For these mammals, there is documented evidence of homosexual behavior of one or more of the following kinds: sexual behavior, courtship, affection, pair bonding, or parenting. Bruce Bagemihl writes that the presence of same-sex sexual behavior was not officially observed on a large scale until the 1990s due to possible observer bias caused by social attitudes towards LGBT people making homosexuality in animals a taboo subject. He devotes three chapters; Two Hundred Years at Looking at Homosexual Wildlife, Explaining (Away) Animal Homosexuality and Not For Breeding Only in his 1999 book Biological Exuberance to the "documentation of systematic prejudices" where he notes "the present ignorance of biology lies precisely in its single-minded attempt to find reproductive (or other) "explanatio