Some people might expect that, after copulation, a male’s battle to reproduce is over - he can turn his attentions elsewhere or simply roll over and go to sleep. But in fact, the fight has just begun. Females may receive sperm from several males in a short period, and if one males’ sperm are somehow better at reaching the female’s eggs than the rest, he will be the one to father the offspring - there’s no prize for second place.
New research has shown how, just as families co-operate in the outside world, one male’s sperm may help each other to reach their goal and beat the sperm of other males.
When fishflies Parachauliodes japonicus mate, the male attaches a package of sperm called a spermatophore to the female’s genitalia. The sperm race out of the package and into the female, travelling through viscous seminal fluid, but rather than go it alone, they group into bundles of several hundred individuals, their heads held together by a special protein.
At Tokyo Metropolitan University, Fumio Hayashi has found why it pays a sperm to be a team player. Larger groups - with many tails moving at once ñ can travel faster in viscous fluids than smaller groups or solo sperm. And fishfly sperm need to be especially quick - they not only have a race to win against the sperm of other males, but a time limit before the female removes the spermatophore. By co-operating, the sperm help each other to reach the female’s sperm storage area (the spermatheca), before it’s too late. (Functional Ecology 12, p347-50).
Background
Competition between sperm has been studied for many years, but evidence of co-operation has been hard to find. One male’s sperm, like brothers and sisters, have more genes in common with each other than with the sperm of other males, hence it may pay a sperm to help its ‘siblings’. Swimming in groups is one form of co-operation, identified in the fishfly Parachauliodes japonicus (opposite) and the opossum (where sperm swim in pairs). Co-operation may also involve ‘kamikaze sperm’. Mutant sperm, previously thought to have no role, may increase the success of their normally shaped fellow sperm by swimming in circles and blocking the path of unrelated competitors’ sperm.