150 years ago, Darwin first discusses sexual selection in The Origin… then comprehensively in Descent of Man. The problem was why some characters seem non-adaptive and not governed by natural selection. A famous example is the tail of the male peacock, who Darwin noticed desperately seeks “a spectator of some kind”, and, “will shew off his finery before poultry, or even pigs”.
What advantage this behaviour has for survival is not obvious: the cost to the male in producing his stunning apparel mustn’t exceed any survival benefits but the way that Darwin carefully constructs a considered and logical argument demonstrates his exemplary working. He shows that the benefit is real and great; not so much involved with individual survival, it is related to the reason for surviving. Reproduction.
Darwin cleverly identified the benefit that outweighs the cost of cumbersome and garish plumage, also predicting some optimal balance in the economy of nature. Costs may define the fine line between getting it right and “going all Liberace”, but the converse of underselling is worse: peahens are a lot fussier than Darwin thought. Recently, the number of ‘eye-spots’ were adjusted without shortening the tail; typically there are about 150 of them, shown to be optimal. Less, and mating success dwindled until males with fewer than 130 had less chance of pulling a peahen than Quasimodo with a feather duster stuck up his arse. In fact, girl power dictates that even two dusters might not be enough; there is an inbuilt, self-perpetuating inflation that drives up costs, but also profit margins. This positive feedback loop is called “Fisherian Runaway” after R.A. Fisher who described it (1915, 1930). For the peahen, choosing a mate who has a beautiful long tail increases the chance of her male offspring growing one, and so also be attractive. Of course, for her there is very good reason to be choosy. Males can and want to spray their sperm around willy‑nilly, but producing an egg and offspring levies a prohibitive tax upon her metabolism.
Whilst researching the evolutionary ecology of African antelopes I couldn’t fail to notice the sexually selective adornments that they boast. Very horny! From the captivating spiral of the Kudu to the seductive Arabian curve of the Giant Sable’s scimitar, it’s advertising with a full page spread. Of course, take out too many column inches like the Irish Elk, and nature’s economy becomes bankrupt, a recession destined for extinction. But get it right, and … well, there was an impala I knew with forty females and territory the size of a nature reserve. He was called “Lucky Boy”.
Darwin could only sketch out the details of how sexual selection works. As with natural selection and genetics, he couldn’t have been privy to the peacock’s immune system, the health of which is strongly linked to his tail. This was also discovered recently, but is likely to be duplicated in other species. For example, stick an extension onto a long-tailed widow bird and other than ending up with a longer-tailed widow bird, you’ll also gain a queue of fawning females. Impressive plumage makes a great advert for virility, because if there’s no parasites depleting your immune system, then you should be a healthy specimen. Of course, it is this underlying adaptive advantage that is really being passed on to any offspring. But there’s more for the female than just genetic imperative. Spotting a brightly-coloured, prancing dandy is adaptive; it saves time searching for a mate reducing her costs and increasing survival.
Avoiding jokes about size mattering, for males given a short straw, there is hope. When costs in choosing are low, females don’t necessarily prefer only one male ornament, selecting between those on offer irrespective of the costs to the males in their production. Again, our peacock’s tail length is an indicator of antibody production while the eye-spots are a measure of phagocytic defences. Sometimes this doesn’t work as well as it might. Embellishments exhibit varied responses to an affliction. The female may then be drawn by an unaffected attribute, costing her dearly if she’s actually selected a pox-ridden impostor. Fisher’s process can produce multiple preferences and the males must follow by ensuring their bits are preferred, whatever and to whomever the price. Rutting and lekking males, like Lucky Boy and Darwin’s peacocks and stags, are good examples of this, strutting their various stuffs. But it can also be introduced artificially: preferring beaks and plumage, female zebra finches also like flashy leg bands, like wearing a Rolex.
Similarly, Darwin had a lot to say about human adornment, specifically beards. He traced the roots of facial hair to “an ancient progenitor” which was then acted on by sexual selection. Yet again modern studies prove that he was right. There is a strong sexual connotation to facial hair because males use it to advertise their social rank and virility. Like muscle mass and penis development, facial hair growth is influenced by testosterone levels, and is like wearing a sign on your chin saying, “hair today, gonads tomorrow”, which must make the World Beard & Moustache champion the sexiest man in the world. At the time of writing, his name is Mr Passion. Oddly enough, occidental society has recently tended away from beards, associating them with antisocial psyches or disturbed pasts, as in “hiding behind a beard”.
Darwin’s own beard has achieved iconic status, the bearded version being more readily called to the minds of most than the young man. However, my own personal experiences provide little support for sexual selection of male facial hair. I have at times grown a beard in various styles, its presence more dependent on my pennies than my penis, and I’m sure it hardly ever contributed carnally. Clearly it was never long enough. I am now habitually clean-shaven, but I wonder what private thoughts Darwin was entertaining as he sat there in his dotage looking like old Father Time. I mean, his was so long that he could have draped it over a shoulder. Not necessarily his own.
UPDATE: Interesting contribution to the peacock tale…
“These feathers weigh about 300g and can exceed 1.5m, so it’s expected that the male birds would be making a significant sacrifice in their flight performance for being attractive—possibly giving up their lives if the train restricts escape from predators such as tigers and leopards in their natural environment …. filmed the take-offs of birds carrying full plumage in 3D, and then filmed the same birds taking off without their trains. The display feathers, which naturally moult at the end of the breeding season, were clipped to judge the change in take-off performance between the two states … found there was no significant difference.“