Sperm Warfare (Or: Why it Takes 1 Billion Sperm to Make One Zygote)
The average man produces roughly 525 billion sperm cells over his lifetime and releases, in one way or another, more than one billion of them per month and anywhere from 40 million to 1.2 billion in a single ejaculation. The males of other species boast some equally impressive numbers: 280 million, 1 billion and 3 billion per ejaculate for rabbits, sheep and bulls, respectively. If it only takes one sperm cell to fertilize an egg, though, why produce so many?
The Seminal Wars
The females of many species mate with and receive the sperm of multiple males, often in quick succession. Deep in the lady’s nether regions, those sperm compete to fertilize the egg. Now, if you’re serious about winning a lottery or a raffle, you don’t buy just one ticket do you? No, you buy several to increase your probability of winning. Sperm, in a way, are a lot like lottery tickets. If you’re serious about passing on your genes, then you want to get as many sperm as possible near a fertile egg cell. (In other ways, they’re not like lottery tickets at all, and I would discourage you from trying to buy them in gas stations or convenience stores.) For a male, the more of his sperm going up against his rivals’ seed, the merrier.
Sperm competition is such a powerful selective pressure, in fact, that it influences the size of the testes and the volume of ejaculate of some animals and causes others to modulate the amount of sperm they produce based on the presence of a rival male. Male chimpanzees, who face high levels of sperm competition, possess the largest testes among the great apes. Gorillas, who face almost no sperm competition thanks to a rigid social structure where the dominant male alone gets to mate with all the females, don’t need to waste precious energy and resources on sperm production and hence have some downright dinky testes—almost 15 times smaller than chimps’ (relative to their body weight).
Male humans would feel somewhat embarrassed if they were naked in a locker room full of chimps, but still pretty good about themselves if they were naked and surrounded by silverbacks (nervous, too, perhaps). Evolutionary biologists are still trying to work out whether our relatively large testes are leftovers from some point in our evolutionary past, or if sperm competition was at one point an important factor in human reproduction.
It’s not a sprint. It’s a marathon
Sperm competition isn’t a prevalent problem among modern Homo sapiens and guys don’t really need a veritable army of sperm to race someone else’s genes to an egg. We still need an awful lot of those squiggly little cells, though, because even if there’s no other sperm to compete against, every man’s little swimmers still have to fight in a battle of the sexes. Females demand only the finest sperm for their eggs, and the war their bodies wage on sperm is one of attrition.
After insemination, the sperm cells of humans, and many other species, have a long trip ahead of them, relative to their tiny size. At every step of the way, many sperm cells run out of energy or die and their surviving brothers are forced to leave them behind: only a portion of the sperm that are deposited into the vagina make it to the uterus, an even smaller group get to the oviducts and a fraction of those make their way to the upper oviduct where the egg is actually located. Once the sperm reach the egg, things don’t get any easier. One does not simply walk into Mordor. The egg is covered by a thick layer of gelatinous, follicular cells called the cumulus oophorus, which acts as a barrier, and it often takes the assault of several sperm cells to break it down enough for one lucky one to get through and fertilize the egg. Charles Lindemann, who researches the mechanisms of sperm motility at Oakland University in Rochester, Michigan, likens the whole ordeal to a “marathon run in a maze filled with mucus followed by an obstacle course.”
The odds stacked against any single sperm cell making the grueling journey to the egg can be offset by producing a large number of sperm. While just a small fraction of the sperm will reach their destination and do the job they were made to do, having a few million more cells backing them up makes for a pretty good reproductive insurance policy.