Role of Hormones and Pheromones | Zoology Optional Notes for UPSC PDF Download

Hormones and behaviour, Pheromones and behaviour

• Operating alongside the nervous system is a specialised group of organs called endocrine glands. These glands provide another means of communication within the animal's body, via hormones. Hormones are secreted by the glands into the bloodstream in response to specific stimuli. Because they operate via the circulatory system, hormonal messages are much slower than the electrical messages of the nervous system. They also have longer-lasting effects on their target organs, some effects persisting for months.
• In vertebrates, the most important endocrine glands affecting behaviour are the pituitary gland, the gonads and placenta, and the adrenal gland. Hormones of the thyroid and parathyroid glands, pancreas, and gastrointestinal mucosa have no direct behavioural effects.

The pituitary gland

• The pituitary gland is situated under the hypothalamus on the floor of the brain. It plays an important role as the central controller of the endocrine system. The pituitary's anterior and posterior lobes secrete hormones which indirectly affect blood pressure and water absorption (vasopressin), gamete production and the secretion of sex hormones (follicle stimulating hormones, luteinising hormone).
• In response to luteinising hormone (LH), testes secrete male hormones or androgens (testosterone and androsterone). One of these, testosterone, influences the development and maintenance of the male reproductive tract, the formation of secondary sexual characteristics, and various aspects of behaviour, particularly aggression. In females, LH stimulates the ovaries to secrete oestrogens and progesterones. Between them, they perform analogous functions to the androgens in males.

Paired adrenal glands

Paired adrenal glands

Hormonal Actions Affecting Behaviour

The effects of hormones on behaviour can be traced to three major sites of action. These are:

• the nervous system,
• sensory perception, and
• effector organs and structures.

Hormonal Effects on the Nervous System

Hormones affect many aspects of the nervous system including its anatomy, biochemistry, and transmission capabilities. Sexual dimorphism in the anatomy of certain neurons in the rat hypothalamus appears to be mediated, and the maturation of reflex connections within the CNS is accelerated by high levels of thyroxin. Corticosteroid and sex hormones, by virtue of their effect on calcium metabolism, indirectly affect nerve conduction in which calcium ions play an important role. Hormones may actually induce inhibition. Oestrogens inhibit aggressive behaviour in female hamsters. Among invertebrates, the sexual receptivity of female grasshoppers (Orthoptera) is inhibited by hormones.

Hormonal Effects on Sensory Perception

• Many studies suggest that hormones affect an animal's sensory capabilities. In doing so, they alter the animal's perception of its environment and the way it responds to certain stimuli. During the spring, male three-spined sticklebacks migrate from the sea to their freshwater breeding grounds. Migration is brought about by changes in hormonal output by the pituitary and thyroid glands. In particular, thyroxin alters the fishes' salinity preference from salt water to freshwater, thus providing the impetus for migration.
• In many female mammals, sensory perception is influenced by the oestrus cycle. Female rats fluctuate in their ability to detect certain odours according to the levels of oestrogen and progesterone. Similarly, visual sensitivity in female humans varies with the stage of their menstrual cycle. Sensory perception in males is also influenced by hormones. Experienced male rats prefer the odour of urine produced by oestrous females to that produced by females in dioestrous. This preference disappears in castrated males. Androgens produced by the testes, therefore, modify the animals' response to standard olfactory stimuli, although the effect is partly confounded with those of experience.

Hormonal Effects on Effector Organs and Structures

• Animals use a variety of their structural components in the execution of behaviour. Hormones may affect these structures and hence the efficacy of the behaviour in a number of ways. Good examples are the secondary sexual adornments of male birds. These may be just bright plumage or bill colorations, or they may be elaborate and gaudy structures like combs and wattles. Secondary sexual adornments appear to serve a variety of functions including the attraction of mates, the deterrence of sexual rivals, and the declaration of reproductive condition.
• The production and maintenance of these adornments depend on androgen levels. For example, comb and wattle size in newly-hatched male and female domestic chicks can be increased to proportions normally found in sexually mature males by injections of testosterone. Bill pigmentation in many species fluctuates in intensity with the time of year. In both sexes of the house sparrow (Passer domesticus), the bill changes from pale brown to black at the beginning of the breeding season. Removal of the gonads in either sex inhibits the deposition of melanin and results in a cream-colored bill.
• Androgens are also important in determining seasonal changes in plumage patterns. Castrated male ruffs (Philomachus pugnox) and black-headed gulls (Larus ridibundus) fail to assume their characteristic breeding plumage. In red-necked and Wilson's phalaropes (Phalaropusspp.), in which females instead of males acquire brightly coloured breeding plumage, pattern changes are also controlled by androgens.

Developmental Effects of Hormones

• Hormones have profound effects on the development of young animals and impart some characteristic features to their behaviour. At a very direct level, hormones affect embryonic development. Hormone-mediated anatomical and physiological changes are important in the development of sexual behaviour. In guinea pigs, testosterone levels influence the development of the genitalia. Treatment of pregnant females with testosterone proportionate results in female offspring with male-like genitals. In rats, analogous effects occur after birth rather than in utero.
• If female rats are treated with testosterone when about four days old, their oestrous cycle and sexual behaviour as adults are suppressed. The CNS in neonatal rats seems to be relatively undifferentiated as far as sexual behaviour is concerned, although there is a tendency towards characteristically female patterns. It is only by the direct action of testosterone that male behaviour develops. The rat and guinea pig examples illustrate an important characteristic of developmental hormonal effects on behaviour. The effects are exerted during species-specific 'critical periods'.
• The presence of an appropriate hormone during the critical period determines anatomical and behavioural characteristics for life. No amount of therapy with oestrogen and progesterone will revive female sexual behaviour in female rats treated with testosterone during their postnatal critical period. Developmental hormonal effects are therefore irreversible.

