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Reproduction
Chapter 7
Sexual Selection

Darwin's theory to explain traits that aren't
obviously advantageous
https://www.youtube.com/watch?v=LL30QtTSz9U
Bower Birds
Bower Birds
She builds the nest, cares for the nestlings and he does….?
What does it mean, to build a
bower?
Females prefer neat bowers.
Hypothesis: These are build by superior males.
Evidence from parasites, brain studies/cerebellum
(conflicting)
Consider the investment in gametes
Gamete: egg or sperm
Female: few
Male: many
Creates competition for access to female gametes
Reproductive Strategies
Female: “few high cost gametes means invest in
parenting those offspring to increase my
fitness”.
Male: “many, cheap gametes means invest in
getting lots of mating opportunities to increase
my fitness”.
Evidence:
Reproductive Success of
Bowerbirds
Therefore, males should not provide
anything beyond gametes!
but...
Operational Sex Ratio
The number of sexually active males: the number
of sexually active females.
The ratio indicates which sex selects a mate
The ratio can change, such as….
Nuptial Gifts
Remember the balloon flies?
Lesser gifts, ratio is high, she selects.
Valuable gifts, ratio is low, he selects.
Why do males chose female dancing
flies with large abdomens?
The ratio can vary over time
Sexual selection is more than mate
choice.
Competition among individuals seeking access,
usually male-male competition.
Look at body size and weapons, correlation with
mating success. Dominance hierarchies.
And if you aren't the biggest/ best?
Make friends with females (baboons)
Make friends with other males (coalition, baboons)
Brief encounters (iguanas)
Satellites (crickets, toads)
Forced copulation
Males can alter these strategies over time.
Are all strategies flexible?
No. Genetic populations of Ruff can be:1 Territory
holder, 2 Satellite, or 3 Female mimic
How do these differences persist?
Must have similar reproductive success if
differences are genetic.
Gamma, Beta and Alpha
Variation among the strategies
depending on conditions in the
sponge.
Beyond direct competition: sperm
competition
Damselfly males remove most of the sperm from
earlier matings.
Female hens can expel semen.
Mate Guarding
Physical barriers: orb web
weaver dies in the female.
Temporal: expensive (why?)
Does it work?
Seychelles warblers and
EPCs.
So, mate choice
What does a female (usually) want?
Resources: territory, nuptial gifts, (spiders)
Paternal investment: blue tits and carotenoids
A “good” quality male: no material benefit
What makes a male good?
1 Healthy and won't make her sick
2 Good genes will benefit her fitness
3 Runaway Selection or “sexy sons”
4 Chase-away selection-no benefit
Healthy mate
Male bowerbirds and high-quality bowers, fewer
ectoparasites
Good genes
Plumage as an honest signal of health
Likely a strong, genetic component to health
Found positive correlation between plumage
brightness and parasite loads across species.
What does this mean?
Runaway Selection
Female choice drives male ornamentation
Links genes for choice with genes for ornament
Goes beyond an honest signal
So, which one?
Chick survival
Conflict between the sexes
Drosophila transfer proteins with sperm that
increase his success and decrease hers!
Females choose large males but they make more
of the proteins and lower her success further.
An arms race! Possibly chase-away selection
Mating Systems-Chapter 8
1 Monogamy
2 Polyandry
3 Polygyny
And the many combinations within!
Why should a male be
monogamous?
1 extension of guarding, little chance of another
mating
2 mate-assistance, big increase to fitness, gryllus
crickets
3 male needed to have any success, seahorse
4 female-enforced, burying beetles
Not common in mammals
More common in birds
Having both parents increases nestling survival in
many birds
But…
90% of bird species studied show EPC.
So…
He might be raising babies
who aren't his!
Explain microsatellite analysis.
Polyandry
High, male-biased sex ratio, females with
territories are rare and can attract multiple
males. Spotted Sandpipers
Female can lay more eggs
Ratio favors males
Locally rich food supply
No fitness effort of 2 parents
Why do females seek additional
matings?
Pro
Assure fertility
disease/parasite
Good genes
predation
Con
Exposure to
Risk of
Genetic compatibility Energy expenditure
Many females show higher
Resources
fitness with EPC!
More caregivers
Male protection
Reduced infanticide
Polygyny
How do you find lots of females?
Female-defense: find the females, guard them
Resource-defense: defend territory with resources
Lek: defend a display territory
Scramble competition: try to find and guard a
receptive female.
Lots of variation in male success
Lek
Males gather, display and few get most of the
matings.
Why would this occur?
Lek
Females are drawn to the location, not
defensible= hotspot hypothesis
Males are drawn to successful males to cash in =
hotshot hypothesis
Females gather to compare males = female
preference hypothesis