Originally posted by adam warlock
What do you mean by functional adaptation?
A beneficial novelty.
Let's deal with bisexuality in animals first.
"In some birds, same-sex unions, particularly between males, might have evolved as a parenting strategy to increase the survival of their young. “In black swans, if two males find each other and make a nest, they’ll be very successful at nest making because they are bigger and stronger than a male and female,” Böckman says. In such cases, he says, “having a same-sex partner will actually pay off as a sensible life strategy.”
In other instances, homosexual bonding between female parents can boost the survival of offspring when male-female pairings are not possible. In birds called oystercatchers, intense competition for male mates would leave some females single were it not for polygamous trios. In a study published in 1998 in Nature, zoologist Dik Heg and geneticist Rob van Treuren, both then at the University of Groningen in the Netherlands, observed that roughly 2 percent of oystercatcher breeding groups consist of two females and a male. In some of these families, Heg and van Treuren found, the females tend separate nests and fight over the male, but in others, all three birds watch over a single nest. In the latter case, the females bond by mounting each other as well as the male. The cooperative triangles produce more offspring than the competitive ones, because such nests are better tended and protected from predators.
"Such arrangements point to the evolutionary fitness of stable social relationships, whatever their type. Biologist Joan E. Roughgarden of Stanford University believes that evolutionary biologists tend to adhere too strongly to Darwin’s theory of sexual selection and have thus largely overlooked the importance of bonding and friendship to animal societies and the survival of their young.“ [Darwin] equated reproduction with finding a mate rather than paying attention to how the offspring are naturally reared,” Roughgarden says."
Is homosexuality in animals genetically determined or is it a bit more fluid, more circumstance-dependent, than that?
(I have no fixed ideas on genetic determinism and homosexuality; for some people, it's a question of a relationship with a person of the same sex (I know one person who had a heterosexual relation, then a homosexual relationship, then another heterosexual relationship, all fairly long-term); for others, it's a sexual preference ('boys turn me on, girls don't'😉 -- a variety of factors seem to be at play there ... We'd have to delve deep into human sexuality in general.)