Sperm Wars

It may be a dog-eat-dog world* out there, but nowhere is the competition fiercer than in the female reproductive tract.

Biologically speaking, the goal of every male is to produce as many offspring as possible. To do this, males need to have some kick-ass sperm, but according to a recent study, too much kick-ass sperm can cause problems.

Human males, thanks to thousands of years of evolution, now over-produce crazy-fast, majorly-aggressive sperm known as “super-sperm.” One super-sperm reaches the egg first. If another sperm binds to the egg after the winning sperm has lodged itself in the egg, the egg will die. Of course, there’s a defense system to prevent this from happening.

As soon as the winning sperm binds to the egg, a biochemical barrier begins to form around the egg. The barrier is complete in just a few minutes, but (BAM!) another super-sperm enters the egg before the barrier is sealed and the fertilized egg dies.

Over those thousands of years of evolution, women’s bodies have evolved as well. To prevent one overly aggressive second place super-sperm from ruining a perfectly good zygote, the female reproductive tract does everything possible to keep sperm out. To survive the sperm will have to become even super-er.

Human sperm are not the only sperm with a mission. Meadow voles, earthworms, damselflies, field crickets and red junglefowl depend on stellar sperm to win the sperm war. You see, in many species, a fertile female will mate with multiple males. All of the males are shooting for the ultimate prize (paternity), but only one—or in some cases, a few—will win.

Different species have different sperm competition strategies. Some, like chimpanzees and rhesus macaques, produce super-speedy sperm. Others vary the amount and quality of sperm they ejaculate based on the situation. If a meadow vole smells other males nearby, he will contribute more sperm than he would in a more private setting. Field crickets and earthworms give significantly more viable sperm to “experienced” partners than to virgins. And red junglefowl produce higher velocity sperm when they mate with attractive females than when they mate with unattractive females.

Some males—like male black-winged damselflies—play it safe no matter what the girl looks like. During mating, the male black-winged damselfly pumps his scrub brush-like penis up and down to remove 90 to 100 percent of the sperm from the female’s spermatheca (her sperm storage tank). Once he’s done cleaning, he deposits his own sperm.

Alas, males and their speedy my-sperm-is-better-than-your-sperm sperm don’t always get to control who scores paternity. A recent study found that female crickets control how much sperm they store from each of their mates. By storing more sperm from appealing males and less from unappealing (related) males, the female determines a male’s chances of fathering her offspring—no matter how “super” his sperm might be.

*Siblings kill each other, lovers eat their mates and parents eat their offspring, scoring a mate isn’t nearly as easy as it is for humans and making a baby is a downright dangerous proposition.

Hasson, O., & Stone, L. (2009). Male infertility, female fertility and extrapair copulations Biological Reviews, 84 (2), 225-244 DOI: 10.1111/j.1469-185X.2008.00068.x

BRETMAN, A., NEWCOMBE, D., & TREGENZA, T. (2009). Promiscuous females avoid inbreeding by controlling sperm storage Molecular Ecology, 18 (16), 3340-3345 DOI: 10.1111/j.1365-294X.2009.04301.x