|Sex between females is not uncommon for bonobos|
Researchers studying communication among the apes found that females made the most noise during sex if the "alpha female" was nearby.
Low-ranking females that were invited to have sex with high-ranking females would also call to tell other group members about the bond.
Experts suggest females communicate the encounters to boost their status.
"It's all about climbing up the social ladder for female bonobos”
"[Sex] is used to reduce stress and competition, develop affiliations, express and test social relationships and for reconciling conflicts and consoling victims in distress," explained Dr Zanna Clay, from Emory University in Atlanta, who has been studying vocalisations in the species for five years.
In order to understand more about communication among the apes, Dr Clay led an international team of researchers to observe a group at the Lola Ya Bonobo sanctuary in the Democratic Republic of Congo, Africa.
Building on earlier work, their findings published in the journal Scientific Reports identified a pattern in the calls made by females during homosexual encounters.
"Using vocalisations, females only advertise sexual contacts with important group members," said Dr Clay, "It's all about climbing up the social ladder for female bonobos."
The team found that calls were most likely to be made by lower-ranking females, particularly if they were "picked" by a higher-ranking female.
The females also appeared to consider their audience - calling more if the most important group member, the alpha female, was present.
"Bonobos appear to be highly aware of the dynamics governing their social worlds," said Dr Clay.
Sex in bonobo society
- Female bonobos have forward facing genitalia and often have face-to-face sexual contact with other females
- Despite being considerably more sexually active than chimpanzees, bonobos do not have more offspring
- Females become sexually mature at 12 years old but will engage in sexual activity from younger ages
- After a conflict, males may make genital contact with their rival, in order to diffuse the tension
She suggests that the females have adopted the calls, usually associated with reproduction, as a strategic tool.
"As a low-ranked female, advertising [a] social-sexual bonding with another dominant group member may serve to strengthen their social position, and signal this to the alpha."
Unlike their close relatives, the chimpanzees, bonobo societies are not male-dominated. Dr Clay suggests that this may be due to the strong relationships between females.
"In bonobos, sexual interactions represent a powerful means to enable females to develop and maintain social relationships, and it is these bonds which lie at the heart of their raised status in bonobo society."