Zoology for IAS

Social Life In Primates


Primates were not social animals when they evolved from the primitive insectivore ancestor in Palaeocene epoch. However, gradually primates became gregarious and social interactions developed in them leading to highly developed social life as in humans.

SOLITARY PROSIMIANS

Prosimians such as tarsiers, bush babies and lorises are mostly nocturnal and highly arboreal primates. Males are found solitary or in pairs with females in breeding. Females live with infants till they become independent. Prosimians are shy animals and hiding in foliage is their means of defence. They rarely come down from trees. The black coloured Aye-aye is nocturnal and lives singly or in pairs.

MONOGAMOUS PROSIMIANS

Tree shrews (Tupaia species) are the most primitive primates alive today and live singly or in pairs in the forests of S.E. Asia. As they are highly territorial and defend their territory with aggression, their social units are broken up into nuclear families, e.g. male, female and juveniles. Males mark their territory by urine and defend it by threat-call and tail-flicking. Intruders are quickly attacked and chased away.

Lemurs occur in the tropical rain forests of Madagascar. They are also monogamous and live in a group male, female and up to 4 young ones. Male and female marry for life and live together. Some species mark territory.

Tamarins are South American monkeys which are also monogamous and live in nuclear families of male, female and juveniles.

SINGLE MALE BISEXUAL GROUPS

Some monkeys, e.g. hanuman langur, howler monkey, red-tailed monkey and blue monkey, live in a social group in which there is a single dominant male having a harem of several females. Young males are chased away by this dominant male and hence they form all male groups outside the other groups. Harem is protected by the overlord male but is constantly attacked by males from the all male groups to unseat him from the dominant position.

MULTIMALE BISEXUAL GROUPS

Baboons (Papio) are terrestrial primates which are found in large groups that may include thousands of individuals. There are several units in which one male and several females with young ones form a small group. These small units forage together. Male protects the group and herds females together and prevents them from meeting other males. Hierarchy is maintained in females for access to male. These small units together form large bands that live together, sleep together and defend them collectively from predators. All males collect together to attack a predator which is usually a leopard.

Rhesus monkeys also form multimale bisexual groups that form large foraging units. One male is dominant and others subordinate. A foraging unit is formed by 3-8 males, each having 5-7 bonded females. Many units form large groups of hundreds of individuals for foraging and for defending.

SOLITARY APES

Orang-Utan lives in the dense forest of Sumatra and Borneo and is completely arboreal, feeding on a diet of fruits and leaves. Males are solitary, seeking females only for mating and shares no family responsibilities. Females are found with the young ones, usually only one young is found with female. For sleeping on the tree female makes a nest with branches and leaves in the fork of tree.

MONOGAMOUS APES

The lesser apes, white handed gibbon and hoolock gibbon are found in the dense forests of eastern India, China and Burma. They are highly arboreal and swing under the branches with the help of excessively long arms. Male, female and up to 4 young are found in one family unit. The units communicate with one another by loud hooting calls. Both male and female share family responsibility together.

MULTIMALE BISEXUAL APES

Gorilla is the largest ape living in the dense forests of Cameroon, Gabon, Congo and Uganda. Males are terrestrial as they are too heavy to be arboreal. Females and young make nests among branches on trees for sleeping. They are found in groups of about 20 individuals. Old dominant males are called “silver backs” which dominate over other males and females. Hierarchy is observed among males as well as females while feeding, drinking or access to females.

DIFFUSED SOCIAL APES

Chimpanzees form diffused social groups of up to 50 individuals. Females are silent and shy. There is hierarchical ranking among males as well as females but females accept several males and there is no conflict. They are omnivorous and sometimes hunt monkeys and share its meat. They also make nests in the fork of trees for sleeping. Both males and females defend their group by screaming, gesturing and by throwing sticks and stones at the intruder.




Social Organisation In Insects


 

In insects social life has evolved only in two orders, namely, Isoptera (termites) and Hymenoptera (bees, wasps and ants) which make a nest and live in colonies of thousands of individuals that practice division of labour and social interaction.

SOCIAL LIFE IN TERMITES

Termites were the first animals which started living in colonies and developed a well organised social system about 300 million years ago, much earlier than honey bees, ants and human beings. Although termites do not exceed 3-4 mm in size, their queen is a 4 inch long giant that lies in the royal chamber motionless, since its legs are too small to move its enormous body. Hence workers have to take care of all its daily chores.

Termite queen is an egg-laying machine that reproduces at an astonishing rate of two eggs per second. Generally the queen of a termite colony can lay 6,000 to 7,000 eggs per day, and can live for 15 to 20 years. The other castes, workers and soldiers are highly devoted to the colony, working incessantly and tirelessly, demanding nothing in return from the society.

Soldiers have long dagger-like mandibles with which they defend their nest and workers chew the wood to feed to the queen and larvae and grow fungus gardens for lean periods.