Seasonal effects

In red deer (Cervus elephus), the administration of testosterone in winter brings about full rutting behaviour. A similar administration in late spring has no effect at all. Even the animal's past experience can influence hormonal effects. Copulatory actions in castrated male cats tend to be more protracted if they have copulated in the past. Inexperienced animals show reduced copulatory vigour. Clearly, hormonal influences on behaviour are not simple and readily predictable.

Juvenile Hormone

• Juvenile hormone (JH), produced by the corpora allata, regulates two important processes in insects. The first is development; as an insect develops, the degree juvenility of the next stage is determined by the amount of JH in the blood; the lower the JH, the more adult the next stage. Experimental removal of the corpora allata results in the premature development of adult characteristics. JH occurs in several forms; the most common is JH-III. The other function of JH is, in many insects, to regulate the production of eggs in the female's ovaries. Because mating behaviour is often synchronized with the ovarian cycle, it makes sense for mating behaviour and pheromone production to be linked with JH. In some insects, including species of cockroach, this is exactly the case. In other species, the role of JH has evolved one step further so that the linkage with ovarian activity is lost. In the most intensively studied species, the honey bee, many scientists think JH regulates the behavioural activities of workers through their life.
• Aggressiveness of guard bees is correlated with their blood JH levels. Even though guards have high JH levels, their ovaries are relatively undeveloped. JH titers in worker honey bees progressively increase through the first 15 or so days of the worker's life. During this period, workers perform tasks inside the hive, such as nursing larvae, constructing comb, and cleaning cells. JH titers peak around day 15; workers this age guard, remove dead bees from the colony, and fan at the colony entrance to cool the nest. Older workers forage for pollen and nectar. This behavior prevents robbing of honey from the nest by bees from other colonies. It is not an easy trick for the guards to tell which incoming bees to attack and which to not. They do this by smelling odors on the surface of the incoming bees; the bees that do not smell like nestmates are attacked.

Allohormones

• In the case of hormones transferred from one partner to another, advocate the use of the term allohormone, which refers to a substance that induces a direct behavioural response, bypassing sensory structures. Allohormones are distinct from pheromones, which are signaling compounds that are detected by sensory structures and then transferred to salient integrating centers of the central nervous system.
• Allohormones alter target tissues such as the reproductive system, just like hormones, and in many cases, allohormones are derived from the same biosynthetic pathways as hormones produced by the targeted individual. An allohormone is found in the garden snail Helix aspersa, which enhances male fertility. In the dusky salamander, Desmognathus fuscus, the male secretes a substance from the mental gland and then transfers it to the female's back, only after having scraped the female's back raw with specialized maxillary teeth. The substance from the mental gland is directly transferred to the female's bloodstream, and it appears to make the female more receptive.

Pheromones and behaviour

• Pheromones are the molecules used for communication between animals. A broader term for chemicals involved in animal communication is semiochemical. Pheromones are a subclass of semiochemicals, used for communication within the species.
• Pheromones were originally defined as substances secreted to the outside by an individual and received by a second individual of the same species in which they release a specific reaction, for instance, a definite behavior (releaser pheromone) or developmental process (primer pheromone). The word pheromone comes from the Greek pherein, to carry or transfer, and hormon, to excite or stimulate. The action of pheromones between individuals is contrasted with the action of hormones as internal signals within an individual organism. Pheromones are often divided by function, for example, into sex pheromones and aggregation pheromones.
• Bombykol, a substance secreted by female silk moth, attracts the male moth for reproduction. One of the best-known pheromones is bombykol, the sex-attractant of the silk moth. A receptive female emits a small quantity of bombykol molecules into the air and sits tight. The molecules drift downwind where they are detected by a wandering male. Because of the extraordinary sensitivity of the male's antennal sense cells, he needs only 200 molecules to strike the antenna within a second to be able to orientate and home in on the female. Unlike the male, the female silk moth is insensitive to bombykol. Her antennae are small and thin compared with the male's elaborate, feathery structures.
• Other well-known pheromones include the complex scented secretions of mammals. Urine and faeces, as well as specialized scent glands, may be used to mark objects, conspecific territory boundaries, or even to scent the air. Species, sex, age, motivational state, may all be coded in various secretions. Semiochemicals acting between individuals from different species are called allelochemicals and are further divided depending on the costs and benefits to signaler and receiver. Pheromone signals can be over smelled by unintended recipients: for example, the predatory beetles use the pheromone of their bark beetle prey to locate them.
• The predators are using the bark beetle pheromones as kairomones. Kairomones are a substance released by one species that benefits members of another. Animals of one species can emit signals that benefit themselves at the cost of the receiving species. Chemical signals used in such deceit (misleading) or propaganda are termed allomones. For example, bolas spiders synthesize particular moth pheromones to lure male moths of those species into range for capture. Semiochemicals benefiting both signaler and receiver in mutualisms, such as those between sea anemones and anemone fish (clownfish), are termed synomones.