I really thought I knew what long meant [cough]...until I came across this study on semen displacement and penis morphology published last year in Evolutionary Psychology. Touching lightly on the most sensitive part, it seems that the shape of human penises may have evolved to increase the chances of insemination by functioning--and here's the rub--not as a semen placer but as a semen displacer.
Under conditions in which several males copulate with a female in close temporal proximity to one another the male who mated with the female last would have an advantage, known as last male precedence (see Birkhead, 2000). In the case of humans, the last male to copulate would be in a position to displace the semen left by previous males before inseminating the female with his own semen. This assumes, of course, that other factors such as sperm quality, ejaculate size, penis length, and female control of mating are constant. As yet, little research has been conducted on this topic...At least not at SUNY-Albany. Have you ever thought of going to South Padre Island for Spring Break, doc?
Actually, they do, more or less. Also on the table: testing the implications of circumcision on semen displacement theory. Still woefully under-researched: whether circumsizing your kid will help him get laid.
One last shot:
To test this hypothesis, Gallup, Burch, Zappieri, Parvez, Stockwell, and Davis (2003) simulated sexual encounters using artificial models and measured the magnitude of artificial semen displacement as a function of phallus configuration, depth of thrusting, and semen viscosity.Which gives me one more idea for a research topic: how do Gallup et al's parents explain what their kids are up to in the family Christmas letters?
Semen Displacement as a Sperm Competition Strategy in Humans [human-nature.com via robotwisdom]