Scientists have found that male flies influence the behaviour of females after mating, namely improve their long-term memory. This involves a protein from their seminal fluid that acts directly on the female’s neurons, increasing their activity.

Mating affects the physiology of females from different angles: for example, reduces the willingness to mate further or changes food preferences. It also has an impact on behaviour and cognitive abilities, but it is still not clear how.

On the one hand, there is a widespread belief that females of different species become stupid after mating, as energy resources from the brain are redirected to the fetus. On the other hand, it would be logical to assume that not all brain functions suffer during pregnancy, and some are useful for the expectant mother and, on the contrary, are strengthened. Such functions include, for example, spatial orientation, risk avoidance, and the ability to remember where the sources of water and food are.

A team of scientists from the Institute of Science and Humanities Studies in Paris, led by Thomas Preat, tested the effect of mating on the memory of Drosophila. Previously, researchers found that the effect of sperm on the body of flies is indirectly mediated by SP – seminal protein. Receptors to it have been found in the uterus and nervous system of drosophila, but their functions – especially in the nervous system – are still unclear.

Scientists conducted a standard experiment on flies to develop avoidable behaviour: they were offered a certain smell, which was accompanied by a stroke of current. Then the flies were offered to choose between the branches of the maze, one of which contained a smell-stimulus, and the other did not smell anything.

After mating, flies were twice as good at this task as flies that did not mate with males. But those who were crossed with male mutants, whose sperm was deprived of SP, showed the same result as unpaired females. The researchers then tried to replace the pairing with an SP injection. Females, who did not come into contact with males, but received an injection, passed the maze as well (p = 0.004) as females after mating.

To test how the SP protein affects memory, the researchers blocked the receptor for SP in neurons of the uterine wall. But nothing has changed: the flies still successfully (p = 0.009) went through the maze after mating. Then scientists suggested that SP can affect the functioning of serotonergic neurons – it is known that their activity is associated with memory in flies. Indeed, SP receptors were found on neurons in the abdominal nerve chain and Drosophila brain.

The activity of serotonergic neurons in flies is regulated by the enzyme phosphodiesterase. Scientists measured its activity and found that after mating it is two times lower than that of non-mating flies – that is, it allows neurons to be more active. When her work was blocked, the flies that did not contact the males began to pass the maze as well (p = 0.003) as the females after mating.

The authors of the work believe that a good memory for smells can be of great adaptive value for pregnant flies – for example, it allows them to carefully select a place for masonry so that there is no enemy or any poisonous substance nearby.