern butterfly (Thecla) has posterior extension of hind wing resembling head, while the actual head is small and inconspicuous. Bird attack is always misdirected on the false head.

Alarm Pheromones

Behaviour modifying chemicals are called semiochemicals and pheromones fall in the same category. Pheromones are chemicals which are used for communication among the members of the same species. They form the chemical language of a species. For instance, attractants are pheromones which bind the members of a colony of social insects.

Kairomones are chemicals used to communicate with members of other species. For example, parasites use the host smell for searching and preys use predators’ smell to escape away.

All social insects release alarm pheromones when their colony is attacked. This triggers defensive behaviour in the other members of the colony. Deers emit a special scent when they are attacked by a tiger. Similarly, skin cells of many species of fishes release alarm pheromones when they are damaged by the teeth of predator. The Canadian skunk (Mephitis) emits an extremely foul smelling secretion from the anal glands which not only repels the predator but also sends alarm to other members of species.  Snails, sea urchins and earthworms also release alarm pheromones when injured.

Ants and termites release a complex of several pheromones from mandibular glands, anal glands and Dufour’s glands to send alarm and confuse the attacker at the same time. J.W.S. Bradshaw, R. Baker & P.W. Howse of University of Southampton, England isolated 33 volatile components from the head of worker ants, 4 of which were specific alarm pheromones. Alarm pheromones of ants spread to a distance of 6 cm in 13 seconds. Pheromones that spread to long distances elicit alerting behaviour in other members of colony, while short range pheromones serve as attacking and biting behaviour releasers.

When honey bees sting, their sting along with its glands is left on the skin of victim and keeps pumping venom into its body. The sting as well as the dying bee releases alarm and distress pheromones from Nassanoff’s glands to attract and stimulate the other bees to attack the intruder.

Pheromone perceiving structures are located on the antennae of insects in various shapes and sizes and are always connected to the nerve.

Drive, Urge Or Motivation in Animals

The term Drive was introduced by Woodworth (1918) as motivational concept. Animals experienced drive as biological needs such as eating and drinking and alteration in their behaviour. Drive theories were later given by Sigmund Freud (1915) and Clark Hull (1943). Freud, who was physiologist by training, believed that drives and urges such as hunger were recurring conditions in the body of animal that produced energy build up in the nervous system.

This energy build up caused psychological discomfort and restlessness that kept on increasing unless the urge was satisfied. Drive arose from a range of bodily disturbances, such as deprivation of food, water, air, sleep or temperature regulation, injury or activities like nest building. Freudian drive theory was based on the following three principles:

  • Drive emerged from bodily needs of the animal.
  • Drive energized the behaviour of animal due to restlessness.
  • Reduction of drive by satisfying needs produced learning.

Konrad Lorenz (1950) proposed the Psycho-hydraulic model or Flush toilet model to explain the drive and consummatory behaviour, which has three steps:

  1. Drive causes action specific energy to accumulate with time and causes increased restlessness in the animal, which results in searching behaviour for food, water, mate etc.
  2. Consummatory behaviour starts after achieving the goal such as food or any other sign stimuli. The innate release mechanism releases the accumulated energy in the animal.
  3. After consummatory behaviour there is a quiescent period in the animal as the accumulated energy has been released and the action stoped. This is called refractory behaviour of the animal.


Hunger drive is controlled by lateral hypothalamus and ventro-median nucleus, the former is stimulatory in function while the latter is inhibitory. Glucocorticoids inhibit the hunger drive. Lateral hypothalamus can be stimulated by epinephrine. The hunger and thirst drives depend on hours of deprivation of feeding on dry food.


Many mammals such as male gerbil and squirrels possess hoarding drive as the lean season approaches. Low estrogens and testosterone levels stimulate hoarding drive in mammals. Castrated individuals show increased hoarding drive, which can be reduced by giving testosterone treatment.


