Essay on Intraspecific and Interspecific Interactions | Population Density

Many of these interactions act to regulate density or, along with climate cause fluctuations or even more drastic changes in a number of individuals of the populations from time to time and so constitute biotic environmental factors.

Intraspecific Interactions:

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It is only under extremely rare and unusual circumstances that an individual is the only representative of his species existing in a given region. The individual generally has many types of intra­specific interactions with old and young males and females of the same species and this interaction involves associations between individuals including social organisations (societies), territorial segregations and communication between individuals.

Types of Association:

Organisms vary in their ability to move about in their environ­ment. Many animals such as cephalochordates, vertebrates, most arthropods, many invertebrates and few plants and microbes are free-living in that they can crawl, swim, walk, fly, or otherwise move.

Others are sessile, being attached permanently to a rock, or other structure in their environment. Among the free-living forms, there are many different grades of mobility. Many marine invertebrates are sedentary, remaining in place for most of their lives. Other animals and micro-organisms are plank tonic in that float in the water and are carried about by the action of currents and winds. Many, however, are motile, actively moving about in their environment, often with relative rapidity.

The type of locomotive ability often influences the types of association existing between individuals. Some kinds of animals are solitary, living alone except perhaps during the reproductive seasons. Others are gregarious, living together in various groups such as schools, flocks (of birds), packs (of wolves), herds (of ungulates), swarm (of locusts), colonies (of various inverte­brates) and similar other associations. Different kinds of associa­tions of organisms can be studied as follows:


An early manifestation of intraspecific cooperation in the evolution of animals is the grouping of free-living protozoan’s to form colonies, and the further development of such colonies into multicellular metazoans that thereafter behave and respond as unit organisms.

Whether the first gathering of protozoan cells to form colonies developed for better protection from some predator or environmental condition, improved utilization of food supplies, or more efficient reproduction, it is impossible to say (Kendeigh, 1974). The colonial form, however, must have had some survival value.

Further, there are various sessile metazoan animals which frequently occur in colonies often with structural connections bet­ween the different members of the colony. Such colonial forms include many coelenterates (Cnidarians) such as Ceratella, Obelia, Physalia, Porpita, Millepora, Tubipora, Alcyonium, Gorgonia, Corallium, etc.; ectoprocts (Bugula, etc.) and ascidians. Among the plants also, grasses, sedges and several herbs form dense stands.

Outcomes of colonization:

During the course of evolution the colonisation quickly led to division of labour between somatic and reproductive cells, as occurs in Volvox, and later to division of labour between somatic cells themselves, so that different cells or organs become specialized to serve particular functions of digestion, respiration, circulation, excretion, and so on. Cooperation among cells, tissues, and organs gave greater metabolic efficiency to the whole individuals and resulted in evolution to the highest types of animals.

The colonial hydroids, also exhibit polymorphism and division of labour in different individuals, called zooids or pulypsol the colony. For examples Physalia is a floating colony of polyps or zooids specialized into four major polyp types—for floating (pneumatophore), feeding (gastro zooid), fighting (dactylozooid) and reproduction (gonozooid). Such specialization of polyps is called polymorphism.


Many associations of individuals appear to be the result of random groupings that furnish some survival advantage for the group. The term aggregation or temporary aggregation is usually applied to a group of animals which come together for some external reason.

For example, a group of insects is more aptsurvive a dry environment than is a single insect, since, in a limited space, each would lose less water before the relative humidity of the surrounding environment would be raised to a less dangerous level.

Other similar examples of so-called “unconscious coopera­tion” include migration of locusts in swarms and of birds in flocks. Many species of membracid insects (Homoptera) exhibit good example of the gregarious habit; sometime the same host plant (e.g., Prosopis spicigera) lodges two different gregarious species (e.g., Oxyrhachis tarandus and Otinotus oneratus), the individuals of both crowding together and mingling with each other very closely.

