Whereas most females mated at least once but fewer than four times, some males were able to mate many times, and some others were unable to mate at all. There is much stronger selection on males to maximize matings, generating a difference in selective pressures on the sexes, and consequently sexual dimorphisms. While touring the farm, Mrs. Coolidge asked about the sexual capabilities of a rooster and was told that he would mate several times a day. Upon hearing Mrs. Coolidge's remark, President Coolidge inquired whether or not it was with the same hen. Among the polyandrous jacana, Wilson's phalarope, and some species of seahorses, the females are slightly larger and more brightly colored than the males.
Males tend the eggs, whereas females jealously compete for additional mates.
The traditional coyness of the female is thus easily attributed to adaptive mechanisms by which she can discriminate the ideal moment and circumstances for assuming the burdens of motherhood. One of the most important circumstances is the inseminating male. It is to the female's advantage to be able to pick the most fit male available for fathering her brood. Unusually fit fathers tend to have unusually fit offspring. Inevitably there is a kind of evolutionary battle of the sexes.