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Lecture 11

ANTHROP 3HI3 Lecture Notes - Lecture 11: Coevolution, Parasitism, Evolutionarily Stable Strategy

Course Code
Priscilla Medeiros

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Chapter 5
1. How do individuals interact with other species? Compare and contrast
predator/prey interactions, host parasite interactions and competitors for
the same resource for type of aggression, relative generation time, and
To a survival machine another survival machine (that is not its own child or
close relative) is part of the environment, similar to a rock or river
o It is something that can be exploited
o But other survival machines can fight back
o Interact directly; predator hunts prey
Aggression: Direct?
Relative generation time:
2. How do between species interactions differ from within species
Between species and within species interactions differ in terms of mating,
habitats, interaction for food, predator/prey interaction, etc.
BS interactions occur a lot less than WS
BS interactions are more indirect
WS interactions are more direct
In some cases, survival machines impinge very little on each other’s lives
Do not eat same thing/mate w. each other/compete for
They may compete for something indirectly (ex; earthworms);
so you will never directly see a mole and blackbird engaging in
a direct tug-of-war with a worm, but if you wiped out the
population of moles, this could have a dramatic effect on
Other times, survival machines could definitely influence each other
Lions want to eat an antelope but an antelope has other plans
for its own body. This is not usually regarded as competition for
a resource, but logically, it is. The resource is ‘meat’ lions
want it for food and antelopes want it to house muscles and or
their organs. The two uses for this resource are mutually
incompatible, and so, there is a conflict of interest.
Survival machines of the same species have a more direct effect on each
other’s lives for many reasons:
1. Half of the population of one’s own species may be potential mates,
and potentially hard-working and exploitable parents to one’s children
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2. Members of the same species are very similar to each other and
preserve genes in the same place with the same way of life, so they
are direct competitors for all resources necessary for life (ex; food).
a. Ex; a blackbird is a larger competition to another blackbird than
a mole
b. If the two blackbirds are members of the same sex, they will
also compete for mating partners; usually males compete with
each other for females
o Logically, a survival machine should murder its rivals and then,
preferably eat them; murder and cannibalism do occur in nature but
they are not as common as a naïve interpretation of TSG theory
3. Why should an individual refrain from a fight to the death?
The decision to fight is a cost-benefit analysis: does the rival hold a valuable
enough resource? Is the attacker strong enough? Does the attacker have
enough time and energy? If the answer to these questions is ‘no’, an
individual will likely refrain from a fight to the death.
1) Requires time and energy
2) If you, A, have rivals B and C, it might make more sense to allow B and C
to fight to the death instead of getting involved
o EXAMPLE; If B and C are both my rivals, and I meet B, it might be
sensible as a selfish individual to kill him. But C is also my rival, and C
is also B’s rival. So by killing B, I am potentially doing a favor to C by
removing one of his rivals as well. It might be better to let B live so
that B and C can engage in combat instead. The moral of this is that
there is no obvious merit in trying to kill rivals. In a large system of
rivals, it doesn’t necessarily do any good to eliminate one or two rivals
from the scene other rivals may be more likely to benefit from this
3) You don’t know what repercussions it will have
o EXAMPLE; You exterminate an agricultural pest and find that a
different pest actually benefits from the extermination more than
human agriculture does
4. How long should an individual continue to fight?
Assume ‘symmetric contest’ – we have assumed that contestants are identical
in all respects except fighting strategy
No species will spend an infinite amount of time threatening another
individual. Time is the currency of this two-bidder auction: whichever rival
needs time to do other things will ultimately lose. A mutant who is willing to
spend more time on a fight will always win, so a fixed time limit isn’t a
feasible or plausible strategy.
OVERALL, an individual should continue to fight until their rival backs
down, OR when they have spent more time than the resource is worth
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o Assume that all individuals knew how much time a kind of resource
was worth (ex; a territory is worth 5 minutes, a mate is worth 10
minutes, etc.)
o Selection would favor a progressive extension of giving-up time
until it once more approached the maximum allowed by the
true economic worth of the resource under dispute
o ESS: Each individual goes on for an UNPREDICTABLE amount of time,
but averaging the true value of the resource
o A form of combat in which all disputes are settled by posturing; a
contest always ends in one rival backing down
o A strategy of giving up and not wasting time would be optimal since
time is such a valuable currency, but as a result, individuals who DO
wait until the end would get some kind of benefit
EXAMPLE; Suppose the resource is worth 5 minutes of display
o At the ESS, an individual can go for 5, +5, or -5 minutes, but the
important thing is that the opponent will not know how long his rival
will go on for
o In the ‘war of attrition’ that individuals should not let their rivals know
when they are going to give up
o Ex; if the flick of a whisker was a way to tell that the rival would
retreat within 1 minute, the ESS for the opponent would be “if your
opponent’s whiskers flicker, wait one more minute, regardless of your
previous plans re: giving up. Never flick your own whiskers”
So natural selection would penalize whisker flicking and cause
evolution of the poker face
5. When are fights to the death most likely to occur?
Fights to the deaths are most likely to occur when the other individual has
something valuable, AND when the attacker is strong enough/has enough
energy to endure the fight
If B is an elephant seal with a large harem full of females, and
if another elephant seal can acquire his harem by killing him,
he might attempt to
This is a very valuable resource
6. When should fighting be delayed?
Fighting should be delayed when an individual’s cost-benefit analysis of
fighting his rival yields the costs outweighing the benefits
o If B is an elephant seal with a large harem full of females, and if
another elephant seal can acquire his harem by killing him, he might
attempt to
o But there are costs and risks even in selective pugnacity; it is to B’s
advantage to fight back; if I start a fight, I am just as likely or more so
to end up dead
o But he holds a valuable resource. Why does he hold it? Perhaps he
won it in combat and beaten off other challengers; even if I win the
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