From Wikipedia:
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.[1][2] As of 1999, about 500 species, ranging from primates to gut worms, have been documented engaging in same-sex behaviors.[3][page needed][4] According to the organizers of the 2006 Against Nature? exhibit, it has been observed in 1,500 species.[5]
The bracketed numbers are references to journal publications.