New research suggests that male semen can affect the genes and behaviour of females.
The research team, from the University of East Anglia, found male fruit flies selectively alter the chemical make-up of their seminal fluid.
And that when around rivals, the males produce more seminal proteins, New Scientist reports.
One of the proteins is a 'master regulator' of genes - and females exposed to it showed a wide range of changes in gene expression.
Gene expression is the process by which specific genes are activated to produce a required protein to help the body function.
These proteins go on to perform essential functions as enzymes, hormones and receptors, for example.
The research was presented earlier this month at the Society for Molecular Biology and Evolution conference in Austria.
It adds to a growing body of scientific evidence showing seminal fluid may have both physical and psychological effects on the body.
Previous studies have claimed semen can elevate mood, increase affection, induce sleep and also contain at least three antidepressants.
The research suggests it is not just that women who are having sex are simply happier, but that happiness levels might be related to the quantity of semen within their body.
Seminal proteins can directly affect the behaviour of flies because they enter the circulation and get to the brain, says Professor Chapman.
This, in turn, helps her to reproduce as soon as possible.
It references the work of Sarah Robertson of the University of Adelaide. When it comes to seminal signalling, she believes 'semen causes the cervix to produce molecules that influence the rest of the body - and influence a women in many other ways than the region involved in reproduction.