Same-sex sexual behavior is well documented in monkeys and apes, and bonobos, our closest relatives (alongside chimps) are a study in bisexuality. A whopping 50 percent or so of bonobo sexual encounters across all ages are same-sex. Female bonobos partake in same-sex genital rubbing once every two hours or so, often leading to orgasm, Stanford University biologist Joan Roughgarden writes in Evolution’s Rainbow: Diversity, Gender, and Sexuality in Nature and People, and an average of two thirds of their sexual encounters are with other females. Males also swing both ways, so to speak, although with less frequency. They participate in oral and manual stimulation, “rump rubbing,” “penis fencing” and even French kissing.
Bonobo society (unlike that of chimps) is female-dominated, and lesbian sex helps form coalitions against males and welcomes new members into the group. These alliances help females of the species reproduce earlier than chimps, leading to higher lifetime reproduction success. “For a female bonobo, not being a lesbian is hazardous to your fitness,” Roughgarden writes. An evolutionary advantage is harder to theorize for female Japanese macaques, which don’t seem to use gay sex to form alliances, to get parenting help, as an expression of dominance or submission, or because opposite sex partners are scarce. These monkeys may do it just for pleasure, and because they still procreate frequently, the effect on the species is neutral.