Homosexual behavior in animals is sexual behavior among non-human species that is interpreted as homosexual or bisexual. This may include sexual activity, courtship, affection, pair bonding, and parenting among same-sex animal pairs. Research indicates that various forms of this are found throughout the animal kingdom. As of 1999, about 500 species, ranging from primates to gut worms, have been documented engaging in same-sex behaviors. According to the organizers of the 2006 Against Nature? exhibit, it has been observed in 1,500 species.