For a mammal to reproduce it must unite its gametes with the gametes of a member of the other sex. Because each gamete provides half of the genetic material of the offspring, the choice of mate has direct implications on the resulting genotype (genetic makeup) of the offspring. For this reason, animals do not mate randomly and instead choose mates.
Mate choice is among the most important pressures affecting the evolution of species because failure to be able to select a “good” mate results in poor offspring quality (offspring that may be less adept to survive or reproduce), or worse yet, no offspring. Animals that fail to reproduce disappear from the gene pool, so mate choice is a critical factor.
In mammals, both sexes produce gametes of different size. Females produce relatively large eggs, and, typically, in limited number. In contrast, males produce tiny, cheap (from an energy standpoint), and extremely abundant sperm. Thus, from the outset, females adopt a “quality” strategy whereas males adopt a “quantity” strategy. This very basic difference in the size of gametes and parental investment in gamete production will trickle down and affect almost all subsequent reproductive processes. The larger initial investment of females will also result in females taking care of growing embryo(s).
Production of offspring in mammals involves placental growth, and this is performed by females. Once born, young are nursed with milk, also produced by the mother. So although the genetic contribution of each sex in the offspring is equal, the investment of females in offspring is greater. Thus, females have more to lose from bad mating decisions. This asymmetry in investment between sexes will be reflected throughout most sexual adaptations, and will lead to females being almost invariably the most selective in mate choice.