The form intraspecific aggression takes is determined in large part by the
relative risks and potential benefits of the aggressive encounter. Some animals
such as male sea elephants will fight to the death over possession of a breeding
harem because defeat is equivalent to genetic suicide. Male American elk, on
the other hand, engage in pushing contests with their antlers. When one becomes
tired he retreats from the contest with the prospect that next year's mating
season may bring better results. The antlers are not used to stab or wound and
are shed at the end of the mating season. Most intraspecific aggression is of
this sort and does not result in significant bodily harm. Because combat is
dangerous and can lead to serious injury or death, evolutionary mechanisms tend
to restrain the intensity of intraspecific aggressive behavior. One way aggression
is minimized is through a genetically programmed tendency to establish territories.
In this way conflicts are usually limited to occasional border skirmishes.
The ritualization of aggressive behavior is another genetically programmed restraint
on combat. Poisonous snakes wrestle without using their fangs; mountain sheep
butt with well-armored heads without inflicting injury; some lizards make threat
displays by expanding a skin fold in the throat; and apes shake branches, gesture,
and shout fearsomely. The advantage of ritualization of combat is that even
a sure winner has a great deal to lose if it becomes debilitated as a result
of a fight. An injured or exhausted victorious animal may not be able to defeat
the next opponent and may become vulnerable to predation by other species. Therefore,
most species have clear signals that indicate acceptance of defeat and terminate
combat before injury occurs: A lizard will crouch; a cichlid fish will retract
its fins; a stickleback will adopt a vertical posture; a dog will expose its
unprotected belly; and a gull will offer the unguarded back of its neck to its
opponent. Each of these maneuvers signals acceptance of defeat and immediately
halts further aggression.
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