Evolution of Reproductive Behaviors

Differences in Sex Roles In general, males court, females choose Video clip

Different styles of bowers

Advantage to males? Convey information about mate quality Advertising Female choice supports this hypothesis But are males that build superior bowers actually superior mates? How to test?

Data on male quality Better bowers= Fewer parasites More disease resistance genes— less able to be infected by lice Bigger brain size (skull comparison bower vs no bower building species)

Discussion Question Females favor robust and aggressive courtship displays, but often flinch or act jumpy in the presence of these displays. Ultimately, the female crouching down is an indication she is likely to mate with the male eventually. Borgia’s lab has proposed that the bower originated to offer the female protection against forced copulation by displaying males. How does the evidence support this hypothesis? How does the male benefit by producing such an elaborate structure that actually might counteract his attempts to mate? Can you predict how a male would react to a robotic female that remained standing, crouched once or twice, or crouched frequently to his display?

Fundamental sex differences Female bower birds spend weeks viewing bowers and courtship displays before settling on a mate, whom she will likely never see again after the copulation.

Male success is dependent on bower quality, and therefore uneven Why is it that males build and females choose? That males chase many mates and females are often content with one or a very few carefully chosen matings.

Sperm vs. Egg Hamster gametes Egg: few produced, huge energy investment, may take up substantial % mass of the female Sperm: Millions to billions produced, far less mass than egg Human example: one man, all eggs So eggs are limiting—sets up competition between sperm to fertilize them. More mates=more eggs fertilized=more genes passed on=more fitness Hamster gametes

Strategies for optimizing fitness Distinguish between new and familiar partners Anolis lizards exhibit far more courtship toward strangers than known companions MALE: Make sure fertilized eggs survive and thrive FEMALE:

Parental Investment Male frog carries tadpoles on his back Male katydid gives female edible spermatophore with nutrients for the eggs and herself Female duck carries babies on her back, protecting them from would be predators Female wasp drags a paralyzed cicada back to the nest to feed her young

Parental investment Good for females? Bad for males? Offspring extremely likely to carry her genes Little incentive to mate again, means greater ratio of receptive males:receptive females Good for females? Paternity uncertain Fertilize as many eggs as possible Greater competition the more females are mated Bad for males? Male biased operational sex ratio (more receptive males than females in a population)

Optimization of the process Sperm optimized for good fertilization ability Fast, numerous, small “number of offspring” strategy Eggs optimized for good development ability Large, immobile, nutrient rich “parental investment” strategy No one gamete could be equally good at both tasks

Sex role differences combine with gamete size differences so that two paths are selected for. Typically males follow the left path, females the right, but there are exceptions…

Sex Role reversal Males are out getting food which they present to the female of their choosing—based on her wing and leg decorations—prior to mating. Females compete for the males and these food gifts.

The nuptial gift Males may reject the mountings of 3.2 gram female katydids in favor of stronger 3.5 gram females which will net them about 50% more eggs fertilized. Since males can probably only produce one nuptial gift in their lifetime, it makes sense for them to be choosy about the female they inseminate. If conditions cause the operational sex ratio to change within a single breeding season, the competition/sex roles can change in accordance.

Pipe Fish: pregnant males Published online 17 March 2010 | Nature | doi:10.1038/news. 2010.127 Male pipefish abort embryos of ugly mothers