A Crash Course on Gender Differences - Session 10
Sperm Competition: Procreation Strategies among human and animal males
Posted Jul 25, 2015
In this last session of our crash course on gender differences I'd like to discuss some intersting findings on sperm compitition.
The evolutionary strategies used by males to increase their chances of successful procreation generally fall into two categories. The first one involves "marketing" strategies used to increase the attractiveness of individual males in the eyes of females. The peacock's tail is a great example for such a strategy and is typically used by biologist as an example for the "handicap principle." The peacock tail has no apparent functional purpose and it seems to be creating an evolutionary burden rather than an advantage. However, doing relatively well with a huge tail in spite of the burden it creates provides females with a proof that the large-tail peacock has an excellent genetic profile that influences other traits. There are many other similar examples for the handicap principal in the animal mating world. The other type of procreation strategies are in sperm competition. The specific characteristics of sperm competition between males vary from one species to another, depending on evolutionary developments. Competition between drones (male bees), for example, comes down to a total of about ten minutes out of their very brief lives. When a virgin queen bee is ready to mate she enters a vigorous dancing state, drawing a swarm of drones. Only the strongest and quickest drones can succeed in mounting the larger queen bee and inserting their sperm into her. The drones die shortly afterwards, while the queen bee stores their sperm for the rest of her life (up to thirty years) for use in fertilizing the millions of ova she produces.
Sperm competition between male mice is no less interesting. Its main expression comes after the act of mating has been completed. After inserting his sperm into a receptive female, the male secretes a sticky substance that essentially blocks the female's reproductive tract to prevent other males from successfully mating with her until his sperm has been fully absorbed inside the female. This strategy, reminiscent of the chastity belts that the knights of the Middle Ages once locked their wives in before going out to battle, increases the male's chances of successfully fertilizing a female with whom he mates and also incentivizes him to care for her offspring because he has greater certainty that her offspring are his.
In some species sperm competition takes place during the mating act itself. Among flies, fleas and several other insects, males will methodically "clean out" the genitals of the females with whom they are mating prior to inserting their sperm. The purpose of this is to remove sperm previously deposited by other males. The strategy of eliminating the sperm of other males from female reproductive tracts is not limited to insects alone. There are mammals who have also adopted it – including humans.
Many researchers studying the evolution of human sexuality have concluded that the shape of the corona at the tip of the human penis developed precisely for the goal of scraping away any residual sperm left in the female vaginal tract from previous sexual encounters with other males. This theory is bolstered by anatomical comparisons with other mammals. Bulls have smooth penises without tips shaped like mushroom caps, and they typically ejaculate immediately upon penetration. Human males, in contrast, have protruding coronas on their penises and usually ejaculate only after performing a succession of pumping actions within the vagina. This characteristic pumping is apparently an evolutionary adaptation intended to remove competing sperms from the vaginal tract prior to ejaculation.
In mammals without pumping acts during sexual intercourse sperm competition is expressed not through the removal of the sperms of rivals but through the release of an immense quantity of sperm. Bulls and male chimpanzees, for example, compete with each other in testicular size. Large testicles cannot guarantee that a male's sperm will fertilize the female with whom he is mating, but the more sperm cells he produces the greater the chances that one of them will successfully win the race to the ovum.
A third category used to increase the chances for procreation, which is less frequently used, is the strategy of rape. Rape is a widespread phenomenon among certain species of monkeys, ducks, spiders and zebras. In most of these species there is no social sanction imposed against rapists. There are many monkey species, however, in which rapists may be punished, sometimes to their deaths.
Thanks so much to all my readers who followed this 10 session course! I am considering starting another one on "Relationships" later this summer.