Such associations may be asexually gregarious, combining individuals without respect their age or sex, or may be sexually gregarious, often bringing together males and females during the reproductive season. For example, mayflies, midges, and mosquitoes swarm for mating purposes.

Social Organizations and Behaviour:

Individuals often actively associate for mutual gain. The term permanent aggregation, society or social organization is usually applied to a group of individuals belonging to the same species and organized in a cooperative manner.

In a social organization, success is often measured in terms of survival of the colony,’ with individual survival having only secondary importance the social organization has evolved separately in several different groups of insects including termites, ants, bees and wasps. According to Eisenberg (1965), the organized societies have the following defi­ning attributes:

1. A complex system of communication. All organized societies have some form of complex communication system.

2. A division of labour based on specialization. In organized societies, animals of different castes, sexes, or age groups have different functions in maintaining the society.

3. Cohesion, a tendency for members to remain together. The individuals constituting a society tend to remain in close proximity to one another.

4. A permanence of individual composition. The individuals making up a society tend to be the same from day to day; there is little migration from the group.

5. A tendency to be impermeable to co specifics that are not members of the group. Most organized societies resist immigration by “outsiders”.

Both kinds of aggregations (viz., temporary and permanent) have following advantages:

1. It is very often the Environment that derives animals into collective groups. Scarcity of space, light, utilization of the same food and shelter, collective breeding and various other conditions induce the animals to aggregate or congregate.

These aggregations are capable of tolerating different ecological hazards more efficiently than they have been living solitarily. Insects and vertebrates survive the acute cold of winter by forming groups which help to generate, retain and conserve heat.

This is one of the basic advantages of aggregations and is termed as the group survival value. Coveys of bobwhite quail roost in close circles, at night. Perhaps this enables detection of predators approaching from any direction but by that behaviour the birds can tolerate lower air temperatures and for a longer time than isolated birds can. Similarly, mice huddle in low air temperatures, a behaviour that reduces heat radiation and consequent need for frequent feeding.

2. Aggregation may have a protective function against predators. The benefits may occur because of mutual protection of the members of the group. Sheep, when threatened, will cluster together, with the rams encircling the young sheep and ewes.

The benefits of aggregation may result because they warn one another of impending danger or because of the confusion caused by the prey scattering in several directions as the predator strikes.

Further, a single musk ox or bison may succumb to a pack of wolves. When in a group, the males form a circle facing outward with the females and young inside, whereby they are usually able to ward off the attack.

3. Aggregation facilitate food gathering. Large numbers of animals seeking food can shorten the time to discovery. Also they may aid each other in capturing and subduing the prey. Coopera­tive hunting by wolves is a well known example.

A single wolf has difficulty in killing a deer; a single coyote in killing a pronghorn antelope. But in packs the wolves can overpower a deer and a pack of coyote can chase a pronghorn to exhaustion.

4. Aggregation of two or more organisms is necessary for sex­ual reproduction. Aside from the obvious need for two animals to participate in sexual reproduction, group activities, sometimes faci­litate breeding by mutually stimulating one another or, as in the case of male frogs that sing together in ponds, it increases the like­lihood that a female will find the pond and be fertilized.

5. Transfer of learning may occur in aggregations. Mammals in general teach their young routes to food, techniques of food cap­ture, places of danger, and so on.

6. Groups of animals can cooperate in building structures for various functions that would be difficult or impossible for one ani­mal alone. The social insects (ants, bees, termites, wasps) gener­ally erect rather substantial nests by cooperative building.

7. Aggregations permit division of labour and much specialization to occur in structure, function and behaviour of the partici­pating members of the group, so the individuals cannot survive outside the colony. Such specialization allows a higher degree of efficiency when performing a task. For example, ants and termites have several castes and are called asocial insects.