Migratory drive occurs in fishes and birds and may be seasonal or related to reproduction. Pineal glands, which is affected by day-light hours, affects migration in birds. In warblers pituitary gland influences migratory urge as well as excessive eating to deposit fat energy in the body. In stickleback fish thyroxin injection caused them to migrate. In Salmon and Anguilla, maturation of gonads produces migratory drive, so much so that they stop eating and set out to the course of migration crossing all obstacles on the way.


Aggression is controlled by amygdala of the limbic system of brain and posterior hypothalamus is also involved to some extent. In most of the male mammals testosterone causes aggression while in females high oestrogen levels reduce aggression and make the female peaceful. Hydrocortisone also increases aggression while hydroxydione decreases it. In ringed dove implants of testosterone propionate at specific sites of hypothalamus causes aggressiveness.


Many vertebrates mark and defend their territory. Dogs, hyenas and some prosimians mark their territory by their own urine. Monotremes and marsupials have anal glands which they rub on the ground to mark territory. In tigers and cheetahs also there are anal glands which spray the secretion on the trees to mark their territory. Gazelles possess orbital glands below the eyes which secrete a tar-like substance that they apply on grasses and bushes. Territorial behaviour is also hormone dependent. Yahr & Thiessen (1972) isolated 11 different hormones that influence territorial behaviour in vertebrates.


Sexual drive involves courtship behaviour such as singing and dancing in birds, croaking in frogs and fighting in males of many vertebrates. In insects courtship behaviour is stopped if corpora allata are removed.

Hormonal levels increase in breeding season. Castrated males and females do not show sexual behaviour in vertebrates while testosterone injections elicit sexual behaviour. According to Johnson (1976) oestrogen enhances female attractiveness and receptivity and causes oestrous in females.

Hypothalamic Releasing Factor (LH-RF) and ACTH are known to affect copulatory behaviour in many animals.


Gonadotropin secretion by pituitary gland cause not only courtship display but also parental care in birds. Progesteron injections made the birds sit on the eggs to incubate within 20 minutes.

In pigeons, secretion of prolactin from pituitary causes enlargement of crop to produce pigeon-milk which is fed to the chicks. Prolactin also acts directly on brain and makes the preoptic nucleus of hypothalamus in birds to respond to chicks calls.


Learning is the ability of the individual to remember and change one’s behaviour in response to earlier experiences. Animals learn a great deal from their surroundings and also from their experiences, particularly during the growing period. Latent learning provides animals with knowledge about their surroundings and escape routes, and also areas where food and water is available. Niko Timbergen (1951) demonstrated by experiments on digger wasp (Philanthus) that it could remember its nesting site by landmarks and got confused when landmarks were changes.

W.H. Thorpe (1951) defines learning as an internal change in the animal causing adaptive changes in the behaviour as a result of experience.

N.E. Miller (1967) called learning a permanent tendency for a stimulus to elicit a response that can be reversed by training.

S.A. Barnett (1981) defined learning as any adaptive change in behaviour as a consequence of experience of repeated stimuli.

There are many types of learning in the animals as given below.


Instinct is the innate behaviour of the animal which is a heritable characteristic. It is the inborn capacity of the animal to perform certain functions. This is also called species memory as it is learned by all members of the species through many generations.

Instinct is build up in the nervous system that controls and modifies the behaviour of the animal and it takes long time to be able to get executed.

Instinct is advantageous to animals that have short life span and no time to learn and no time for parents to teach the juveniles. Sometimes there is no parental care and juveniles are left on their own. Such animals must carry out their activities through instinct in order to survive.

For example, invertebrates in general perform complex activities through instinct. Nectar collecting and hive building by honey bees are done by instinct. Migration of many vertebrates such as sea turtles is by instinct. The catadromous eels and anadromous salmons migrate thousands of kilometres for spawning by instinct.


