Sexual intercourse plays a major role in bonobo society observed in captivity, being used as what some scientists perceive as a greeting, a means of forming social bonds, a means of conflict resolution, and post-conflict reconciliation. Bonobos are the only non-human animal to have been observed engaging in all of the following sexual activities: face-to-face genital sex (although a pair of Western Gorillas has been photographed performing face-to-face genital sex), tongue kissing, and oral sex. In scientific literature, the female-female behavior of touching genitals together is often referred to as GG rubbing or genital-genital rubbing. The sexual activity happens within the immediate community and sometimes outside of it. Bonobos do not form permanent monogamous sexual relationships with individual partners. They also do not seem to discriminate in their sexual behavior by sex or age, with the possible exception of abstaining from sexual intercourse between mothers and their adult sons. When bonobos come upon a new food source or feeding ground, the increased excitement will usually lead to communal sexual activity, presumably decreasing tension and encouraging peaceful feeding.