Without the cries of passion, male macaques almost never ejaculated, the study revealed, with only 2 percent of male macaques ejaculating for more quiet females.
The study was conducted in a nature reserve in Gibraltar by researcher Dana Pfefferle, and the results have been published in online scientific journal Proceedings of the Royal Society B.
Pfefferle also counted the number of pelvic thrusts that would occur during the monkeys' sexual encounters and determined that when the females yelled, the thrusting would become faster and more vigorous.
According to the findings, macaque monkeys are promiscuous. If the males have multiple sex partners, this can result in low levels of sperm. Female macaques will often shout out mating calls at the time when they are most fertile, encouraging the males to make the best use of their sperm.
In an unrelated study, primatologist Kimran Miller of the University of Northern Iowa in Cedar Falls, Iowa, found that male Capuchin monkeys would urinate on their hands and then rub their feet.
The practice is not uncommon among primates. However, with the capuchins, Miller found a correlation between the urine-washing behavior and male monkeys seeking to comfort themselves or make themselves more attractive to females.
Miller and associates studied a group of two dozen capuchins at the National Institutes of Health Animal Center in Maryland.
Researchers found that monkeys that displayed urine-washing behavior seemed to reduce the tendency towards aggression. Placed in an aggressive situation, urine-washed monkeys avoided further confrontation 87 percent of the time.
The study said that the calming effect might have something to do with a lowering of the stress hormone cortisol in monkeys that practiced urine-washing.
The study found also that urine-washing in males increased twofold when in the presence of sexually solicitous females.
Details of the study have been published in the American Journal of Primatology.