There are win­ged fertile males (kings) and females (queens) which are specialized for reproduction; the wingless sterile soldiers that possess large mandibles and irritating glandular secretions and are specialized for fighting and defending the colony against the predacious ene­mies; smaller, wingless sterile workers which are specialized for carrying out the daily normal activities of the nest and so on.

In cases, where such specializations of individuals like the asocial insects do not occur, individuals may take4.urns with a work­load. For example, in some primate groups, the care of the young is partly a community responsibility.

Certainly this is the case in the social insects like bees and wasps, where workers or “nurses” care for the larvae. For example, the colony of honeybee has a single fertile female that is specialized for egg-laying and numerous sterile workers specialized for various activities for the benefit of the colony.

In fact, a beehive is a marvelous society. Just as a mammalian body has elaborate homeostatic mechanisms that main­tain a constant temperature, water balance, and nutrient level, so a beehive has elaborate mechanisms that maintain hive homeo­stasis.

For example, when the hive is hot, worker bees fan air throughout it and cool it off. When the nurse workers attending the larvae are short of water, they turn to the nearest workers and signal a need for water.

The latter give what water they have and then turn to their neighbors; thus, the shortage of water moves from bee to bee until it reaches a worker who leaves the hive and returns with more water. When a predator or the hive, workers rush forward and defend the colony, with the result that many die.

It is almost as if the hive itself, and not the indi­vidual bees, were the organism. Martin Lindauer (1961) found that in a beehive, a horde of workers, as they go through their life cycles; perform one major task after one another.

After emergence an adult worker bee first cleans the hive, then tends the brood, builds more combs, guards the nest entrance and finally forages for pollen and nectar. This behavioral sequence of events is correlated with physiological changes in the nurse glands (which produce nutritive materials for bee larvae) in the head and the wax glands (which produce comb-building material) in the insect’s abdomen (Adcock, 1979).

Thus, the majority of fe­males, (i.e., workers) of the bee society do not reproduce, acting in­stead to care for their sisters. This pattern of behaviour is called altruistic behaviour and it is required for the following three evolu­tionary processes : inclusive fitness (i.e., the sum of an individual’s own fitness plus its influence on the fitness of its relatives other than direct descendants; Hamilton, 1964), kin selection (i.e. differen­tial representation of genes resulting from favoring the survival and reproduction of relatives who possess the same genes by common descent), and reciprocal altruism (i.e., the trading of altruistic acts by two or more individuals at two different times ; in effect, an in­dividual engages in an altruistic act, such as jumping into the water to save a drowning person, in exchange for the promise that the other individual will do the same in like circumstances ; Trivers 1971).

There exist several more examples of the altruistic behaviour animals: In the colony of paper wasp (Polistes), a diploid fer­tile and dominant female performs the work of reproduction, while a horde of subdominant sterile female workers increase the size of the nest, add cells, feed the larvae, detect and drive off parasites and descend en masse on potential predators, stinging them into retreat.

In lions, females of a given social group (called a pride) care for, defend and even nurse all youngsters in the pride. The nest of a social bird, Florida scrub jay has numerous helper birds other than the parents that help in feeding the nestlings and fledg­lings.

On spotting a predator, group living birds (Blue titmouse black bird), Belding’s ground squirrel and primates emit a “alarm call” or “warning cry” which alerts other group members to the presence of a predator but may make it easier for the predator to locate the signaler (Dewsbury, 1978).

Further, the social animals also exhibit the following behavi­oural phenomena.

Dominance hierarchies:

Most social animals recognize pattern of authority or dominance-subordination, the most basic being parent-child or provider-dependent and the relation­ship of the stronger to the weaker. Van Kreveld (1970) has defined dominance as a priority of access to an approach situation or of leaving an avoidance situation that one individual has over other.

Approach situations include access to food, mating partners, oppor­tunities for aggression and space. Avoidance situations include remaining in undesirable areas as well as having to tolerate threats, attacks and punishments. The essential feature of dominance is that there is some resource that is of limited availability and is con­tested one animal has gained privileged access to the limited resource relative to another animal.

