Homosexuality in nature: An evolutionary perspective

2년 전

On the surface, homosexual behavior seems to fly in the face of evolutionary theory. What possible adaptive significance could it have? How does it increase the fitness of an individual to perform behaviors such as displays, mounting and copulation with a member of the same sex? Yet the fact of the matter is, many species (around 1500 documented) engage in some degree of sexual behavior involving same sex partners. Which in turn suggests there must be some clear adaptive benefit to doing so.

Birds of a feather...

Let's start with the birds. Birds have been noted to peform homosexual acts or form homosexual pairs across a wide variety of contexts. One of the most cited examples is the female-female pairings of Laysan Albatrosses, particularly at one colony in Oahu, Hawaii.

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Around ⅓ of the pairings in this particular colony are female-female bonds

What's interesting about these birds is that they form largely monogamous pairs, with some extra-pair copulations occuring - a fact which becomes important for our female-bonded pairs when attempting to secure the best genetic material for their offspring. To do so, one female (and the next season, the other) will mate with the ‘fittest’ male available, regardless of his existing heterosexual couple. Working together, the females then rear their chick, thus ensuring their own Darwinian fitness through reproductive success. The reason behind this strategy lies in a high population skew, with more females than males in the population. In this sense, the pairing of females is entirely facultative. Two females together simply have a higher chance of rearing offspring than a single female would alone (though interestingly the chance is still lower than in heterosexually bonded pairs).

Interestingly, it has been shown that in general, female-female heterosexual pairings tend to occur more frequently in bird species which form monogamous pair bonds AND are even more likely in birds with precocial (well-developed at birth) offspring. Though these two facts may seem to contradict one another, a possible reason lies in that female-female pairings are less successful in rearing, thus work better when the chicks are already nearly ‘good to go’ at birth.

Where male-male homosexual behavior occurs, it tends to be more of the short-lived display and/or mounting behavior variety (go figure). So males of species which are polygamous, with multiple females to a male, tend to show more homosexual behaviors! Again, this seems counter-intuitive, but these behaviors most likely regulate social hierarchies in males and thus establish which males have more access to females.

Ok, but what about primates?

When we get into the realm of our closest relatives, things get more complex. Primates, it seems, just like having sex. They're not always too fussy about who they're having it with either so, rather than the predominantly homosexual bonds seen in monogamous birds, we find plenty of evidence for a mix of homosexual and heterosexual behavior. The classic example here is Bonobos:

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There isn't much that can't be solved with sex if you're a Bonobo

For these apes, sex is the glue that holds society together. It can be used to establish rank, reconcile feuds and bond members of the group. Two male Bonobos will often settle a score with some well-timed ‘penis-fencing.’ The advantage here lies in the fact that when the group is well-bonded, they function better as a cohesive unit, thus increasing everyone’s fitness. Sexual behavior between females, males and even young Bonobos is very common, with the little ones often actively climbing the social ladder by performing ‘favors’ for their elders.

Another primate group whose sexual exploits are well-studied are Macaques:

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Who knows what's going on in that hot-tub?

Amongst these monkeys, it's very common for females to copulate with one another and interestingly, it often occurs with males freely available. Though this fact initially puzzled researchers, close observation revealed that sexual acts between female Macaque pairs were often more varied in terms of the positions and movements used than heterosexual Macaque pairings. The conclusion? That though these ladies mate with males they often turn to their fellow females for gratification purely because it's more… um, fun. Phew! Again, sex in the Macaque world is still largely bisexual however, and likely also serves to reinforce social bonds and hierarchies in their female dominated society.

Bar Ram Ewe?