Imprinting is strongly controlled by genes. Young chicks must learn from parents’ activities such as singing, nest building or following their mother. There are three types of imprinting:

Filial imprinting involves activities that are learned by the young ones from their parents in early stages of life. Hunting by cheetahs, leopards and tigers is learned from parents in younger age.

Sexual imprinting is the recognition of the opposite sex in the adult stage. Courtship display, such as singing, feather display and dancing must be learned during the growing stage by watching the adults. Peter Marler of the University of California by experiment demonstrated that chicks reared in isolation failed to develop courtship songs which they tried in adult stage. The intricate nest building by weaver birds must be learned by the individuals in the younger stage by watching the older individuals.

Social imprinting happens during the brief but sensitive early period of life but it has great stability and influences the behaviour of the animals towards the others for the rest of life. Adult behaviour is the result of learning during the early stages of life.


Habituation is the decrease of responsiveness upon repeated exposure to a stimulus. This is stimulus-specific decline in the response resulting from repeated stimulation. For instance, animals in urban areas are not alarmed by the loud sound of vehicular traffic as they get accustomed to it, while their wild counterparts get extremely agitated by the same stimulus.

Habituation is not fatigue but learning not to respond to stimulus that is neither rewarding nor punitive to the animal. Habituation saves time and energy of the individual for more important activities.


This is increased responsiveness to a repeated stimulus. Sensitization is opposite of habituation. Here the stimulus irritates or annoys the animal and hence the animal shows increased responsiveness each time the stimulus is applied. For example, if frog’s skin is touched with a needle, it will wipe the area with its hind legs. This response will keep on increasing in intensity every time the needle is touched on body.


Conditioning is flexible learning in which a stimulus elicits a specific response from the animal. I.P. Pavlov (1927) conducted experiments with a dog, in which dog was given food after the gong of a bell. The dog got conditioned to the gong of bell and food and salivated immediately.

Later, the dog salivated at the gong of the bell even if no food was presented to it. If animals are presented with a choice of two or more stimuli, the animal gets conditioned to chose the best option. Birds get conditioned to choose the edible butterflies from the unpleasant ones.

Sign Stimuli

Sign stimuli, also called releasers or key stimuli, are those stimuli that are capable of releasing Fixed Action Pattern (FAP) or consummatory behaviour of the animal. They are signals that evoke instinctive patterns of behaviour in animals, such as fighting behaviour in the territorial animals, triggered by the entry of another male. Lehrman found that courting male dove began to bow and coo to a stuffed model of female in the absence of a living female.

Konrad Lorenz (1972) was the first biologist to identify sign stimuli which he called key stimuli because they function as keys to release and unlock the fixed action pattern of the animal. He proposed the Innate Release Mechanism in response to sign stimuli.

Niko Tingbergen (1952) conducted experiments on stickleback fish in which bright red colour of the belly and neck is a sign stimulus for the other males to attack it, while it elicits attraction in female fish.

Bird chicks respond to jerks in the nest, which is a sign stimulus for them to open their beak for eating food. Similarly distress calls given by chicks are a sign stimuli for hens to release rescuing behaviour.

Bright red colour of the oral cavity of a cuckoo chick is a sign stimulus for the foster parent, warbler to feed it. Otherwise the chick is distinctly different from the foster parents.

Sign stimulus is often not one character but a combination of many stimuli, namely, shape of the bill, colour and patches on body or even actions and auditory signals.

Sign stimuli or releasers can be of three types:

Visual Releasers. They are morphological characters that are displayed to elicit response, as display of feathers or dancing in birds. Nest making behaviour in birds such as weaver bird, not only attracts females but also repels other males. Fire flies emit light signals that bring about response from females. In three-spined stickleback fish (Gasterosteus) males get nuptial coloration during breeding season in which neck and belly become bright red in colour to attract females and also to warn other males not to enter the territory.