Such a behaviour pattern is simply adaptive, ensuring that the strongest will be best fed and so remain strong and successfully reproduce, while the weaker are less likely to survive (Alcock. 1979).

The classical examples of dominance hierarchies are from the so called “peck orders” of chickens and pigeons. The organization of an established flock of chickens is based on a set of dominance relationships. If food is given in a restricted location, it becomes apparent that one animal has priority of access to the food. It will exclude other chickens from the area of the food, perhaps by deli­vering a peck at them. The other animals never return the peck.

Such pecking is particularly prominent in newly established flocks; hence Schjeidenip-Fbbe, an early student of dominance hierarchies, referred to dominance hierarchies as “peck orders”. Typically, in a group of a dozen or so chickens, there is but a single despot or alpha animal who has priority of access all other animals in the flock and who literally can peck any other animal, the beta individual, has priority over all animals in the flock except the alpha animal.

In an ideal linear hierarchy, there is a perfect straight line ordering of all animals in the flock, so that each is dominant to all animals below in the hierarchy and subordinate to all animals above it in the hierarchy. The hierarchy gives dominant animal’s priority of access to food, water, roosting places, mates, and any other appropriate resource.

The peck order, thus, can be an excee­dingly effective means of pegging the population size to the current state of the environment, generally the availability of food, water, or nesting space, while insuring that a number of individuals in the population both survive and reproduce.

Sometimes, as in rabbits, there are two linear hierarchies, one for males and one for females. In other cases, for example in cer­tain monkeys and rats, the hierarchy for food may be different from that for access to reproductive females.

Finally, not all hiera­rchies are linear but rather a complex web. For example, in com­plex monkey troops, triangular relationships may occur, in which animal A dominates B, B dominates C, but C dominates A. Domi­nance hierarchies are found to exist in a variety of animals such as crabs, crayfish, cockroaches, lizards, rodents, wolves, hyaena, dairy cattle, reindeer and various non-human primates (Dewsbury, 1978).

Social hierarchies or leaderships:

The term “leader­ship” generally is used to refer to the ability of a given individual to influence the movement pattern of the group as it goes from place to place. Thus, the leader determines the time, rate and dire­ction of the group’s movements. The most effective “leader”, in this case, is not necessarily the most dominant individual.

Experi­ence often counts more than physical prowess in determining leadership. In many primate troops, it is the females that play an important role in controlling group movements. Among the red deer protection of the herd from danger falls to the most experie­nced female, rather than to a dominant stag. Leaders may be of any age and of either sex.


Many species of animals establish “ownership” and defend specific areas of land called territories. This is called territorial behaviour or territoriality. Kaufmann (1971) has defined a territory as an area in which the resident enjoys priority of access to limited resources that he or she does not enjoy in other areas.

The male stickleback fish provides an excellent example of territoriality. During the breeding season, the male leaves the school and stakes out a distinct territory, defend­ing its borders by displaying to intruders. Examples of such terri­torial defense are found in a wide variety of insects, fish, amphibian’s reptiles, birds and mammals.

In birds, the territorial behaviour is generally exhibited by males early in the breeding season. When the females arrive in the area, they are courted by various males and eventually settle down in one territory to build a nest. The territory that is established is sufficiently large to provide food for the family of birds for the season.

The fighting between contending males seldom leads to blood­shed; it is largely ritualized and stylized behaviour filled with bluff it and display. In this way, the entire suitable habitat is par­celed out among the aggressive healthy males. If too many birds are present in an area, all the suitable sites are taken and many animals (especially the young) are forced to try to establish territo­ries in less hospitable regions or will not take up residence at all.

The chances that these subordinate animals will successfully raise a brood are very low. Territorial behaviour assures that the natural resources of the bird population are not overexploited by over­population. A certain area of habitat is required for a pair of birds to raise a brood to maturity. In the years of low population den­sity, all birds will find suitable real estate, and breeding success will be high for all, tending to drive the population up.

