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Well, I decided to get a little more information about animal
aggression.  I knew, really, that the idea that "animals never
kill" is outdated.  Here's some further info, from the
Encyclopedia Americana (1989) and Academic American Encyclopedia (1981).

Instinct is unlearned, inherited and species-common.  Closed
instincts cannot be modified.  Open instincts can be modified.
An example of a closed instinct is that certain fish and birds
will respond aggressively to a red belly or chest and that
behavior cannot be extinguished.
An example of an open instinct is nest-building in some birds.
The instinct is to pick up any material to build the nest.  But as
the bird learns, it ignores materials in the area that did not
work well and picks up only those materials that have worked well.

Aggression and territoriality are not necessarily related.  Some
advantages of territoriality:
 1. Territorial species will have flexible dimensions of each
individual's or group's territory, depending on the amount of food
available each year.  These species have less fluctuation in
population densities than non-territorial animals.
 2. Being more spread out lends protection against diseases and
parasites.
 3. Group defense cooperation leads to social organization.

In lower animals, aggression is inherited and laboratory rats can
be bred to be more docile or hostile.  In higher animals,
including humans, learning plays a role in aggression-looking
behavior.

13 types of aggression have been identified and each is elicited
by different stimuli and each is associated with different neural 
and endocrine bases: 
 1. predatory - in response to presence of prey
 2. anti-predatory - in response to presence of predator
 3. territorial - in response to intruders
 4. dominance - in response to challenge to rank or possessions
 5. maternal - in response to threat to offspring
 6. weaning
 7. parental discipline
 8. sexual
 9. sex-related
10. intermale - in response to presence of competitor
11. fear-induced - in response to being confined or cornered
12. irritable - in response to signals such as red belly
13. instrumental - change in environment making any of above more likely

On the one hand, the encyclopedia says that threats of aggression
and withdrawal are much more common than actual fighting.  On the
other hand, langurs will kill the offspring of other males.  Also,
male lions will go as far as to kill one another when they are
fighting for possession of a pride, and if the challenger wins, he
might kill the cubs sired by the deposed male.

The encyclopedia points out that a certain amount of aggression
can give a breeding advantage to individuals, which then passes on
the level of aggression to the offspring.  On the other hand, too
much energy spent in aggressive behavior can be a disadvantage,
for instance, in the case of birds whose time would be better
spent caring for the young.  Hence, a balance is maintained for
each species and probably for a particular environment.

Well, this is grisly stuff, as Mickey said.  Personally, I think
humans can do better than nature in the sense of putting justice
and compassion into practice.  Nature is competitive.  Sorry if
this is a depressing topic.

Then again, how do we know that the chimps, gorillas and lions
that killed did not feel unusual pressures due to the presence
of humans?  If lions and apes have been killing all along, why
didn't someone notice it sooner?  Weren't the "Born Free" people
studying lions for a long time (before they got murdered)?

There are certain mechanisms I've heard of that take effect under
crowded conditions.  Lemmings go nuts and run into the sea in
their panic.  Mice will have spontaneous abortions or the males
may eat the offspring, I seem to recall.  Guppies will eat
newborns under crowded conditions, I've heard.  And as Barbara
said, the alligators can afford to eat the young because there are
so many compared to the area available.

I'm not sure where these mechanisms would fit into the 13 types of
aggression above.  "Instrumental", I guess.  Apologies if this is
too depressing or grisly.
-- 
Krista A.
HONOR Our Neighbors' Original Rights!