© copyright 2015- all rights reserved strange traits 2. flashy males 1. copulatory suicide 4. egg...

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Strange traits2. Flashy males

1. Copulatory Suicide

4. Egg Mimics

3. Effeminate males

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Sexual Selection and Trait Evolution

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Male

Female

The presence of some traits is puzzling.

- Some traits cannot have evolved by natural selection, because they actually decrease survival.

- Others are clearly different between males and females

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Sexual selection: selection on an organism’s ability to obtain mates

When does sexual selection occur?

Reproduction can be more costly for one sex (usually females):Metabolic cost of eggs vs. spermGestation period- harder to obtain resourcesParental care

Unequal parental investment females more choosy when selecting a mate© Copyright 2015- all rights reserved www.cpalms.org

American pronghorns:females are picky when choosing a mate

Females often travel great distances before choosing a mate.

When females take the time to choose attractive males, their offspring grow faster and better survive the winter.

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How can males evolve to obtain more matings?

1. Outcompete other males: For example, direct combat may determine who gets to mate.

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How can males evolve to obtain more matings?

1. Outcompete other males: Or, an alternative “sneaker” male strategy may evolve. For example, in the isopod Paracerceis sculpta:

These male isopods are large and aggressively try to monopolize mating with groups of females.

These males mimic females and go undetected by the larger, aggressive males

These males are so small that they sneak by the large males and mate with the femalesFemale

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Alternative mating strategies – males have multiple morphologies and/or behaviors

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2. Become more attractive to females.

One way to do this is to provide direct benefits to females in the form of food or other resources

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2. Become more attractive to females.

The ultimate sacrifice is when a male sacrifices himself as food. This is called copulatory suicide.

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2. Become more attractive to females.

Good genes – males can also advertise that they have good genes through their traits or behaviors. Females choose healthier-looking males that should produce healthier offspring.

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2. Become more attractive to females.

Sensory bias – males can display something that females are naturally attracted to, even if it doesn’t indicate anything about that male.

Theis A, Salzburger W, Egger B (2012) The Function of Anal Fin Egg-Spots in the Cichlid Fish Astatotilapia burtoni. PLoS ONE 7(1): e29878. doi:10.1371/journal.pone.0029878http://www.plosone.org/article/info:doi/10.1371/journal.pone.0029878

Egg mimics on the anal fin of Astatotilapia burtoni

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2. Become more attractive to females.

Sensory bias – males can display something that females are naturally attracted to, even if it doesn’t indicate anything about that male.

Theis A, Salzburger W, Egger B (2012) The Function of Anal Fin Egg-Spots in the Cichlid Fish Astatotilapia burtoni. PLoS ONE 7(1): e29878. doi:10.1371/journal.pone.0029878http://www.plosone.org/article/info:doi/10.1371/journal.pone.0029878

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Sexual selection can explain the evolution of many traits that are seemingly not adaptive but increase mating success.

There are several different strategies that animals may evolve to get more mates.

Good genes

Direct benefits Alternative mating strategies

Sensory bias