From Frans de Waal's Our Inner Ape: A Leading Primatologist Explains Why We Are Who We Are:

They [bonobos] simply have no shame, no modesty, no inhibitions other than the wish to avoid trouble with rivals. When two bonobos couple, the young sometimes jump on top of them to take a peek at the details. Or another adult may move in and press her swelling against one of them to take part in the fun. Sexuality is more often shared than contested. A female may lie on her back masturbating in the open, and no one will blink an eye. She moves her fingers rapidly up and down through her vulva, but can also assign a foot to the job, keeping her hands free to groom her infant.