Pheromones Represent

PHEROMONES

In recent years it has been appreciated that the behavior of animals may be influenced not only by hormones (chemicals released I nto the internal environment by endocrine glands that regulate and coordinate the activities of other tissues) but also by pheromones-substances released into the external environment by exocrine glands that influence the behavior of other members of the same species. We are accustomed to thinking that information can be transferred from one animal to another by sight or sound ; pheromones represent a means of transferring information, by smell or taste. Pheromones evoke specific behavioral, developmental or reproductive responses in the recipient; these responses may be of great significance for the survival of the species.

          Some pheromones act on the recipient’s central nervous system and produce an immediate effect on its behavior. Among these releaser pheromones are the sex attractants of moths and the trail pheromones and alarm substances secreted by ants. Other primer pheromones trigger a chain of physiological events in the recipient that affect its growth and differentiation. The regulation of the growth of locust and the control of the number of reproductives and soldiers in termite colonies are controlled by primer hormones.

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          The sex attractants of moths provide some of the more spectacular examples of pheromones. The male silk moth has an extremely sensitive receptor in his antennae for sensing the attractant. When an investigator records the nerve impulses coming from the antennae, he finds that these electroantennagrams show specific responses to bombykol and not to other substances. The male silk moth cannot determine the direction of the source by flying up a concentration gradient because the molecules are nearly uniformly dispersed except within a few meters of the source. Instead, he responds to the stimulus by flying up-wind to the source. With a gentle wind, the bombykol given off by a single female moth covers an area several thousand meters long and as much as 200 meters wide. An average silkworm contains some 0.01 mg of bombykol. It can be shown experimentally that male responds appropriately when as little as 10,000 molecules of attractant are allowed to diffuse from a source 1 cm away from him. He can have received only a few hundred of these molecules, perhaps less. Thus, the amount of attractant in one female moth could stimulate more than one billion males !.

          Sex attractants have been tested as possible specific insecticides. By putting sex attractant on stakes placed every 10 meters in a large field, investigators could blanket the air with sex attractant, thus confusing the males and greatly decreasing the probability of their finding females and mating with them.

          The fire ants, when returning to the nest after finding food, secrete a trail pheromone that marks the trail so that other ants can find their way to the food. The trail pheromone is volatile and evaporates within two minutes, so that there is little danger of ants being misled by old trails. Ants also release alarm substances when disturbed, and this (rather like ringing the bell in a firehouse) in turn transmits the alarm to ants in the vicinity. These alarm substances have a lower molecular weight than the sex attractants and are less specific, so that members of several different species respond to the same alarm substance.

          Worker bees, on finding food, secrete geraniol, a 10-carbon, branched chain alcohol, in order to attract other worker bees to the food. This supplements the information conveyed by their wagging dance. Queen bees secrete 9-ketodecanoic acid, which, when ingested by worker bees, inhibits the development of their ovaries and their ability to make royal cells in which new queens might be reared. This substances also serves as a sex attractant to male bees during the queen’s nuptial flight.

          In colonial insects, such as ants, bees, and termites, pheromones play an important role in regulating and coordinating the composition and activities of the population. A termite colony includes morphologically distinct queen, king, soldiers, and nymphs or workers. All develop from fertilized eggs. However, queen, king, and soldiers each secrete inhibitory pheromones that act on the corpus allaturm of the nymphs and prevent their developing into the more specialized types. If the queen dies, there is no longer any “antiqueen” pheromone released and one or more of the nymphs develop into queens. The members of each colony will permit only one queen to survive, devouring any excess ones. Similarly, the loss of the king termite or a reduction in the number of soldiers permits other nymphs to develop into the specialized castes to replace them.

          Primer pheromones occur in mammals as well as in insects. When four or more female mice are placed in a cage, there is a greatly increased frequently of pseudopregnancy. If their olfactory bulbs are removed, this effect disappears. When more females are placed together in a cage, their estrous cycles become very erratic. However, if one male mouse is placed in the cage, his odor can initiate and synchronize the estrous cycles of all the females (the “whitten effect”) and reduce the frequency of reproductive abnormalities. Even more curious is the finding (the “bruce effect”) that the odor of a strange male will block pregnancy in a newly impregnated female mouse.

Sumber : Buku Pembelajaran Hor to prepare Toefl  “About Little Animals”