The odd scoop shaped penis of primates has been the object of fascination by biologists for ages. Why is it that the head of the penis is shaped the way it is?
A background in cuckoldry
Turns out that cuckoldry has been a part of human evolution for as long as humans have existed. Two of the top ten reported sexual fantasies of females is having sex with multiple males at the same time, and having sex with males other than their partner.
This act of double-mating appears to be a real outcome as well. 17.5% of females that have had fewer than 50 copulations reported that they had engaged in double-mating at least once. The figure seems to increase steeply with sexual experience. 71.8% of females that have had more than a 1000 copulations, also reported having double-mated at least once.
Naturally, some might say, this leads to a lot of offspring genetically unrelated to their social father. Current estimates are that 3.1% of children in Western countries are genetically unrelated to their social father.
The tendency for females to double-mate has led to the need for males to adapt and develop sperm competition tactics, and anti-cuckoldry tactics.
Semen displacement theory
Current evidence suggests that one of the anti-cuckoldry tactics is the ability to displace semen within the female's reproductive tract with their own. Using artificial human penises (realistic dildos essentially), and artificial reproductive tracts (fleshlights), scientists were able to provide evidence that the human penis may be able to displace semen within the female. Combining the overall scoop-shape and deep thrusting, the scoop-shaped penises could remove a rivals sperm far more efficiently than non-scoop-shaped penises.
The results seems staggering. Penises with a mushroom-like appearance displaced upwards of 90% of the sperm already within the artificial female, as opposed to only around 30% in the smooth penis. Being able to scrape out a rival male's sperm and replace it with your own seems to be one of the significant anti-cuckoldry and sperm competition tactics employed by males.
Sources
DeLecce, T. (2022). Sperm Competition Theory. In T. K. Shackelford (Ed.), The Cambridge Handbook of Evolutionary Perspectives on Sexual Psychology (pp. 242–263). chapter, Cambridge: Cambridge University Press. https://doi.org/10.1017/9781108943567
Gallup, G. (2003). The human penis as a semen displacement device. Evolution and Human Behavior, 24(4), 277–289. https://doi.org/10.1016/s1090-5138(03)00016-3
