Female choosiness (and the male-male competition it leads to) tends to increase the likelihood that a female will select only those males that have plentiful resources or relatively high-quality genes, either of which is beneficial to the female, allowing her to produce more or better offspring—where “better” may mean increased disease resistance or physical traits that will be found attractive by future mates. Female choosiness is manifested by one or more of four general rules.

1. Mate only after subjecting a male to courtship rituals. In many bird species, the female requires the male to perform an elaborate and time-consuming courtship dance before she will mate with him. For the western grebe, for example, this courtship dance involves fancy dives into water, graceful hovering, various head movements, and flamboyant twists and turns. The courtship process can go on for several days. But if the male passes the time-consuming audition, he can generally be counted on to stick around to see a brood through hatching and early care (FIGURE 9-21).

Figure 9-21Four factors that influence a female’s choice of mate.

2. Mate only with a male who controls valuable resources. Territorial defense is a common means by which males compete for access to females. Among arctic ground squirrels, for example, a female chooses a mate based, in part, on the territory he defends, which is where she will reside after mating. With greater quality and quantity of resources in his territory, a male is better able to attract females, whose reproductive success can be increased if the territory is rich in resources.

3. Mate only with a male who contributes a large parental investment up front. Better than a believable pledge to commit resources to future offspring is an actual exchange in which a female requires a male to give her his parental investment up front, in the form of resources that will help her maximize her reproductive success. In the hanging fly, for example, a female will not mate with a male unless he brings her a big piece of food, called a nuptial gift—usually a dead insect. The larger the food item, the longer she will mate; and the more she eats, the larger the number of eggs she will lay. After about 20 minutes of mating, though, when a male has transferred all of the sperm that he can, he is likely to break off the mating and take back whatever remains of the “gift,” which he may use to try to attract another mate. Nuptial feeding is common among birds and insects.

4. Mate only with a male that has a valuable physical attribute. Male-male competition for the chance to mate with females can also take a more literal form: actual physical contests. Across the animal kingdom, from dung beetles to hippopotamuses, male-male contests determine the dominance rankings of males. Females then mate primarily with the highest-ranking males.

In a similar process, rather than choosing the best-fighting or largest males, females sometimes base their choice on some other physical attribute, such as antler size in red deer, the bright red chest feathers of frigate birds, or the elaborate tail feathers of the male peacock (see Figure 9-21). In each case, for the female, the physical feature serves as an indicator of the relative quality of the male, possibly because the feature is correlated with the male’s health.

With so many examples of male-male fighting as part of the courtship rituals that have evolved for attracting a mate, it is reasonable to ask: why is it so rare for females to fight? And why do females generally not have to advertise their health with flashy feathers or other ornamentation? The answer is that as long as females are making the greater investment in reproduction, nearly any male will mate with them. Consequently, there is nothing more to be gained by trying to outcompete other females or otherwise attract the attention of males.

Moreover, among humans, social and cultural values have powerful influences over mating behavior, complicating interpretation. We are not lumbering robots, destined to follow some genetic program. Researchers have noted some subtle manifestations of female health and fertility, including waist-to-hip ratios and patterns of facial and body symmetry. There is a rich and complex world of mating tactics, many of which we do not fully understand.