The problem pages of teenage magazines and tabloid newspapers tell us that sexual relationships are full of conflict. But for the gory details of the war between the sexes we must look in a different type of publication, one whose readers’ sensibilities are not so great as to prohibit the frank descriptions of the sordid facts about what is, after all, a natural phenomenon.
We will find what we are looking for, not on the newsagent’s top shelf, but in the libraries of academic institutions. In recent months the scientific journals have carried reports of a sexual arms race that has escalated to the point where chemical weapons are in routine use. They also provide an analysis of the problem in terms of a fundamental driving force of biology – the pressure to maximise the return on an investment.
The object of the investment in question is to preserve genes by passing them on to the next generation. Of course sexual reproduction usually involves cooperation between male and female but there can also be a conflict of interests. The problem is that the sexual partnership is unequal. The genetic return from successful reproduction is practically the same for male and female, but the size of the investment is very different. At the very least the female’s investment in producing the egg far outweighs the cost of producing the sperm that fertilises it, and in many species she also has to bear and rear the young.
Consequently it pays a female to select her partner very carefully and even to demand an increased investment from him in the form of a long-term commitment to rear the offspring. It may also pay her secretly to cuckold him if a better male offers her the chance. Males on the other hand can increase the chance of a return on their tiny investment by repeating it as often as possible. They may also fight each other for access to individual females, or sequester them in harems.
The bottom line is crucial in this enterprise. Unsuccessful investors are removed by the grim reaper. So natural selection may drive females to be fussy and demanding, and males to philander, to coerce, to build harems or even to commit infanticide to improve their chances of reproducing. The set of strategies adopted by each species will depend on many factors. But some of the clearest analyses of sexual conflict have been carried out on insects who also seem to use some of the dirtiest tricks.
Getting the chance to mate with females is not the only problem for males. They must also mate in a way which maximises their chance of fertilising her. This may be complicated if she has recently mated with somebody else. Geoff Parker of Liverpool University first described this problem, which is known as sperm competition, in the 1970s. Recently he has analysed it in some detail in the common yellow dungfly.
A male dungfly copulates with a female for long enough to flush out most of the sperm from her previous matings, and then stays with her until she lays all her eggs, which are fertilised as they are laid. “The last male to mate gets 80 per cent of the eggs” says Parker. But an unlucky male may be displaced by a bigger rival, who will then attempt to do the same thing all over again. “It’s a wretched business for the female” says Parker who uses marginal value theorem (developed by economists) to predict how long males of different sizes should copulate to maximise the return on their investment. The optimal time depends on size because on the one hand bigger males transfer sperm faster, but on the other hand they get more takeovers. According to Parker all except the smallest males copulate for the length of time that the theory predicts.
Sperm is not the only important component of seminal fluid. Work on the fruit fly shows that it contains a number of chemicals that affect the female in different ways, disabling the sperm of previous matings, making her less likely to remate with another male and increasing the number of eggs that she lays. Not only do these extra components of seminal fluid work to the advantage of the male, they have the unfortunate side effect of killing the female. “It’s one of the few chances that male fruit flies get to push females around, and they are rather good at it” says Linda Partridge of University College London.
The chemical warfare tactics of the male could be turned against the species of fruit fly that cause damage to crops. Partridge is investigating the possibility of using genetic engineering to develop male flies that are not only sterile, but have semen that kills the females with whom they mate, as an advanced technique of pest control..