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In the evolution of sociality among animals, different degrees are acknowledged. Some species present overlapping generations, some division of labor, and the minority present reproductive division. Those that present the three traces above are called "eusocial" (e.g. some wasps, some bees, all ants and termites, and an african mole).

Maybe our own species could be eusocial? After all, homosexuality is present in virtually every culture studied by anthropologists. Some say it may bring an advantage to the group via group selection (not having kids of their own, they could study plants, medicine, weapons, and so on). What do you think? Are we an eusocial species?

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I wouldn't go so far as to say homosexuality is eusocial - some still become parents by reproduction. asexuals might be, but if environmental conditions vary, they might reproduce. habitual sexual practice is not non-reproductive behavior I think. it might be very low chance of reproduction, but reproduction only needs to happen once really. –  shigeta Feb 9 at 16:16
    
Well, some wasps and some Paraponera ants sometimes reproduce, even not being the "queen" in the nest. It doesn't mean there is no reproductive division, though. In our species the "reproductive division" (if that's the case) goes together with our high intelligence, that teaches us to have sex just for fun. But my point is that the lack of sexual attraction for the opposite sex may have evolved similarly as eusociality in other species. –  Rodrigo Feb 10 at 18:16
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it depends on your definition of eusociality - if eusociality does not mean caste reproduction where some castes cannot reproduce, then by all means call it eusocial. en.wikipedia.org/wiki/Eusociality#In_mammals –  shigeta Feb 10 at 22:41
    
Thanks for the link. If we are not eusocial, perhaps something in that sense may have triggered the development of homosexuality, that being so widespread in human societies, asks for an evolutionary explanation. –  Rodrigo Feb 11 at 2:18

2 Answers 2

The question of homosexuality and its role in nepotism is interesting.

According to this wiki article, less than 10% of the population is homosexual. If you consider that homosexuality makes human eusocial because homosexual do not reproduce but help their brothers and sisters to reproduce, then you would have to consider any species where a few percentage of the population produce some kind of altruism to be eusocial. Given that all phenotypic traits theoretically have social impacts, then I would bet that you might call eusocial a lot of species.

It seems wiser to me to limit the definition of eusocial when an important part (maybe 98% minimum) of the population do not reproduce but help others to reproduce. One might also want to consider some other restrictions for a the use of the word "eusocial"

Therefore, using the argument of "reproductive division" I would rather not include Homo sapiens in the list of eusocial species. But it is nothing more than a question of definition. You're free to define your words the way you want and call human eusocial but make sure your interlocutor knows your definitions.

Now, let's say we place the limit to the definition of eusociality at 1%. When more than 1% of the population does not reproduce but help their kin to reproduce, then the species is eusocial. Then, humans would seem to fit in this definition at first sight. But do homosexual in our modern society really help their kin to reproduce? In other words, do siblings of homosexual reproduce more (or have offspring that have a high fitness)? This study investigated the question (as pointed out by @oreotrephes in the comments) and they indeed found some pattern but the difference in reproduction between relatives of homosexual and non-relative of homosexual is is quite weak. But if homosexual don't reproduce and don't help their kin reproducing then, humans is definitely not a eusocial species.

Finally I want to add that individuals from lots of different species have been found to perform homosexual behaviour (see this wiki article). If you want to consider human as eusocial because some homosexuality, then you will have to consider sheeps, labradors and pigeons to be eusocial as well!

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Thanks for your comment, Remi.b! I'm using the definition from E.O.Wilson in Sociobiology and other books (The Insect Societies also use the same, if my memory is right). They say nothing about how big is the "sterile caste", only about the 3 traces ocurring together. And if homossexuals change their habits, it doesn't change the reason that behaviour might have evolved in first place - and I think that's the objective within biological reasoning, not to put rules of what we might or not do with ourselves. –  Rodrigo Jan 9 at 23:07
    
Indeed, I have never heard either talking about the number of sterile individuals. But it seems to make sense to me that the whole point of eusociality is that many individuals work for the benefit of one. And therefore, you might see a continuum from some individuals helping some others to many individuals helping one. I guess one would need to define eusociality as an arbitrary threshold along this axis (and some others). –  Remi.b Jan 9 at 23:26
    
In my last paragraph, I just wanted to say that even if one want to consider that humans were eusocial in the past, he might not be able to say the same for the modern species of human if siblings of homosexual do not have greater fitness than average anymore. –  Remi.b Jan 9 at 23:26
    
Actually, in social hymenopterans, the queen alone works for the benefit of the worker caste. Because of the genetics of haplodiploidy, when a worker gains a sister, it has a copy of 75% of her DNA. When the queen gains a daughter, it has a copy of only 50% of her DNA. So, no, there's not a "sense" like that "whole point" you mention. Indeed, in different species the logic may vary wildly, according to genetic and/or ecological constraints. –  Rodrigo Jan 10 at 1:27
    
About your last paragraph, I disagree. The cultural time is almost irrelevant close to genetic/evolutionary time. We probably have the same instincts we had 100,000 years ago. Even if homosexuals change their behavior for the next 1000 years (what is highly improbable), this would hardly change the genetics that cause homosexual behaviour. And even if in industrial-western-societies this change happens and is sustained for so long, still there are other societies that would not change alike. That's the reason our cultural behavior are not supposed to define our biological terms. Do you agree? –  Rodrigo Jan 10 at 1:50

I'm going to hazard turning my comment into an answer....

Even if the answer might be no, you know this is a good thought and there is something there... lets dig a little.

Eusociality in evolutionary biology is by many like Dawkins and Wilson pointing to the evolution of sterile castes of animals, which are very closely related to the breeding caste. This is how hymenoptera are famed as examples of the importance of kin selection. Worker ants are entirely dependent upon the queen to reproduce the next generation, but they can do so because the queen is close enough to being them so that ants continue to exist as they do. Not only is this a reasonable way to exist, but kin selection has produced eusocial animals in every

The application of this definition to mammals has been done, but its not a universally agreed upon definition of eusociality. To a great extent its because the animals can still breed, even if they are inhibited from doing so by a social structure. Wolf packs and ape troops where the alpha male sires all the offspring are not eusocial - the other males are only deferring mating (sometimes forever) but they are biologically fertile. This is not eusocial, just social biology.

But to your question, I think the point to be considered is that the dynamic that allows the evolution of eusociality does shape social structures. Primates are so rational and their brains are so big that their brains allow them to adapt their social matrix such that we can have different roles in our groups depending on the conditions of environment at any given moment, whereas sterility and castes defined by adaptation will change very slowly, we can change our society in just a generation. Social animals are eusocial in a way, but they are more adaptable by the use of their social reprogrammability.

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