In years when population is dense, the extra birds (without territories) will fail to raise young and will probably have a low survival rate themselves; this will drive the population down.

Territorial behaviour has similar regulatory function in contro­lling population size in most of the animals for example, seabirds, such as gannets, nest on rock cliffs but feed from the ocean. The adults defend nesting territories on the cliffs symbolically might be said to establish fishing rights.

Only birds with territories will raise young, thus limiting the population in that area. Likewise, mature males of fur seals which breed on the Prlbilof Island, establish terri­tories along the beach, collecting females into their “harems”.

Young males (bachelors) cannot hold territories, so they do not breed their first year. Only so many seals can occupy the beaches, thus population is limited. Certain colonial species of the primates often set up communal territories, which they defend jointly. Group size is roughly limited, and “tribes” will compete with each other for dominance over selected feeding areas.

Forms of territoriality:

Wilson, (1975) has recognized the following five forms of territorial organisation:

Type A:

A large, defended area within which sheltering, court­ship, mating, nesting and most food gathering occur (e.g., benthic fishes, arboreal lizards, insectivorous birds, some small mammals)

Type B:

A large defended area within which all breeding acti­vities take place but which is not the site for most food gathering (e.g., nightjars and reed warblers).

Type C:

A small defined area around a nest (e.g., many colo­nial birds, such as ibises and herons, and specie wasps).

Type D:

Pairing and/or mating territories (e.g., insects such as damselflies and dragon flies such as sage grouse and ungulates such a Uganda Kob)

Type E:

Roosting positions and shelters (e.g., many species of bats, starlings, and domestic pigeon).

Home range:

The home range of an individual is the area that it habitually travels in the course of its normal activities. Many species, such as most rodents, occupy relatively small home ranges for their entire lives. Home ranges may be completely unde­fended and convey no privileged access to resources; they should not be confused with territories.

Individual distance and flight distance:

Many species act so as to keep a certain distance between themselves and other species (such as a distance between birds and primates). Such a distance is referred to as “individual distance”.

The flight distance refers to the distance to which an individual will permit a predator to approach before it flees.


Communication between members of a species is almost universal. It generally occurs during repro­duction in sexual species; however, it plays a daily role in the life of many social animals. Communication is said to have occurred when an animal performs an act that alters the behaviour of ano­ther organism. Such interactions are often quite specialized and usually are adaptive for one or both organisms. A communication system involves the following seven essential components (Dewsbury, 1978):

1. Sender: an individual which emits a signal. 2. Receiver: an individual whose behaviour is altered by signal. 3. Message or signal: the behaviour emitted by the sender. The variety of signals animals use to communicate a behavioural state such as aggressiveness or an intention to take flight or attack includes odors (chemicals), postures, colors, sounds, shapes and motions. These are meaningful to friend and foe of the same; as are raised eyebrows denoting surprise among humans. 4. Channel: a pathway through which the signal travels (e.g., vocal-auditory channel). Several different sensory receptors are involved in animal communication. 5. Noise: background activity in the channel that is unrelated to the signal. 6. Context: the setting in which a signal is emitted and received. 7. Code: the complete set of possible signals and contexts. The accurate trans­mission and reception of social signals can mean life or death to an individual or its offspring.

Some important methods of animal communication can be studied as follows:

1. Chemical signaling:

Odours maybe the most primitive communication techniques chemicals produced by animals and serving either as gustatory or olfactory signals that convey infor­mation between, different members of a species are called pheromones.

Pheromones may be released into the air or water or depo­sited onto or near the ground by animals when they are in certain behavioural states. Pheromones then generate complementary behaviour when they are detected by other members of the species. These chemical signals play a part in behaviour comparable to that played by hormones as chemical messengers between different groups of cells within the body.

