Animal Behavior - biology.

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Publié le : 11/5/2013 -Format: Document en format HTML protégé

Animal Behavior - biology.
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Animal Behavior - biology.
I

INTRODUCTION

Animal Behavior, the way different kinds of animals behave, which has fascinated inquiring minds since at least the time of Plato and Aristotle. Particularly intriguing has
been the ability of simple creatures to perform complicated tasks--weave a web, build a nest, sing a song, find a home, or capture food--at just the right time with little
or no instruction. Such behavior can be viewed from two quite different perspectives, discussed below: Either animals learn everything they do (from "nurture"), or they
know what to do instinctively (from "nature"). Neither extreme has proven to be correct.

II

NURTURE: THE BEHAVIORISTS

Until recently the dominant United States school in behavioral theory has been behaviorism, whose best-known figures are J. B. Watson and B. F. Skinner. Strict
behaviorists hold that all behavior, even breathing and the circulation of blood, according to Watson, is learned; they believe that animals are, in effect, born as blank
slates upon which chance and experience are to write their messages. Through conditioning, they believe, an animal's behavior is formed. Behaviorists recognize two
sorts of conditioning: classical and operant.
In the late 19th century the Russian physiologist Ivan Pavlov discovered classical conditioning while studying digestion. He found that dogs automatically salivate at the
sight of food--an unconditioned response to an unconditioned stimulus, to use his terminology. If Pavlov always rang a bell when he offered food, the dogs began slowly
to associate this irrelevant (conditioned) stimulus with the food. Eventually the sound of the bell alone could elicit salivation. Hence, the dogs had learned to associate a
certain cue with food. Behaviorists see salivation as a simple reflex behavior, something like the knee-jerk reflex doctors trigger when they tap a patient's knee with a
hammer.
The other category, operant conditioning, works on the principle of punishment or reward. In operant conditioning a rat, for example, is taught to press a bar for food
by first being rewarded for facing the correct end of the cage, next being rewarded only when it stands next to the bar, then only when it touches the bar with its body,
and so on, until the behavior is shaped to suit the task. Behaviorists believe that this sort of trial-and-error learning, combined with the associative learning of Pavlov,
can serve to link any number of reflexes and simple responses into complex chains that depend on whatever cues nature provides. To an extreme behaviorist, then,
animals must learn all the behavioral patterns that they need to know.

III

NATURE: THE ETHOLOGISTS

In contrast, ethology--a discipline that developed in Europe but that now dominates United States studies as well--holds that much of what animals know is innate
(instinctive). A particular species of digger wasp, for example, finds and captures only honey bees. With no previous experience a female wasp will excavate an
elaborate burrow, find a bee, paralyze it with a careful and precise sting to the neck, navigate back to her inconspicuous home, and, when the larder has been stocked
with the correct number of bees, lay an egg on one of them and seal the chamber. The female wasp's entire behavior is designed so that she can function in a single
specialized way. Ethologists believe that this entire behavioral sequence has been programmed into the wasp by its genes at birth and that, in varying degrees, such
patterns of innate guidance may be seen throughout the animal world. Extreme ethologists have even held that all novel behaviors result from maturation--flying in
birds, for example, which requires no learning but is delayed until the chick is strong enough--or imprinting, a kind of automatic memorization discussed below.
The three Nobel Prize-winning founders of ethology--Konrad Lorenz of Austria, Nikolaas Tinbergen of the Netherlands, and Karl von Frisch of West Germany (now part
of the united Federal Republic of Germany)--uncovered four basic strategies by which genetic programming helps direct the lives of animals: sign stimuli (frequently
called releasers), motor programs, drive, and programmed learning (including imprinting).

A

Sign Stimuli (Releasers)

Sign stimuli are cues that enable animals to recognize important objects or individuals when they encounter them for the first time. Baby herring gulls, for example,
must know from the outset to whom they should direct their begging calls and pecks in order to be fed. An adult returning to the nest with food holds its bill downward
and swings it back and forth in front of the chicks. The baby gulls peck at the red spot on the tip of the bill, causing the parent to regurgitate a meal. The young chick's
recognition of a parent is based entirely on the sign stimulus of the bill's vertical line and red spot moving horizontally. A wooden model of the bill works as well as the
real parent; a knitting needle with a spot is more effective than either in getting the chicks to respond.
Sign stimuli need not be visual. The begging call that a chick produces is a releaser for its parents' feeding behavior. The special scent, or pheromone, emitted by
female moths is a sign stimulus that attracts males. Tactile (touch) and even electrical sign stimuli are also known.
The most widespread uses of sign stimuli in the animal world are in communication, hunting, and predator avoidance. The young of most species of snake-hunting birds,
for instance, innately recognize and avoid deadly coral snakes; young fowl and ducklings are born able to recognize and flee from the silhouette of hawks. Similar sign
stimuli are often used in food gathering. The bee-hunting wasp recognizes honey bees by means of a series of releasers: The odor of the bee attracts the wasp upwind;
the sight of any small, dark object guides it to the attack; and, finally, the odor of the object as the wasp prepares to sting determines whether the attack will be
completed.
This use of a series of releasers, one after the other, greatly increases the specificity of what are individually crude and schematic cues; it is a strategy frequently
employed in communication. Most animal species are solitary except when courting and rearing young. To avoid confusion, the signals that identify the sex and species
of an animal's potential mate must be clear and unambiguous. For example, a minnowlike fish called the stickleback uses a system of interlocking releasers to
orchestrate its mating. When its breeding season arrives, the underside of each male turns bright red. This color attracts females but also provokes attacks by other
males; red objects of almost any description will trigger male stickleback aggression. A female responds to the male's red signal with a curious approach posture that
displays her swollen belly full of eggs. This incites the male to perform a zigzag dance that leads the female to the tunnel-like nest he has built. The female struggles
into the nest, whereupon the male touches her tail with his nose and quivers. The resulting vibration causes the female to release her eggs for the male to fertilize. If
the male fails to perform the last part of the ballet, the female will not lay her eggs; vibrating the female with a pencil, however, which she can plainly see is not a male
stickleback, works perfectly well, although the male in this case, not having gone through the last stage of the ritual, refuses to fertilize the eggs and eats them instead.

B

Motor Programs

A second major discovery by ethologists is that many complex behaviors come prepackaged as motor programs--self-contained circuits able to direct the coordinated
movements of many different muscles to accomplish a task. The dancing of sticklebacks, the stinging action of wasps, and the pecking of gull chicks are all motor
programs.

The first motor program analyzed in much detail was the egg-rolling response of geese. When a goose sees an egg outside its nest, it stares at the egg, stretches its
neck until its bill is just on the other side of the egg, and then gently rolls the egg back into the nest. At first glance this seems a thoughtful and intelligent piece of
behavior, but it is a mechanical motor program; almost any smooth, rounded object (the sign stimulus) will release the response. Furthermore, removal of the egg once
the program has begun does not stop the goose from finishing its...


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Animal Behavior - biology.

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