If you thought the macho male enjoys an upper hand in the mating game, you have got it awfully wrong. New research shows that female mammals are completely in command and can decide whose sperm they want for fertilisation after having mated with several partners.

The study — by a team of researchers led by Alireza Fazeli of the Academic Unit of Reproductive and Development Medicine at the University of Sheffield, the UK — upsets the conventional theory that suggests competition for the egg is male-oriented: that the healthier sperm that swims faster fertilises the ovum. It shows the final word in selecting the winner rests with the female.

During their study on pigs, the scientists found that the reproductive system of female mammals can “sense” the presence of sperm and react to it by changing the uterine environment. The scientists decided to study the sperm recognition system in the Fallopian tubes of pigs because their reproductive system is similar to that of humans.

The study clearly shows that the sperm’s arrival in the female reproductive tract triggers a cascade of changes that leads to alterations in the Fallopian tube’s protein profile. “This is mainly done to prepare the oviduct environment for storing the sperm, fertilisation and early embryonic development,” Fazeli says. However, this can also be used as a detection and selection system that alerts females to the presence of different kinds of sperm and then triggers mechanisms that control sperm transport, binding and activation for fertilisation.

“We know sperm selection exists in Nature, especially in promiscuous species, in which females mate with several males,” Fazeli says. “Baboons are a good example. During one reproductive cycle, if the female mates with several males, the offspring usually belong to one of the males — not a spread between all of them… We are now seeing what the molecular basis could be for this effect.”

While sperm interaction with oviduct cells has been studied in laboratory cultures, the new research provides evidence for this poorly understood process in living animals. Using minimally invasive techniques, the researchers compared the protein changes in the oviductal fluids before and after sperm introduction.

The molecular insights into this post-coital “ladies’ choice” has profound implications for infertility treatments such as in-vitro fertilisation (IVF), cloning and animal breeding.