Sperm Warfare
To continue our discussion of sperm we are going to explore some of the more controversial discussions surrounding both sperm morphology (shape & form) and its implications for a concept known as “sperm competition.”
One of the foremost experts leading the research on sperm and thus changing and challenging existing reproductive thought is a British biologist, Robin Baker. By sampling the post-sex excretions of men and women, he has developed some theories around the nature of sperm competition and how it has encouraged sperm to develop - and how it has promoted the reproductive responses of a woman’s body.
Baker has demonstrated that there may be autonomic (unconscious) adjustments to sperm counts depending on whether a man feels as though his partner has been faithful or not. He also has linked the likelihood of a women practicing infidelity during their peak fertility period - and their staying faithful during their most infertile phases. Aside from this interesting and provocative research, the parts of his theories that apply more directly to the areas we are discussing are his two ideas of physiological spermatozoa evolution.
First, the opposing ideas of the history of spermatozoa: Baker’s analysis of his data is based on a model of society that assumes a multi-male reproductive system, supported by the fact that 10% percent of children have fathers that are not the primary partner of the man acting as their father. This would suggest that even in a climate of monogamy there is still a sufficient amount of sperm competition. If women in a given society usually engage in intercourse with more than one man, then they are likely to encourage the evolution of an aggressive and plentiful type of sperm, that each fight for supremacy during the struggle to get to the egg first. The other theory suggests that since women are less likely to practice multi-male sex in modern society and may have evolved uterine protective measures to ward off bacteria and unwanted pregnancy, sperm has changed not because of the competition but because of the need to pass said defenses.
While going deeper, we can see that Baker is redefining the way that we think of sperm we once thought had bad morphology (structure). More contemporary ideas are suggesting that the majority of sperm are not designed for impregnating but for running interference, so that other sperm can reach their intended destination (as seen in many sports where one or more player may sacrifice themselves to neutralize an opponent by way of helping keep a team-mate going for the goal). Baker says that because men are competing for a small window of time to fertilize an egg that the oddly shaped sperm are there to defend against competing sperm. His research has led him to the conclusion that deformed sperm can function as both blockers (defending the routes towards the egg), or killers (halting the path of other sperm in such a way that they die).
This theory has been challenged on the basis that, like gorillas, the presence of so many deformed spermatozoa is a result of lazy sperm production that is resultant from a specific lack of sperm competition. Gorillas, as opposed to chimpanzees that have quite active sex lives, also have high levels of sperm deformation, perhaps because of the females’ lack of multiple partners.
An alternate explanation for the deformed sperm acting as sperm killers or blockers is proposed by Dr. Alexander Harcourt, an expert on gorillas at the University of California at Davis. He notes that women have evolved many means to protect the uterus from invading bacteria, and that these defenses will also harm sperm. It may be the sperm's need to co-evolve counter defense mechanisms, not sperm competition, that explains the signs of evolutionary pressure seen in the DNA data, he suggests.
Clearly more research needs to be done, but these are the first steps in what will hopefully be an interesting and informative line of research.