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From an early age Barry knew he just wasn't like the other Rams

Perhaps one of the most fascinating examples of ‘strict’ homosexuality that occurs is that of the male domestic sheep. For some Rams, there simply is no use for ewes. Studies show that 10 % of male sheep, in fact, exhibit a lifelong preference for homosexual interactions. Various developmental and socialization factors have been shown to influence this and intriguingly, examination of the brains of these males shows that they posses a smaller hypothalamus (a region of the brain involved in regulating sexual behavior) than their heterosexual counterparts. Clusters of neurons responsible for sex hormone regulation also differ between ‘gay’ and heterosexual sheep. One theory of how this comes to be lies with different levels of exposure to androgen hormones in these males during development. Fascinatingly, similar brain structure differences have been noted between gay and straight men!

Of course, the obvious question here is how such genes persist in the sheep population, given that these males don't reproduce. A possible mechanism lies in the expression of the gene in females related to these homosexually oriented males. Though not yet proven, it's possible the same gene in related ewes governs functions such as increased fertility or sexual activity thus ensuring these females produce more offspring and thus more Rams with the same ‘gay’ gene.

So what about Homo sapiens?

While all of this provides compelling evolutionary mechanisms for the development and propagation of homosexuality in nature, the full range of factors influencing homosexuality in humans are still being uncovered. Many of the behaviors and situations discussed here are more facultative in nature and of course there is a strong argument for innate homosexuality in humans. In a later article, I'll be looking at these factors as they apply to humans specifically, as our understanding of the genetics of gayness within our own species deepens.

Well, that's it for this one. Stay tuned for my delve into the complex factors affecting human sexuality!

Happy Steeming people,
The Wise Fox
Images from wikimedia commons

Paper and article sources:
https://academic.oup.com/beheco/article/18/1/21/209396
https://www.news-medical.net/news/2006/10/23/1500-animal-species-practice-homosexuality.aspx
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3866402/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3085551/
http://www.bbc.com/earth/story/20150206-are-there-any-homosexual-animals
http://community.lovenature.com/wild/why-do-some-animals-exhibit-homosexual-behaviour

And for an in depth look at mating systems and sexual behavior in birds, check out this paper:
https://academic.oup.com/beheco/article/18/1/21/209396

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This was a really informative post! It does debunk those who claim that homosexuality is created by humans. I'm not that much of an expert on homosexuality in animals; I am much more of an expert when it comes to homo sapiens. I dedicate my personal life to research and that's why I have a homosexual boyfriend. Just for research purposes ;)

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"I dedicate my personal life to research and that's why I have a homosexual boyfriend. Just for research purposes ;)"

Omg, I am so stealing this 😂 Research purposes indeed!

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hahahaha. give credit where it's due!

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This is the best comment I have ever seen on the platform. Love it 😂

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hahaha thank you! Am honoured :P

I think this is the first Steemit Post I've shared on Facebook - that's how much I love it! haha... I love the photo of the monkeys having sex!

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😱 Not Facebook.... Just kidding 😄 Glad you enjoyed it!

For those of you who are interested in this topic I recommend the book by Bruce Bagemihl called Biological Exuberance.
Cheers!
ian

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Hmm, yes. That one is on my 'to read' list :)

You write really well. I thoroughly enjoyed reading this post and would have never thought of this topic on my own. I look forward to seeing more from you 😊

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Thanks so much! Been wanting to write this one for a while actually :)

Fascinating post. I've known about the Bonobos for a while now. Was not familiar with the other examples.

I think the more fascinating thing here, in some respects, is not so much the homosexual behavior but the fact that other animals are having sex for pleasure at all. Though I guess it really shouldn't be too surprising to anyone who's had a dog hump his leg...

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Yeah, it's a pretty fun peek into our evolutionary history when we realise the social/bonding function sex serves in other species. Even weirder, the Macaques have been found interacting sexually with deer recently...

Resteemed holo. Sorry for the delay, been busy off platform. I'll resteem your other work as well. Let me know if there is anything else I can help with.

