The female ocellated wrasse usually prefers a house-proud male with his own nest and good child-caring skills, so that she can be safe in the knowledge that any offspring she has with him will be well looked after. But her ideas may be thwarted when a so-called “sneaker male” rushes in at the last moment of intimacy and sprays his sperm all over her eggs, before the respectable male gets a look in. Yet a new study has found that the females may be fighting back.
While it is common for the females of species that have internal fertilization to evolve ways for them to influence which male sperm gets to fertilize the eggs, from the labyrinthine vagina of the duck to female bonobos' near-month-long sexual swellings, for obvious reasons this is a little trickier when fertilization occurs outside of the body and therefore outside of the female's control. It was usually thought that this would be impossible to achieve, but it seems that the female ocellated wrasse has come up with a solution, which has now described in Nature Communications.
Females will tend to pick the males that are willing to build a nest, and then guard the eggs he fertilizes, looking after the young fishlets to increase their chances of survival. While that is all fairly admirable, there is another type of smaller male who is a bit of a cad, known by researchers as a “sneaker male”. He’ll hang around a courting pair and, just as the female releases the eggs for her good house-proud male to fertilize, will rush in and spray them all with his own sperm, in a rather sticky form of hit and run.
Interestingly, the sneaker males have been found to produce around four times as much sperm as their more honest counterparts, on the logic that the more sperm they flood the eggs with, the higher the likelihood that they will father the offspring. Yet it now seems that rather than sitting back and letting the spunky events unfold, the females have a trick of their own. As she lays her eggs, she takes a precaution by coating them in “ovarian fluid”, which it turns out has unique properties.
While the sneaker males may be producing bucket loads more happy juice than their honorable counterparts, it seems that the nesting males produce faster-swimming sperm. The researchers think while it may be influencing the velocity and motility of the nesting males’ semen, it is not quite that simple, and the ovarian fluid may be somehow removing the number advantage of the cheater males, reducing their heavy load advantage.