Bruce Bagemihl, PhD wrote in his 1999 book Biological Exuberance: Animal Homosexuality and Natural Diversity that homosexual and/or transgender activity has been observed in 478 species. He states in his book's introduction:
"Any book on homosexuality and transgender in animals is necessarily unfinished, a work in progress. The subject is so vast, the types of behaviors so varied, and the number of species involved so large, as to defy any attempts at comprehensiveness. And the scientific research in this area is only in its infancy: new developments and discoveries are continually being made, and the extent of uncharted and as yet unknowable terrain is so great as to render any attempt at completeness hopelessly premature."
Homosexual / Transgender Behavior Observed in 478 Species