After all homosexuality is pretty rare in humans and couple this with a less restricted (hump everything) attitude among many animals it would take some fairly sensitive observation to suss out the existence of homosexuality. After all, if members of the species are willing to hump members of the non-preferred gender for kicks it makes it tough to pick out some animal equivalent of homosexual orientation.

However, if one really (after careful examination) can’t find evidence of homosexuality in almost any non-human animals it substantially increases the probability I assign to it being an adaptive response. It could certainly be that basic design trade-offs in core biological functions, e.g., immune functions, or selfish genes (non-sex chromosome linked gene which aids daughters enough to balance out) end up leaving some small percent of non-reproductive seeking individuals. However, this seems much less likely if it doesn’t obtain in other animals…they manage to avoid this reproductive roadblock so given strong selective pressure why haven’t we.

On the other hand humans are unique in having large social groups and a high degree of altruism for extended family members. Thus, if it is a uniquely human effect it seems more likely to be an adaptive strategy for this situation.

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Then again has anyone done any good work on estimating the rate of homosexuals becoming fathers in hunter-gatherer societies. Lots of people engage in sex with the less preferred gender for a variety of reasons and absent sperm donation women may have asked homosexual men to father their children. It is one solution to the marriage problem in a small community (what do you do if all the guys are taken except for some nasty ones and you don’t want to draw the wrath of other women by mating with their men?)