When Darwin wrote of evolution he saw two processes unfolding, the struggle between species for resources and the struggle within species for mating opportunities. This intraspecies sexual struggle he further said is of two kinds: ‘In the first case it is between the individuals of the same sex, generally the males, in order to drive away or kill their rivals, the females remaining passive; while in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners.’
But these two kinds of sexual struggles can take place in the same species in ways that are not so obvious. The female choice Darwin wrote about is often about mating with as many males as possible. This ensures the actual battle is not just between males but between sperms to fertilise the egg. Males in various species have responded differently to such demands. In butterflies almost 90 per cent of the sperm of male butterflies is non-fertile, and is meant to fool the female sperm storage organs into thinking enough sperm has been obtained from various males to fertilise the eggs, thus ruling out competition from sperm of other males.
In a recent work on butterflies, evolutionary biologist Nina Wedell of the University of Exeter in England and her colleagues have found that males that produced more non-fertile sperm had sisters that mated less frequently with different males. These results showed that the two traits are genetically correlated. Since mating with more males is better for the females, here the sexual struggle has led to males evolving a trait that is actually detrimental in their female relatives. Explaining how this could be possible, Wedell said, “The existence of a genetic correlation between sperm production and storage means that, provided the benefit to one sex is larger than the cost in the other sex, the trait can rapidly be elaborated.’’
The battle of the sexes is by no means a zero-sum game.