Why do males exist? If you look at any standard biology textbook, you will probably read that the point of having males as well as females is to promote variation by the exchange of different mutations, and hence to increase the chances of species survival. Unfortunately, most evolutionary biologists stopped believing in this explanation over 20 years ago. From a reproductive point of view, no individual is interested in anything beyond donating genes to the next generation, while species survival happens more or less at random, according to the whims of climate and geology. You don't actually need sexes in order to mutate and produce variation. In any case, most mutations have no effect, or mainly deleterious ones. John Maynard Smith talks of ‘the twofold cost of males’.1 Firstly, it is incomprehensible that any female should want to chuck away half her genome. Secondly, the males of many species are useless at doing anything except sitting around, getting fat at the females’ expense, and—in the words of Richard Dawkins—duffing up other males.2 Among some animals, such as elephant seals, the vast majority die as wasteful, disappointed virgins. Given the cost of males, it is perhaps not surprising that there are at least 40 species where the female kills the male during or after sex. In the case of the praying mantis, she literally bites his head off as part of foreplay, and he carries on in a delighted reflex of posthumous orgasm. Females of other species …