Pheromones may be of the following two basic types: signaling Pheromones that result in a more or less immediate effect on the of the recipient animal; priming pheromones that hormonal activities that may become manifest in overt be only at a later time.

Pheromones may be deposited by means of object marking, marking of a social partner, self-marking, substrate marking, or release of the pheromone into the air. Pheromones convey a variety of different kinds of information in different spe­cies, including species and racial identity, sexual identification and reproductive state, individual identification, age and mood.

Phero­mones are functional in affecting reproductive behaviour (sexual and maternal) and other forms of social behaviour (withdrawal and submission, aggression and dominance and scent marking).

Best known pheromones are the chemical markers serving to bound the territories of individual animals. Various mammals produce a “personalized” secretion from anal or facial glands that is rubbed into trees or rocks. For example, a male dik-dik deer marks its territory by depositing secretions from an eye gland onto twig ends.

Cat and dog species generally urinate to mark the edges of their territory. Any incursion into the territory evokes a res­ponse. In prairie dogs the emission of strong, musky odours is often associated with the defense of territory from other individuals of the same species.

Various female mammals such as cats and dogs are known to re­lease a species-specific sex pheromone, when they are in “heat”, which attracts males from considerable distances. Such a pheromone signals the sexual state of the female and acts to excite the males.

Other striking examples of potency of sex pheromones are the sex attractants released by virgin female moths of many species, including the silkworm moth and the gypsy moth. These phero­mones are secreted by special glands on the female abdomen and are detected by receptors on the male antennae.

A “calling” female releases minute amounts of the sex attractant, which is then carried downwind. When it is detected by males, often kilometres away, it induces flight activity and a tendency to travel upwind in a search pattern for as long as they can detect the odour. These built-in Instructions bring the males close to the calling female.

When we realize that the abdomen of a silkworm moth (Bombyx mori) contains a total amount of only 1.5 ? 10-6 g of sex phero­mone called bombykol and that only a fraction of this amount is dituted in an air stream that may be several kilometres in length, the extreme sensitivity of the male moth’s olfactory sense is not surprising.

Dietrich Schneider, (1974) has estimated that a male moth will react when 40 out of 40,000 receptors on the antennae receive one strike of a pheromone molecule per second.

The best known and most elaborate systems of chemical com­munication are found in social insects such as termites, ants, bees and wasps. Honeybees and ants are found to have- upto a dozen different exocrine (externally secreting) glands, each producing chemicals that elicit specific forms of behaviour in nest matesexample, queen production in a honeybee colony is carried out by the workers, future queens. To do this they build rather larger cells than usual— the queen cells.

In each of these the queen deposits an egg; the future of the egg is upto the workers. But they do not make queen cells so long there is a healthy queen in the hive and the hive po­pulation is not too large. Such a peculiar behaviour of worker bees is found to be regulated by a pheromone which is chemically known as 9-oxodec-2-enoic acid and is secreted by the mandible glands of the healthy queen. Shi spreads this pheromone over the surface of her body as she clears herself. The workers lick it off her as they tend her.

This pheromone then enters food-exchanged stream and is passed from bee to bee round the hive. It produces a direct physiological effect on the workers making them sterile (i.e., this pheromone inhibits ovarian development in worker bees). The effects of this pheromone disappears a few hours the queen’s death, releasing queen-raising behaviour in the worker bees. This pheromone also acts as sex attractant, for drones are drawn to virgin queens by it.

Beekeepers are well aware of the fact that if one bee stings, many others will attempt to. An alarm pheromone (isoamyl acetate) is left with the sting of the bee in the wound. It diffuses in the air and excites other bees to aggregate and sting. Likewise, when their colony is threatened; ants produce various kinds of alarm phero­mones that cause aggregation, excited running and readiness to attack. An injured fish also gives off a chemical that causes other fish to leave the vicinity and hide.

