Behaviour of animal
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.
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”
