General Ecology
Life History Strategies
We are interested in a few
basic questions.
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When should you reproduce?
How often should you reproduce?
With whom should you reproduce?
How much energy should you invest in
reproduction / offspring?
To answer these questions we need
to understand our ultimate objective.
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Maximize inclusive fitness.
If evolution / natural selection
works, we expect:
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Organisms should behave
reproductively in a way that maximizes
inclusive fitness.
Do organisms so this?
We can make a basic
observation:
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Parental care and fecundity are
inversely related.
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Organisms that invest a lot of energy in
their offspring, generally have fewer
offspring. Think of elephants.
Organisms that invest very little energy in
their offspring, generally have very many
offspring. Think of flies or rabbits.
What are the consequences of
the investment strategy?
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Offspring that receive little parental
investment generally have a poor
probability of survival.
Offspring that receive a great deal of
parental investment, generally have a
high probability of survival.
Again, think of bunnies and elephants.
Which is better?
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This depends on the availability of
resources (as well as other factors).
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In general, if you supply organisms with
more resources, they invest more in their
offspring, and offspring survival increases.
Similarly, if you reduce the availability of
resources, they invest less in their offspring
and offspring survival decreases.
Compare Peromyscus truei
and Dipodomys panamintinus.
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The pinyon mouse lives on average
about 6 mo. It produces litters of about
5 or 6. The young are weaned within a
few weeks, and leave the nest in about
4 weeks.
Panamint K-rats live about 8 years, they
produce 2 offspring, sometimes they
delay reproduction, and the offspring
don’t leave the nest for months.
Are there other differences?
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K-rats have remarkable hearing and
easily detect snakes and owls. They
are saltatorial, and if confronted by a
predator, accelerate rapidly.
Pinyon mice have no special predator
avoidance techniques.
What are the implications?
WITH WHOM SHOULD YOU
REPRODUCE?
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How do birds select mates?
How do lizards select mates?
How do fish select mates?
How do deer select mates?
Notice, in each case, females evaluate
some quality in the male. Mate choice
is for the most part, practiced by the
female. Why?
When should you reproduce?
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Does it make a difference?
Does reproduction early in life influence
reproduction later in life?
In terms of inclusive fitness, is it better
to delay reproduction, or as the saying
goes, vote early and vote often?
Consider green wood-hoopoes in Africa.
When to reproduce
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If individuals engaging prematurely in
sexual competition lose out to larger,
more experienced competitors in
addition to suffering a reduction in their
liklihood of future reproductive success,
then sexual bimaturism will be
selected…
David Barash.
How aggressive should a male
be?
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Male aggression is not just a matter of
testosterone.
Is there a cost and or benefit to
aggression?
When should parents terminate
parental investment?
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First, what is parent – offspring conflict?
Is a cost/benefit analysis possible?
Are humans different?
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Do we fit the patterns?
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Do we exhibit ornamentation?
Does the ornamentation advertise quality?
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Permanent breasts?
Pubic and facial hair?
Male pattern baldness?
Does size matter?
Who selects whom?
What happens when the
environment is unpredictable?
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Do environmental fluctuations influence
juvenile survival or parental survival?
If the fluctuations reduce juvenile
survival probabilities, what should the
parent do?
If the fluctuation reduce parental
survival, what should the parents do?
What kind of mating system
should be used?
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Monogamy?
Polygyny?
Polyandry?
Promiscuity?
Which System is Best?
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How do we decide?
What is best for females?
What is best for males?
Monogamy
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1 male and 1 female.
Pair bonds may be permanent or serial.
What environmental conditions favor
monogamy?
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Think about distribution of resources.
How about energy required to rear young?
Would you expect altricial or precocial
young?
How common is monogamy in primates?
Monogamy
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Selection must favor an equalization of
reproductive effort. Monogamy is the
derived condition.
Note: most birds are monogamous.
Why?
Polygamy
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There are 2 forms of polygamy:
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Polygyny
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Polyandry
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1 male and several females.
1 female and several males.
Is polygyny a good system for males?
Is polygyny a good system for females?
Conditions for Polygamy
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Local or seasonal superabundance of
food.
Risk of heavy predation.
Precocial young.
Sexual bimaturism.
Nested territories due to niche division
between sexes.
Superabundance of food
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If food is superabundant, female can
rear young by herself and male is free
to pursue other copulations.
If male has high quality territory, it may
be advantageous to female to join the
harem of this male, rather than pair
bond with a single male with a low
quality territory. This is the polygyny
threshold.
Risk of heavy predation
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Heavy predation of territorial animals
favors monogamy, assuming parents
can ward off predators.
Precocial young
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If female can guide young to best
feeding areas, safe from predation,
male is free to pursue other mates.
Sexual bimaturation &
extended longevity
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If courting sex is long-lived, it can delay
reproduction to gain dominance.
Dominance can provide courting male
with access to enough females fo
compensate for delayed maturation.
Nested territories
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Males and females have different
niches, but male niche/territory includes
several female feeding territories.
If female is capable of caring for young,
male is free to pursue other females.
Polygyny
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What kind of males are found in
polygynous systems?
Are these high quality males?
What constitutes high quality?
Do females select high quality?
What are the costs and benefits of
polygyny for females?
Polygyny
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Fundamentally, organisms are
polygamous. To be otherwise requires
something special.
Males and females rarely have equal
reproductive success, reproductive
effort, or variance of reproductive
success.
Group size
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In some sense, maintenance of a harem
is the ultimate in group sociality.
Think about non-reproductive groups.
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What influences group size?
What is necessary (in terms of behavior) to
maintain groups?
Are you better off alone, or in a group?