Further, an ant returning from a rich food source touches her stinger to the ground every few steps, leaving a chemical trail that is followed by other ants that in turn reinforce the trail on their return to the nest. Oleic acid is formed in the decomposing corpses of dead ants.

This chemical causes a live ant to pick up the source and carry it out of the nest to the nearby refuse pile. The unity and stability of these insect colonies are reinforced by mutual licking and a continuous interchange of material regurgitated from the crop. This is called trophallaxis and serves as social bond by distributing a common “nest flavor” among nest mates.

2. Tactile communication:

In prairie dogs and numerous other mammals and certain insects tactile signals such as a ‘kiss’, nose-rubbing or rubbing of antennae (in bees) are used to identify members of a group and perhaps, to communicate other information.

3. Visual communication:

Visual communication is particularly among certain fish, lizards, birds, mammals and arthro­pods including insects. Visual messages may be communicated by variety of means, such as through color, posture or shape, movement, or timing. The octopus is capable of rapid color angels that are involved with sexual display. Plumage coloration many male birds (e.g., peacock) serve the same purpose. The eye ring in certain sea gulls is the major- key in maintains- g species separation.

Some male butterflies are attracted to the female by her performance of a specific flight pattern. Fireflies are attracted to each other on the basis of their flash intervals; each species has its own frequency. Some spiders have males which raise their appendages in a specific ordered sequence much like a sema­phore signal: these are required for proper species identification and they prevent the female spider’s attack.

Different postures of the trunk, ears and head of the African elephant display different messages. Trunk in a forward position indicates threat. Trunk, curled inward indicates fear and submissive or defensive postures. Rising of ears or head indicates increased aggressiveness. Simi­larly, body positions such as are used by dogs and wolves transmit such signals as anger, fear and joy. Male baboon exposes his long canine teeth to signal the threat.

Visual alarm signals are common in flocking birds which flash their brilliant tail and wing feathers when danger is near. Mammals, too, use alarm signals. Species of deer have a white patch under their which they flash during danger. This behaviour alerts the other deer in the area and they bound off together. Visual signals sometimes have temporary nature such as the reddened rump areas in female chimpanzees and baboons during estrus.

4. Auditory communication:

Auditory signaling (acoustic and vocal) is the most highly developed means of communication. Sounds are produced in a wide variety of ways. For example, some insects (crickets, cicadas, grasshoppers, etc.) rub parts of the body together (wings, legs), while many fish vibrate their swim bladders by muscle contraction.

The modified skin forming the vibrating rattle of a rattlesnake is well known and various mammals vibrate their tails or stamp their feet on the ground, thus producing sounds. Vocal cords are well developed in most frogs, salamanders, some lizards, birds and mammals.

For example, sound plays a significant role in reproductive, food-gathering and defensive behaviour in various animals. Most birds, insects and amphibians use sounds to attract mates the sound patterns are generally specific. In cases where several species look similar and live in the same area (sympatric), their calls are rather distinct. In areas where they live apart (allopatric), the difference in their calls may be small. Herring gulls and gibbons have food-finding calls which announce the discovery of food to other individuals.

Buzzards signal the location of dead animals to other buzzards by circling above the site where the carrion is located. Alarm calls are common in birds. Small European finches have two types of alarm calls. One is used when they sight hawks overhead; it is a high frequency sound, starting and stopping gra­dually. The other call is used when they see danger on the ground, it is loud, sharp, staccato sound.

Some birds (such as U. S. eastern crows) have mobbing calls or assembly calls to draw member of the group together to deal with predators like owl or a cat. Some animals have developed departing calls. This call is used danger is present, so their sudden departure from a group will not cause alarm. Eastern crows produce a departing call before they fly off. If this is not given, which is the case when danger is near, the flock is alerted and they all fly off when one abruptly leaves.

