© Copyright all rights reserved Strange traits 2. Flashy males 1. Copulatory Suicide 4. Egg Mimics 3. Effeminate males

© Copyright all rights reserved Sexual Selection and Trait Evolution

© Copyright all rights reserved 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

© Copyright all rights reserved 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. sperm Gestation period- harder to obtain resources Parental care Unequal parental investment females more choosy when selecting a mate © Copyright all rights reserved

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. © Copyright all rights reserved

How can males evolve to obtain more matings? 1. Outcompete other males: For example, direct combat may determine who gets to mate. © Copyright all rights reserved

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 females Female © Copyright all rights reserved

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

© Copyright all rights reserved 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

© Copyright all rights reserved 2. Become more attractive to females. The ultimate sacrifice is when a male sacrifices himself as food. This is called copulatory suicide.

© Copyright all rights reserved 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.

© Copyright all rights reserved 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): e doi: /journal.pone Egg mimics on the anal fin of Astatotilapia burtoni

© Copyright all rights reserved 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): e doi: /journal.pone

© Copyright all rights reserved 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