Work those heads, guys!
I have been monitoring this forum for a few weeks now and sees a lot of emphasis on girth and length of the shaft. I think I once saw something on the head. I found the following by coincidence in the line of my work. This article speculates about the form of the head and why it evolved to what it is. It is thought to be to remove semen from a previous male copulator. Sperm competition certainly is an important factor in animal breeding, but up till now not so much seen as important in humans.
Gordon G. Gallup, Jr et al. 2003. The human penis as a semen displacement device. Evolution and Human Behavior, Volume 24(4), Pages 277-289
The apparently large student community on this forum may be able to find the article in their university libraries. It is also electronically available on the platform ScienceDirect. Below is the abstract.
“Inanimate models were used to assess the possibility that certain features of the human penis evolved to displace semen left by other males in the female reproductive tract. Displacement of artificial semen in simulated vaginas varied as a function of glans/coronal ridge morphology, semen viscosity, and depth of thrusting. Results obtained by modifying an artificial penis suggest that the coronal ridge is an important morphological feature mediating semen displacement. Consistent with the view of the human penis as a semen displacement device, two surveys of college students showed that sexual intercourse often involved deeper and more vigorous penile thrusting following periods of separation or in response to allegations of female infidelity.”