Communication in honeybees:

Honeybees of species Apis have developed a complex language that utilizes chemo- reception as well as tactile, visual and auditory stimuli. The German ethologist Kail Von Frisch and his colleagues have decip­hered the language of bees. They found that honeybees leave the hive and forage for food. When they return, other bees gather around the scout to get the information about the food source.

Scout bees inform other bees of a source of nectar by performing the following two types of dances on the wall of the hive: a round dance for nearby food and a tail wagging dance for distant food. Tail wagging dance also gives information of direction of food source.

Both dances are performed in the dark hive, so that the kind of food is determined by chemoreceptors on the antennae of the other bees identifying the pollen or nectar on the dancing bee, and the location by placing the antennae on the dancing bee and following the pattern of the dance.

Sometimes a low humming noise is also associated with the dance, apparent also indicating distance. After repeating the dance a few times the bee then returns to the food source for more nectar. As long as the supply remains large, the dance is repeated after each trip, but when the supply is limited the bee continues to make trips until the supply is exhausted but does not perform the dance in the hive.

Psychological Factors:

There are certain reactions by populations to unusual condi­tions most commonly crowding, which do not come into play under normal conditions, and which seem like breakdowns in the normal psychological balances of the organisms. These include cannibalism, inability to breed or care for young, increase in rate of spontaneous abortion and similar problems.

In many species, they act to control the population size when the more normal controls have failed. Certain evidences have suggested that the social stresses act on the individual through a physiological feedback involving the endocrine system.

In vertebrates, this feed­back is most closely associated with the pituitary and adrenal glands (Christian, 1963). Increasing populations of mice held in the laboratory resulted in the suppression of somatic growth and cur­tailment of reproductive functions in both sexes.

Sexual maturation was delayed or totally inhibited at high population densities, so that in some populations female reached normal sexual maturity. Intrauterine mortality of fetuses increased. Increasing population density resulted in an inadequate lactation and subsequent stunting of nurslings.

An example of a relatively simple psychological factor acting control population size is shown in the fish guppy, Silliman and Gutsell, 1958). When stable experimental populations are maintained by regularly removing a per­centage of the fish, signifi­cant numbers of the young survive, although some may be cannibalized by their par­ents. When no artificial predation is practiced the guppies breed continuously, but they remove the entire bree­ding surplus through canni­balism.

Evidently, the nor­mal control mechanism for guppy populations is predation from other fish higher in the food chain. But if, for any reason this mechanism is lost the population can regulate its own number thro­ugh cannibalism.

Such psychological regu­lation of population has also been studied in albino rat (Calhoun, 1962). When rats were raised in densities higher than normal, they showed psychological abnormalities such as hyper sexuality, homo­sexuality, asexuality, canni­balism, inability of a mother to raise her pups successfully to weaning, increased rate of spontaneous abortion, breakdown in the predating rituals and abnormal crowding around certain places such as food hopper and water fountains. Different biotic factors of the environment thus tend to modify the activities of animals like the abiotic factors.

Inter specific interactions:

An individual in nature interacts not only with others of the same species but with individuals of other species. Many different types of interactions occur when, as in few cases, the presence of one species appears to have no measurable effect on a second species (i.e., no interaction) a state of neutralism is said to exist.

When the interactions result in a benefit for one or both of the species and harm to neither, the results are termed positive interactions. Several degrees of such interactions can be recognize commensalism wherein one specks jains and the second is helped nor harmed; protocooperation, wherein both gain, though each is also able to survive separately; and mutualism, wherein both gain but neither can survive without the other.

Negative inter­actions occur when one of the species is harmed by the relation­ship- Various levels of negative interactions are also recognized : amensalism, wherein one species is harmed by a second, but the latter neither gains nor is harmed by the association; parasitism, wherein one species gains (the parasite) while the second (the host) is harmed but not killed by the association; predation, wherein one species (the predator) kills and feeds on the second (the prey); and competition, wherein each is adversely affected by the presence of the other in their search for food, shelter, living space, or other requirements for existence.