Reproduction
Chapter 7

Sexual Selection
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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 Con
Assure fertility disease/parasite
Good genes predation
Exposure to
Risk of
Genetic compatibility Energy expenditure
Resources
Many females show higher 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