Homosexuality amongst animals

There was an article in a local newspaper recently about the evolutionary advantages of homosexuality in the animal kingdom (which apparently is very widespread). They wrote about the black swans in australia and how males of the species would often spend their lives with another of the same sex (though they would mate with females when they felt the need to procreate and chase her off after the laying of the eggs). This would offer the baby swans a 10 times higher chance of survival than if they would have been raised by a heterosexual couple. What’s your take on this? Are there other examples of advantages of homosexuality amongst animals? I’m just trying to wrap my mind around the concept. I guess I’m just old fashioned darwinian :wink:

It’s not just homosexuality but homosexuality and necrophilia combined that icks me out

That’s not homosexuality. Here’s an article from MSNBC that isn’t too bad, listing exampled of animals that engage in homosexual acts (which is different from being a homosexual). Note:

That article also mentions black swans, although it says that only 1/4 of them engage in the behavior you describe. We’ve had lots of threads about this in GD, and while there are many animals that display homosexual behavior, I think you have to be careful about anthropomorphizing.

The animal that people love to mention in this arena is the bonobo, which is known to have sex almost any way possible. But I don’t believe that any wild bonobo would be classified as a homosexual, i.e. a being solely attracted sexually to its own sex.

It’s like Margaret Cho says about herself when reflecting on the idea that she likes sex with both men and women…

(quote paraphrased from memory) :smiley:

I am not aware of any widespread advantage to homosexual behavior among animals. Such behavior, as you mention, is widespread among species. However, in most species the frequency of actual long-term homosexual pairs is quite low. In certain circumstances it may not be disadvantageous, since the individuals involved may still be able to reproduce. Certainly if the individuals involved will be selectively disadvantaged if they engage in exclusive homosexuality, even if they manage to adopt an offspring from another pair (since their own genes won’t be passed on). If they do on occasion mate with members of the other sex, then they may pass on their genes. To the extent they have a lower reproductive rate than an average heterosexual pair, however, they will be disadvantaged.

I don’t know offhand how frequent male homosexual pairings might be among Black Swans, but if the situation is as you describe it is likely it is a rather special case. If males are much more aggressive than females, then perhaps two males might be better able to drive off predators than a male-female pair. In many, probably most, species a male-male pair would be unlikely to have an advantage over a mixed pair, and more likely would be at a disadvantage. If homosexual pairing was generally advantageous, you would expect to see it more commonly.

Among humans, it has been postulated that homosexual siblings of parents might contribute more to the upbringing of their nephews and nieces since they would not have offspring of their own. This would in part explain the persistence of homosexuality in the population through kin selection. This seems rather far-fetched to me. The frequency of exclusive (and I stress exclusive) homosexual behavior is sufficiently low in humans that it probably does not require any special explanation through kin selection.

Would you be less “icked out” by heterosexual necrophilia?

Wouldn’t that put whichever swan did not mate at a genetic disadvantage? He is spending all his energy raising someone else’s offspring.

And why is it so much of an advantage to have two males raising baby swans rather than one of each sex? If this behavior were genetically based, then I would expect this strategy to have outcompeted everyone else, and all black swans would act like this.

Got a cite?


It’s possible that both males mated with the female and the clutch is a mixture of offspring from them. That’s not all that unusual in birds.

Many bird species pair bond and cooperate in raising their young. One stays with the young while the other gets food. Emperor penguins take this to an extreme. It’s also possible for a population to have a mixture of survival strategies.

n.b.: I’m not so much taking a position on this (although the cite I gave in my first post does seem to confirm it), as I am pointing out that your objections aren’t insurmountable.

Sure, but what I was asking is why it was so much of an advantage to have two males rather than one male and one female?


It seems to be in part because two males, being more aggressive, are able to defend larger territories.

From here

If in fact many of the eggs are taken from other swans, then even if the males are more successful in raising the chicks, they may be less successful in evolutionary terms, since they are not passing on their own genes.
Unfortunately the actual papers this is based on don’t seem to be available on the web for free, so I can’t really tell what the real situation here is. Note that the figures are given for only a single study population. This situation could be a local anomaly, not found in the species as a whole.

That’s what I was thinking, which would offset the greater success of raising offspring (assuming the offspring had the gay gene and survived to reproduce). Wouldn’t it depend on how closely related the breeding group was to one another? So if they stole eggs from a sibling, that would tend to reinforce the behavior, and if they stole from some less-closely related swan, that would tend to extinguish it (assuming a genetic cause/influence).


Yes, if the group was closely related this could enable the behavior to persist through what is known as kin-selection. If males stole eggs from their siblings, or perhaps even cousins, the behavior might persist. This would especially be so if the heterosexual siblings themselves were able lay again to replace the stolen eggs and also reproduce.
As I mentioned above, some biologists have proposed kin-selection as an explanation for the persistence of homosexuality in human populations, but I find this unlikely.

What about necrosadobestiality?

Or, as it’s more commonly known, beating a dead horse.

This approach, while not spreading the genes of an individual, would benifit the species as a whole. If, for example, X percent of the species is inclined to join a homosexual pair, regardless of the parent’s “orientation”, the species would be allowed to propagate advantageously. A symbiotic relationship, if you will, as the hetero parents produce viable offspring that are raised by a homo couple, and the homo individuals are still produced by the heteros.

This is merely speculation. Feel free to rip my theory to shreads.

In general, selection at the level of the individual is believed to be the dominant factor in evolution. In some cases, kin selection can also operate where close kin (siblings, perhaps close cousins) are involved or inbreeding is high.

Whether group selection occurs (selection at the group level, of groups not consisting of close kin) is controversial, but at best it is not very common. Selection at the species level may possibly occur, and is thought to be the reason why sexually reproducing species are more common than parthenogenetic ones, at least among animals.

In order to become established in a population, selection generally has to be operative at the individual level, or at most kin level. If it is just beneficial to a species, but not to individuals or kin, it is difficult to explain how a trait would become established throughout a species.

Does egg laying take a significant amount of resources from the female? Because if the female swan is exhausted or just weakened by egg laying, it might be to the male’s advantage to find a fresh replacement partner. If they can both impregnate her, it works. And she would reap the benefits, reinforcing the “have a fling with a gay swan” gene.

Doesn’t explain the adoption of other eggs unless perhaps swans can identify close family members?