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Hey Mudcat, thanks and no worries. How are things coming along with your on-platform projects? :)

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Busy mate. Damn busy. If at any point you have a post going up you want a hand with then give me a shout. I started supporting you when you first arrived and as you are an excellent writer/mind I consider it a delight to continue to do so. You are always welcome on my blog and promoting you has always been great fun as you produce awesome content. I understand you work with students all the time and I know how annoying/trying this can be. Particularly if you are working with young MSc students. But if at any point you want to start a once a week writers class on Steemit I would see to it that we brought you in, got you votes, exposure, and writers to come to you for advice. Think of it as mentoring. You wouldn't have to wear a prozac permagrin throughout either, you could be yourself in your feed back. Sharp wit with constructive criticism is a great way to stimulate creative growth in others. Positive pressure to spur on evolution if you will. Give it some thought and if interest you we can chat on discord at some point.

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I'll definitely give that some thought! :)

It's been a while since I looked into the research, but I thought at last count, there were well over 1500 species that exhibit at least some level of same sex sex and as for relationships? Those species with a higher order of intelligence show a tendency to monogamous relationships, with some pair bonding for life.

Dolphins, Humans, big cats (lions, etc), most primates.

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Yeah, the numbers vary according to different sources. It depends on what each study classifies as 'some form of homosexual interaction.' The most consistent figure I've come across is around the 1500 mark for confirmed homosexual behaviors, but I'm always happy to stand corrected :). Do you have any links to the studies quoting higher numbers? I'd be keen to have a look.

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I can check for some recent studies.

About 10 years ago there was some interesting research looking into population density and homosexuality. The theory was that if a population grow to large more homosexual/lesbian births seem to occur. I could see where this would be the case with Barry the sheep. Just because we humans like to keep sheep in large herds does not mean that the herd size is seen as normal or beneficial in sheep culture or biology. There might be something there.

As for the albatrosses it does not surprise me that the males do the bird equivalent of the gay bar darkroom. The sexual act might or might not have a social component next the dominance trigger in this case but the parallels to most male spices including humans really is not all that surprising lest we forget that males are sort of biologically programmed to "spread it around". That kind of hardwired coding probably bleeds in to pure sexual interactions as well. Hence why gay culture can be at times very sexually aggressive.

Bonobos are a matriarchal society and sex seems to be very much a part of normal sexual interaction with them. I mean it’s as common place with them s us shaking hands LOL. That being said I am not sure if you noticed the parallels between human matriarchal societies and that of the bonobos. Most societies based on matriarchy seem to have been very sexually open and liberal opposed to patriarchal societies who always seem to want to reign it in. It is also interesting to observe a primate patriarchal society like that of chimpanzees next to that of a matriarchal one like that of bonobos. While there are the occasional “social “ sexual encounters between same sex partners in chimpanzee culture most same sex sexual encounters are dominance driven and are often there to display rank and remind the lower ranked member where their place is. Bonobo culture is completely opposite while there are rank fights like in most cultures sex is just a way of life. Its like hey bob I feel sad lets have sex , oh Margery this food was good lets have sex , Mary I am bored lets have sex :P There is a reason Bonobos are considered to be the happiest apes LOL :)

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There are certainly a lot of fascinating questions regarding sexual behaviour and social/adaptive relevance in animals. I'm starting to think that this may be a very interesting research field to pursue should I ever desire to do a PhD...

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Yes absolutely! - go for it :D

Good stuff! I believe Dolphins enjoy relations for fun as well. Shows how smart they really are LOL.

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They do indeed! Although dolphins are pretty evil with one another as well...

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Yeah when some of the esoteric junkies tell me how enlightened they are I always point to the fact that they are known to gang rape ...so much for spiritual beings :P

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Indeed. And don't get me started of bloody meerkats...

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Wait, who's gang raping who?

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Dolphins sometimes rape and it is not limited to the male female dynamic either.

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Ahhh. Gotcha. The animal kingdom is remarkable, rape or not. Crows, elephants, octopus, rats, dogs.. Humans only understand minute capabilities other creatures truly have., ie. multi-layered problem solving skills. Let alone outside this planet. :)

Thanks for the informative post!

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