Posts Tagged ‘dimorphism’
stuff on sexual selection, and our humanity
As perhaps mentioned, I’ve started reading The descent of man, or to be accurate, I started reading it a while ago, then stopped, what with all the other pleasurable and thought-provoking reading I’ve been doing. In fact one of those books is Matt Ridley’s Birds, sex and beauty: the extraordinary implications of Charles Darwin’s strangest idea, to give its full title, and it’s maybe a bit of a spoiler, because I’ve heard that The descent contains a section on sexual selection, that ‘strangest idea’ aforementioned, an idea that, as Ridley tells me, was found more questionable than general evolution by many scientists of his time, and even afterwards. Wallace, who developed his evolutionary theory independently of Darwin, and became a close associate afterwards, was quite skeptical, along with most of Darwin’s other backers, and Ridley, quite rightly I think, points to the reason. Evolution was obviously a rather serious assault on religion – and is still felt that way – whereas sexual selection was an assault on patriarchy, though many of its detractors may have been oblivious of the fact. Patriarchy’s so insidious that many people still think it’s normal! Duh…
So what exactly is sexual selection? Well, just about everyone points to the obvious case of the peacock, with his extravagant tail feathers. Can such a stunningly colourful but ungainly mass to drag around you be anything but a hindrance? It’s kind of exhausting just to look at them. Can a peacock actually fly? Well, yes, but certainly not like an eagle. And how does it sneak up on its prey? Does it only eat blind mice? Actually they’re omnivores, but probably easier to be veggo with all that get-up. And of course they can be prey as well as predators, and you should be able to guess the rest – one minute you’re a mate, next you’re dead meat. It’s just a matter of getting things in the right order.
So sexual selection is about males so impressing females, whether by looks, dancing displays or some other spectacle, that the female offers herself happily, or complacently, to him. And males are often in competition with other males for the honour, the point being that it’s the female who sexually selects. This type of selection goes on in species of birds, fish (eg guppies), insects (eg fruit flies), mammals (eg elephant seals) and reptiles (eg most lizards). It also goes on in humans (eg party hardcore, referenced below), but not generally for the purpose of reproduction. As for bonobos – well, that’s another story, and it’s complicated, much as with humans.
So what about humans? We like to think we’re too complex and sophisticated for that sort of categorisation, and anyway, we in the WEIRD (non-Catholic) world don’t connect sex so tightly with reproduction. Even so, when we do decide on reproduction, we surely, male and female, do it on the basis of having selected, more or less mutually, the best partner available. This is of course a best-case scenario – we can be fooled, perhaps by our hormones, into being convinced that this person will be the best father/mother for our child/children, not to mention a variety of other scenarios which result in pregnancy. If we think of sexual selection in terms of pairing rather than pregnancy, it may be that neoteny, the preservation of youthful characteristics, including by artificial means – make-up, cosmetic surgery, mode of dress and so forth – has played a role, but this doesn’t appear to be a scenario in which only the female does the selecting. Certainly it’s less clear ‘who’s zoomin who’, if anyone. Darwin, I think wisely, considered sexual selection in humans has worked in both ways, with hairlessness in females being selected for by males, and males’ reduced hair ‘going along for the ride’ to some degree (also because of ‘genetic correlation’, though Darwin didn’t know about genes, and so could not consider genotype-phenotype examples), while reduced sexual dimorphism and, in contrast, the greater difference between the voices of adult males and females, are being selected for by their opposites. The reduced sexual dimorphism argues for greater monogamy (compare gorillas). Even so, females may in general be selecting for height, muscularity, and deeper voices. I give up.
Seriously, the scenario is complex for humans, which is to be expected. From Wikipedia:
In a study measuring female attraction to males with varying levels of masculinity, it was established that women had a general masculinity preference for men’s voices, and that the preference for masculinity was greater in the fertile phase of the menstrual cycle than in the non-fertile phase. There is further evidence from the same study that in fertile stages of the menstrual cycle, women also had a preference for other masculine traits such as body size, facial shape, and dominant behavior, which are indicators of both fertility and health. This study did not exclude males with feminine traits from being selected, however, as feminine traits in men indicate a higher probability of long-term relationship commitment, and may be one of several evolutionary strategies.
I don’t want to think too much about male dominant behaviour being indicative of fertility or health – especially mental health – but this finding that the menstrual cycle affects female preference seems just too ‘beyond our control’ for my liking, which isn’t to say it’s not true. As other sources have pointed out, there’s a sort of more-or-less understandable ‘muddle’ between selecting for ‘tough guys’, in both physical and emotional senses, or for caring-and-sharing softies who’ll be reliable providers – they hope. But isn’t that typical of women – they can never make up their effing minds.
Clearly, though, there are two strategies that are more or less blindly operating in humans. A video by ‘Brainstorm’, linked below, puts it neatly:
Sexual selection is an extremely powerful tool to explain much of evolutionary change in behaviour in animals – in humans it becomes a bit more complicated – we’re neither clearly a tournament species [males fighting for dominance (e.g gorillas)] with huge extremes re sexual dimorphism, nor are we sexually identical in behaviour, aggression or life expectancy… we do see dimorphism in bone structure, muscle mass and levels of aggression…
These male-female differences are not extreme, and we might just be able, with work, to make them less extreme. Wouldn’t it be great if we could actually work toward this. The sexual dimorphism among bonobos is minimal, and it would make sense, given their society, that it is diminishing, though we have no substantial evidence that this is so. Humans, of course, still have a way to go, given all the warfare and violent confrontations that are still occurring.
Anyway, once again, vive les bonobos. We have so much to learn…
References
Charles Darwin, The descent of man, and selection in relation to sex, 1871
Matt Ridley, Birds, sex and beauty, 2025