Archive for the ‘sexual selection’ Category
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
Tanah Papua, the bird paradise
There are few simple pleasures to compare with watching birds of paradise display and perform, even if it’s only on video – and it would be pretty hard to see them otherwise. The forests of Papua are a wonderland for birds, and 39 of the 42 known species of birds of paradise are found only there. My attention was drawn to these very striking birds – in their very various ways – when I read Peter Raby’s biography of Alfred Russel Wallace several years ago, and I’ve associated them with exotic forest regions, miserable weather, malaria and an almost toxic other-worldliness ever since. A sort of anti-paradise doused with bewildering colour and cacophony.
The lineage of these birds has been hard to reconstruct, and apparent similarities to other birds, such as bower birds, have led ornithologists astray in the past. And to be scientific about it all, I’d have to master such concepts as order, family and clade, but I won’t go there in this post, except to say that the whole classification system looks a mess from the outside.. The most recent mitochondrial DNA research has set their ‘moment’ of emergence at about 24 million years ago. They’re quite close, genetically, to corvids, always a plus.
To an amateur, the most striking features of these birds are the plumage and displays of the males. And the females surely agree, even when they try to appear unimpressed. One immediately thinks of sexual selection – and Darwin. Here’s what Darwin had to say about the process, in The Origin of Species:
Sexual selection… depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring.
…when the males and females of any animal have the same general habits… but differ in structure, colour or ornament, such differences have been mainly caused by sexual selection
On the origin of species, 1st edition, pp88,89
One has to wonder why it’s the males who grow their weird and wonderful plumage and create their elaborate dances to attract the females, and not the other way around. Generally the female coloration – and that of both sexes when young – has evolved so that they match and blend in with their surroundings. The males are taking serious risks in drawing attention to themselves in this way, which is why the differences appear late – as secondary sexual characteristics. In fact, males generally mature later than females, by quite a wide margin. But since they don’t bear the offspring the males are more dispensable. It’s interesting to note, in this light, that in polygamous species, birds are sexually dimorphic both in appearance and lifespan. That’s to say, monogamous birds tend to look alike and to have the same lifespans, whereas in polygamous species, the females live longer. In fact this is the case in all species, not just birds. The San Diego zoo, which specialises in this family, tells us that birds of paradise can live for some 30 years in captivity – but surely it would be tougher in the wild, especially for males.
Not all birds of paradise are polygamous though – they have a range of breeding systems, of which the most interesting is ‘lek-type polygamy’. Lek is a Swedish term which refers to fun activity bound by the loosest of rules. It was first used in a book about the avian life of that region back in the 1860s, and is now adopted worldwide, and applies to a bewildering variety of species, including birds, bees, butterflies and bats (and various species of reptiles, amphibians and fish). And not just because they all like to have fun; the term has been refined to refer to specific behaviours. Two general types of lekking are identified – classic and exploded. They refer essentially to the physical space occupied – the lek. In a classic lek (a more or less circular region), each of the males is within sight of at least some of the others, whereas in an exploded lek, this isn’t ‘necessary’, as long as they’re in earshot. In fact, with some booming birds, like the New Zealand kakapo, they can be kilometres apart. In these exploded leks, the variety of behaviours is greater, as if the woo-ers are less inhibited by their neighbours.
Of course the leks in thickly forested Papua would be very different from Sweden. Sometimes the males share and compete in a common ground such as a forest clearing, while in other species they display and dance in the trees, controlling their territory vocally. Displays involve spinning, charging, freezing, hopping and skipping, hanging upside down, making show of their most irridescent plumes, all the kind of stuff imitated by the Folies Bergère, but with males rather than females as the beauties.
Breeding behaviour, though, is very varied. Not all male species are brightly festooned, and male-female relations run from bim-bam-thanks-mam polygamy to the sharing of nest-building and child-rearing among monogamous species such as the manucodes (they don’t all bear the bird-of paradise name – apart from manucodes there are sicklebills, riflebirds, parodias, astrapias and others).
These birds are, unsurprisingly, and more than other types, very much tree-dwellers, preferring the high canopies and living largely off their fruits, so they need to have their forests protected. They’ve been hunted for centuries, and Wallace had no hesitation in killing as many as he could himself in the 1850s. Of course, being in straitened circs in those years, his income was largely dependent on producing exotic specimens for the home trade, and birds of paradise were like gold. But local tribesmen also valued the bright plumage as a status symbol. In recent times the popularity of the birds for twitchers has helped in their protection.
They’re a fairly raucous lot, and the striking, machine-like notes of riflebirds and sicklebills are very much worth listening to online. One might assume that in dense forest, loud calls to indicate location are a must.
In conclusion, I began writing this to find out more about a particular family of birds, but got sidetracked by a lot of fascinating stuff I’ve only touched upon here, such as sexual selection, sexual dimorphism and anisogamy. More about all that in later posts, I hope.