Bruce Bagemihl did research in 1999 . It shows that homosexual behavior has been observed in close to 1500 species , from primates to gut worms , and is well documented for 500 of them . Homosexual and bisexual behavior are widespread in the animal kingdom : a 1999 review by researcher Bruce Bagemihl shows that homosexual behavior has been observed in close to 1500 species , ranging from primates to gut worms , and is well documented for 500 of them .