Female Barbary macaques emit ear-piercing calls when mating, and now researchers have determined other males listen to these sounds with apparent interest.
Since the female calls vary, depending on whether or not the male partner has successfully mated, it's believed the eavesdropping males use the sounds to figure out what's going on "in the bedroom" and may even rate the happenings.
"The fact that copulation calls are loud and distinctive gives other males of the group the chance to listen in and 'judge' copulations," lead author Dana Pfefferle told Discovery News.
Pfefferle, a primatologist at the German Primate Center in Gottingen, and her colleagues previously discovered that female Barbary macaques act a bit like cheerleaders when mating, using their vocalizations to cheer on and stimulate their mates, causing their partners to increase their thrusting rates.
The scientists documented two basic types of female mating calls: those linked to partner ejaculation and those linked to no ejaculation.
"The peak frequency is higher and the interval between the single units of the call is shorter in ejaculatory compared to non-ejaculatory calls," explained Pfefferle.
For the new study, she and her team recorded these two types of calls from a female who was in the fertile stage of her monthly cycle. The scientists played the recordings for unsuspecting male Barbary macaques at an outdoor enclosure called La Foret des Singes, located in Rocamadour, France.
Playbacks were carried out when the test males were engaged in quiet activities, such as resting, self-grooming or feeding, and when these males were facing away from the speaker.
For the most part, the listening males ignored the non-ejaculatory copulation calls, even though they were emitted by a fertile female. When vocalizations associated with ejaculation were played, however, the males looked around and approached the speaker.
The findings have been accepted for publication in the journal Animal Behavior.
While the researches did not conduct the experiment with actual mating couples, they theorize that in the wild, eavesdropping males would be more inclined to mate with a female who has just received another male's sperm. Since sperm itself "competes" within the female for access to her egg, the noisy process may promote competition, result in fertilization by the fittest male and therefore be supported by natural selection.
Stuart Semple, senior lecturer in the School of Human and Life Sciences at London's Roehampton University, studies primate socializing and welfare.
Semple told Discovery News the he thinks the new study presents "very exciting results, which further our understanding of the complex information content and function of female copulation calls," he said.
Semple added, "I would, however, be very hesitant to extrapolate the findings to copulation calling in humans."
Pfefferle agrees on that point.