Male Barbary macaques spy on their mates having sex so they can take part in a so-called 'sperm competition' and deposit their sperm in as many females as possible, a new study has indicated.
Hearing an ejaculatory call is a signal to other males that "sperm competition" for fertilisation has commenced, the study found.
Keen to get in on the race, other males will approach the female.
"There's a conflict between male and female reproductive success," New Scientist quoted Dana Pfefferle of the German Primate Center in Gottingen and study's lead researcher, as saying.
"Females should try to be choosy and get only the best male genetic material," she added.
Males, on the other hand, may be best off trying to fertilise as many females as possible.
When the females are infertile, they slightly modify the structure of the call, making the male less likely to ejaculate. Fertile females sometimes make non-ejaculatory calls as well.
In the study, the researchers Dana Pfefferle of the German Primate Center in Gottingen and her colleagues recorded ejaculatory and non-ejaculatory calls produced by both fertile and infertile female Barbary macaques (Macaca sylvanus) in "La Foret des Singes", a primate visiting centre in France where Barbary macaques roam freely.
The scientists, then, hid a speaker in the park foliage, not far from a resting male, and played the recorded cries.
After listening to the cries, the males showed strong responses to ejaculatory calls. They turned around and looked in the direction from which the call came for roughly twice as long, and in some cases rose and approached the microphone.
Pfefferle made sure the recorded female was never around during the playback experiments, so the other males could not find her. Upon hearing the ejaculatory calls, males did however approach other females and checked their genitals for the swelling that indicates they are fertile.
Pfefferle said that the strategy adopted - whether females are choosy or promiscuous - depends on the species' social structure.
In case of Barbary macaques, females are all fertile at roughly the same time. During that period, females and males mate frequently and indiscriminately.
"It's all about sperm competition. I think the female wants to get as much sperm as possible to 'choose' which genetic material is passed on to her offspring," Pfefferle said.
The study is published in the journal Animal Behaviour.