Greying Researcher@Australian National University Australian National University Research School of Biology Evolution, Ecology & Genetics Building 116 Daley Road, Acton Canberra, 0200 Australia Website
Compelling evidence from many animal taxa indicates that male genitalia are often under post-copulatory sexual selection for characteristics that increase a male’s relative fertilization success under sperm competition. There could, however, also be direct pre-copulatory female mate choice based on male genital traits. Before clothing, the non-retractable human penis would have been conspicuous to potential mates. This, in combination with claims that humans have a large penis for their body size compared to other primates, has generated suggestions that human penis size partly evolved due to female choice. We presented women with digitally projected fully life-size, computer-generated animations of male figures to quantify the (interactive) effects of penis size, body shape and height on female assessment of male sexual attractiveness. We generated 343 male figures that each had one of seven possible values for each of the three test traits (7x7x7 = 343). All seven test values per trait were within two standard deviations of the mean based on a representative sample of males. We calculate response (fitness) surfaces based on the average attractiveness rank each of the 343 male figure received. We also calculated individual response surfaces for 105 women (each women viewed 53 figures). Both methods yielded almost identical results. We discuss our finding in the context of previous studies that have taken a univariate approach to quantify female preferences. We discuss the hypothesis that pre-copulatory sexual selection might play a role in the evolution of genital traits.
Why do some female marine animals release their eggs into the water to be fertilised, while others retain them on, or inside, their bodies? Why do terrestrial plants disperse pollen but not ovules, and why do most terrestrial animals hold onto both sperm and eggs until they find a mate? Sex role research has largely overlooked the significance of where gametes are located when fertilisation occurs. Parent-gamete proximity during fertilisation is not only a basic difference between the sexes in many species; it is also essential to the evolution of parental care and most forms of mate choice. I explore the evolutionary logic behind gamete release and retention using mathematical models. I focus particularly on marine invertebrates, in which evolutionary changes in these behaviours are common. Along the way, I provide a new explanation for why egg retention, small body size, and large egg size are correlated in sessile marine invertebrates. I also explain why there are no species in which males retain their gametes while females release them.