Scientists have shed new light on monkeys' sexual behaviour, by finding that female monkeys may shout during sex to help their male partners climax.
A new study by scientists at the German Primate Center in Gottingen discovered that male Barbary macaques (Macaca sylvanus) almost never ejaculated without these screams.
Female monkeys regularly give out loud, characteristic yells during or after sex. However, the exact purpose behind this behaviour has remained heavily debated.
Thus, to find an answer, the German scientists thoroughly studied Barbary macaques for two years in a nature reserve in Gibraltar.
They found that females shouted during 86 percent of all sexual encounters, reports LiveScience.
When females shouted, males ejaculated 59 percent of the time, they discovered. However, when females did not yell, males ejaculated less than 2 percent of the time.
To determine whether yelling was caused by how vigorous the sex was, the scientists counted the number of pelvic thrusts males gave and timed when they happened.
They found that when shouting occurred, thrusting increased, suggesting that hollering led to more vigorous sex.
Lead researcher Dana Pfefferle, a behavioural scientist and primatologist at the German Primate Center, said that counting monkey pelvic thrusts is 'quite weird, but it's science.'
Male and female Barbary macaques are promiscuous, and often have sex with many partners, meaning that sperm levels can get quite drained. The females yell when they are most fertile, so that males can make the most use of their sperm.
Pfefferle said that her research suggests these calls might also make females more attractive to other males, adding that these shouts might play different roles in other species.
The findings are published online in the journal Proceedings of the Royal Society B.