Nasutes are specialized soldiers which specialize in chemical warfare. They produce a jet of highly corrosive chemical from their bodies that can dissolve the skin of enemies and can also help in making galleries through the rocks.

SOCIAL STRUCTURE OF A BEE COLONY

The population of a healthy bee hive in spring and honey flow period may contain 40,000-80,000 individuals but the population declines in winter and extreme summer. There is remarkable order in the hive and no conflicts are seen among the members.

Queen is one and a half times larger than the workers and is the only fertile female in the hive. Queen keeps the colony together by secreting a pheromone called queen substance from its mandibular glands. In multiqueen colonies, young queens after emergence attempt to sting and kill the rival queens.

Generally only one queen stays in the hive and other queens along with their army of workers swarm out and seek new places for building their own hives. Queen takes one to several nuptial flights and after mating with drones settles in the hive and starts laying eggs.

Drones are haploid fertile males of the colony, whose only job seems to be to mate with the queen and transfer their sperms in her spermatheca. There are 2-3 dozen drones in a bee hive all of which energetically pursue a queen in her nuptial flight. Once the breeding season is over drones are driven out of the hive by workers and die of starvation, since they are unable to forage for themselves.

Workers in a hive are 20,000-80,000 in number, which are genetically sterile females that build, maintain and protect the hive. A worker attends to cleaning and maintaining the hive and feeding the larvae with honey and bee bread. It also secretes wax from the abdominal wax glands and participates in building honey comb cells. The workers function as foragers of nectar and pollen and in later part of life as water carriers, and eventually die while working.

SOCIAL ORGANISATION IN WASPS

Insects known as wasps belong to family Vespidae, about 75,000 species of wasps are known, most of them parasitic or predators. Wasps are characterized by two pairs of membranous wings, three pairs of legs and an ovipositor that may be modified as sting in sterile females.

The abdomen is narrowly attached to the thorax by a petiole. In addition to their compound eyes, wasps also have three simple eyes known as ocelli, arranged in a triangle on the top of the head. Females have diploid number of chromosomes and develop from fertilized eggs. Males are haploid and develop from unfertilized eggs.

Yellow jackets and paper wasps prey on caterpillars and other larvae that can destroy crops. Wasps feed on flower nectar and play a role in pollination. Wasps can be solitary or colonial and social insects that exist in colonies numbering up to several thousand strong and build nests.

The type of nest produced by wasps can depend on the species and location. Many social wasps produce paper pulp nests on trees, holes in the ground or in other such sheltered areas. Unlike honey bees, wasps have no wax producing glands.

Many instead create a paper-like substance primarily from wood pulp, which is gathered locally from weathered wood that is softened by chewing and mixing with saliva. The pulp is then used to make combs with cells for brood rearing. Mud daubers and pollen wasps construct mud cells in sheltered places typically on the sides of walls. Potter wasps similarly build vase-like nests from mud, often with multiple cells, attached to the twigs of trees or against walls.

ANT SOCIETIES

Ants are cousins of honeybees and wasps but while bees and wasps are diurnal and sleep in the night, ants are busy working day and night. Ants have no wings, except in sexual forms in breeding season, and therefore their job of travelling to long distances in search of food is very difficult, but addicted to work as they are and having never-say-die spirit, make them excellent foragers that work round the clock, apparently without any rest.

Ants have the highest developed social system, next only to man, with no apparent conflict seen in the society. A colony may have few thousand to over 500,000 individuals. The nests are built in various designs and are called formicaria. Extreme devotion to duty and “Work is worship” attitude binds them together.

Like honeybees, they have polyethism, which means castes are specialized to carry out specialized duties in the colony. For example, the queen has large abdomen to lay a lot of eggs (2-3 million in a year), males fertilize her, workers have broad, sharp mandibles for cutting and chewing and the soldiers have large head that bears sharp dagger-like mandibles for fighting. Workers and soldiers are sterile females.

Ants have poor eyesight and are deaf but have a highly sophisticated chemical language for communication. They possess glands that secrete pheromones for communication. The mutual attraction among the members of a colony is maintained by endless antennal caressing, licking and nuzzling during which they trade food, glandular secretions and enzymes, which is called tropholaxis.

Most ant species excavate nests in the ground or wood but some construct suspended nests on trees made of earth, carton, wax or silk, while some, like safari ants, do not build nests at all. Desert ants build crater-like nests or mounds in which they are able to maintain temperature much below the outside heat. The tropical ant Oecophylla makes nest by webbing the leaves with silken thread that is produced by their larvae.




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 & THIRST DRIVE

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.

HOARDING DRIVE

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

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 DRIVE

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.

TERRITORIAL DRIVE

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.

HORMONES IN SEXUAL DRIVE

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.

PARENTAL CARE DRIVE

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


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

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

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

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.

SENSITIZATION

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

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.




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