Auditory Releasers. The sign stimulus is auditory signal like humming sound of male insects. Song birds such as cuckoos, nightingales, starlings establish their territory and attract the female for mating by singing, often for several days. The song is a signature stimulus of individual bird as it is specific. Birds of different areas sing in different dialect and elicit different response.

Stridulatory organs of some insects, such as cicadas and crickets are strong sign stimuli during breeding season.

Co-qui calls given by the tree frog, Eleutherodactylus are strong auditory releaser for males as well as female frogs of the same species.

Chemical Releasers. Pheromones are different types of volatile chemicals released in the atmosphere that act as releasers on the individuals of the same species or other species. Pheromones affect the individuals of the same species while allomones affect other species.

Sex pheromones in insects are produced by the females and perceived by the enlarged antennae of males of the same species, sometimes from the distance of a couple of kilometres.

In vertebrates, the sex pheromone, Copulin is secreted by the female in estrus.

Alarm pheromones are produced by skin cells of several species of fishes that have schooling or shoaling behaviour. Alarm pheromones are also released by the stinging honey bees and wasps which attract other individuals of hive to attack.

Ants and termites produce trail pheromones from the posterior end of abdomen which help them to follow each other.

Sign stimuli bring about responsiveness in target individuals which show consummatory behaviour. The responsiveness diminishes as the consumption proceeds and energy is released.

Sensory filtering

Animals receive much more sensory information than they could possibly register in their brain and respond to. Therefore brain has to be selective and filter out certain information that is not so necessary. Sensory filtering or stimulus filtering takes place at several levels, namely, at the level of sense organs, nerves or different parts of brain.

Sensation is the basic data sent by sense organ to brain, and sense organs have their limitation and hence filter out much of the information. For example, human eye filters out ultraviolet and infrared rays from the spectrum.

Peripheral filtering is done by receptors because of their mechanical ability to receive and transmit information. Receptors are often highly specialized and respond to a narrow range of stimulus. For example caloreceptors can perceive sense of heat but not cold and frigidireceptors can only transmit the sense of cold. Bats can perceive ultrasonic sounds for echolocation but sense organs of other mammals do not possess that ability.

CNS filtering is done by different parts of brain by selective attention or because the part is not well developed. Perception is the interpretation by brain of sensory information in the light of earlier experience. A lot of information is received by brain but is not perceived.

Reticular Activating System located inside the medulla oblongata if inactivated, stops lots of nerve impulses coming through the cranial nerves.

Stimuli that reach respective areas of brain such as optic lobes, auditory lobes etc. can get filtered out if not important. Epithalamus, which functions as the central switch board, selects and sends only necessary nerve impulses to cerebral hemispheres.

When the nerve impulses arrive in different areas of cerebral cortex, they are analysed and interpreted and if found worthless can be rejected without perception.

Only information that is considered important is selected by the areas of cerebral cortex and interpreted, and motor action travels through the nerves to muscles to act. Neurochemical information coming from cerebral cortex affects hypothalamus, which stimulates endocrine system to alter the behaviour of the animal.

Muller’s Law of specific nerve energies: Sensation perceived depends on the part of nervous system activated, and not on the sense organ stimulated.

Examples: The male of the South American tree frog (Eleutherodactylus coqui) produces co-qui call to attract female and also to repel other competing males. The tympanic membranes of male and female are adapted differently. Males can hear only the co note and get warned and repelled. Males cannot hear the qui part of the call, while females hear only the qui part of the call and therefore get attracted to males.

Olfactory cells located on the antennae of male moths (Lepidoptera) can perceive only specific pheromones which are released by the female of the same species. These pheromones cannot be perceived by the receptors of males of another species.

European Robin (Erithacus rubicula) attack red-breasted male robins. Only red feathers of the competitors are perceived and the other colours are filtered out for attack and for defending the territory.


Animal Behaviour


Ethology is the science of the study of animal behavior. This is a comparatively new science developed during the last century. It deals with the response of animals to various stimuli and factors operating in the environment.

The following chapters are included in this section: