Now we know why your female partner plays her musical score while having sex: it boosts your fertility. Judging on a lower rank on the evolutionary scale, female monkeys could shout while having sex to help their male partners ejaculate!

This is the results of a research carried by a team at the German Primate Center in Göttingen and published in the journal Proceedings of the Royal Society B. The study focused on Barbary macaques (Macaca sylvanus) in a natural reserve in Gibraltar and found that if the female does not shout the male almost never ejaculates.

It was known for long that female monkeys often make loud, specific sounds before, during or after sex, but none could explain their purpose.

During the two years of observations, the team discovered that females shouted in 86% of all sexual encounters. In this case, males reached climax in 59% of the cases, while in case of silent sex, males ejaculated in less than 2% of the encounters. The researchers also determined the number of pelvic movements correlated with the shouts. Noisy female sex was found to increase the number of pelvic thrusts executed by the male; the female's yelling was connected to more vigorous sex.

Like in most macaques, the male and female of Barbary macaques are highly promiscuous, each having sex with several partners. Such solicited males can quickly run out of sperm, that's why females have noisy sex when they are at the peak of their fertility, so that males use at best their sperm. That's because in many macaque species (the Japanese macaque and stumptail macaque are two examples), like in humans or bonobos, sex does not have an exclusively reproductive role, but can also serve for social bonding.

"These calls might also make females more attractive to other males. These shouts might play different roles in other species." co-author Dana Pfefferle, behavioral scientist and primatologist at the German Primate Center, told LiveScience. The Barbary macaque is the only European monkey, found only in Gibraltar in Europe (but even there it is colonized from northwestern Africa, even if it inhabited the old continent in warm interglacial periods).