I suppose I am like most people who have a model of conception as a great race, involving a male emitting a vast number of sperm that then race towards the egg waiting for them in the female and one of the sperm, the victor, gets to fertilize the egg. Robert D. Martin, emeritus curator of biological anthropology at the Field Museum in Chicago and a member of the Committee on Evolutionary Biology at the University of Chicago, writes about the history of studies into sperm and that this idea of ‘macho sperm’ is one of the many myths that surround conception.
One question that immediately comes to mind is why so many sperm are produced. The realization of so many sperm led to the swimming competition model of fertilization.
Whether in the popular or scientific press, human mating is commonly portrayed as a gigantic marathon swimming event in which the fastest, fittest sperm wins the prize of fertilising the egg. If this narrative was just a prejudicial holdover from our sexist past – an offensive male fantasy based on incorrect science – that would be bad enough, but continued buy-in to biased information impedes crucial fertility treatments for men and women alike.
Eventually, powerful microscopes revealed that an average human ejaculate, with a volume of about half a teaspoon, contains some 250 million sperm. But a key question remains unanswered: ‘Why so many?’ In fact, studies show that pregnancy rates tend to decline once a man’s ejaculate contains less than 100 million sperm.
Clearly, then, almost half the sperm in an average human ejaculate are needed for normal fertility. A favoured explanation for this is sperm competition, stemming from that macho-male notion of sperm racing to fertilise – often with the added contention that more than one male might be involved. As in a lottery, the more tickets you buy, the likelier you are to win. Natural selection, the thinking goes, drives sperm numbers sky-high in a kind of arms race for the fertilisation priz
He says that the idea of a sperm competition in which the most vigorous swimmer gets to fertilize the egg is based on the study of promiscuous chimpanzees that may not be applicable to humans and other primates.
Convincing evidence has instead revealed that human sperm are passively transported over considerable distances while travelling through the womb and up the oviducts. So much for Olympic-style racing sperm!
In fact, of the 250 million sperm in the average human ejaculate, only a few hundred actually end up at the fertilisation site high up in the oviduct. Sperm passage up the female tract is more like an extremely challenging military obstacle course than a standard sprint-style swimming race. Sperm numbers are progressively whittled down as they migrate up the female tract, so that less than one in a million from the original ejaculate will surround the egg at the time of fertilisation.
So why are so many sperm produced at all? There are possible alternative explanations.
For species not regularly exposed to direct sperm competition, the only promising alternative explanation for high sperm counts concerns genetic variation. In a couple of rarely cited papers published more than four decades ago, the biologist Jack Cohen at the University of Birmingham in the UK noted an association between sperm counts and the generation of chromosome copies during sperm production. During meiosis, the special type of cell division that produces sex cells, pairs of chromosomes exchange chunks of material through crossing over. What Cohen found is that, across species, sperm counts increase in tandem with the number of crossovers during their production. Crossing over increases variation, the essential raw material for natural selection. Think of sperm production as a kind of lottery in which enough tickets (sperm) are printed to match available numbers (different genetic combinations).
An interesting read for someone like me who knows little about the process.