 Evolutionary game theory was first developed by R. A. Fisher [see The Genetic Theory of Natural Selection (1930)] in his attempt to explain the approximate equality of the sex ratio in mammals. The puzzle Fisher faced was this: why is it that the sex ratio is approximately equal in many species where the majority of males never mate? (See, for example, the Northern elephant seal Mirounga angustirostris.) In these species, the non-mating males would seem to be excess baggage carried around by the rest of the population, having no real use. Fisher realized that if we measure individual fitness in terms of the expected number of grandchildren, then individual fitness depends on the distribution of males and females in the population. When there is a greater number of females in the population, males have a higher individual fitness; when there are more males in the population, females have a higher individual fitness. Fisher pointed out that, in such a situation, the evolutionary dynamics lead to the sex ratio becoming fixed at equal numbers of males and females. The fact that individual fitness depends upon the relative frequency of males and females in the population introduces a strategic element to evolution.