Every female animal wants a mate with sufficient genetic quality to make a good husband and good father. Every male animal wants as many wives as possible, and to sometimes find good mothers and dams – rarely to find good wives.
Genetic Sieves
Males act as a kind of genetic sieve: only the best males get to breed and the constant reproductive extinction of bad males constantly purges bad genes from the population.
It has sometimes been suggested that this is the ‘purpose’ of males but that falsely assumes that evolution designs what is best for the species.
Samuel Butler, ‘ a hen is only an egg’s way of making another egg’,
In choosing mates, males are after quantity, while females are after quality. The peacock male will go through their ritual courtship display for any passing female; females will mate with only one male, usually the one with the most elaborately decorated tail. It is the female’s fault that the male has such a ridiculous tale, according to sexual selection theory.
Females evolved the ability to be charmed to be sure of picking the best males.
Darwin suggested that peacocks have long tails (they are elongated rump feathers that cover the tail) because peahens will only mate with peacocks that have long tails. Since then, the peacock has been the mascot of sexual selection.
But what is the explanation behind this? Is it competition between males that determine aesthetic characteristics or is it simply innate female choice?
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There are two techniques – wooing or winning – both are equally likely to sieve out the ‘best’ male.
The difference is that whereas the first technique will select dandies, the second will select bruisers. Thus bull elephant seals and red deer stags are big, armed and dangerous. Peacocks and nightingales are aesthetic show-offs.
Experiments have shown that females choose. And female preference for male ornaments can be a threat to the survival of the males.
Females choose; their choosiness is inherited; they prefer exaggerated ornaments; exaggerated ornaments are a burden to males. That much is now uncontroversial. So far, Darwin was right.
Mimetic Females
In the 1970s, Fisher suggested that females prefer long tails simply because other females also prefer long tails. This sexy-son reason suggests that peahens prefer beautiful males because they seek heritable beauty to pass on to their sons, so that their sons could attract females.
A competing theory is the one based on good-genes: peahens want beautiful males because beauty is a sign of good genetic qualities – disease resistance, vigor, strength, and these are qualities that females want to pass down to their sons.