Martin Daly & Margo Wilson; 1983
In animals such as ourselves, where parental care is an immense undertaking, the energetic cost of the initial gamete is a negligible proportion of total parental investment. The ovum-sperm disparity seems trivial. Yet nurturance is predominantly a female enterprise in many animals including almost all mammals, and the dimorphism of those little gametes is ultimately to blame. This is because the gamete size distinction has set the stage for some other evolutionary developments that have greatly amplified the initial difference in the parental investment of female and male.
Large ova bias organisms toward female nurture wherever fertilization takes place inside one parent's body because the relative immobility of the ovum assures that, whenever internal fertilization occurs, it takes place within the female. Internal fertilization protects the young from perils like desiccation and predation, thereby greatly increasing each individual ovum's prospects for fertilization and survival. But it also has a cost. By increasing her nurture for each ovum, the female can no longer invest in so many. In one season a ten-year-old cod may release as many as 5 million eggs for external fertilization. No fish with internal fertilization can match the cod's fecundity. But neither will an internal fertilizer suffer the same massive destruction from early deaths and fertilization failures as unparented cod eggs. There are trade-offs between fecundity and nurture.
Once the female is harboring zygotes within her, any further investment in nurturant or protective devices that may then evolve tends most often to be paid by the female, although both parents reap the fitness benefits. Birds and reptiles, for example, construct a protective eggshell at considerable material expense. In several types of animals, mechanisms have evolved to feed the infant while it grows within the maternal body; the mammalian placenta is perhaps the most sophisticated such device. In fact, we mammals have attained a pinnacle of concentrated maternal investment. First there is pregnancy: Prolonged internal gestation and placental nurture are enormous commitments of female's resources that pay off in greatly reduced offspring mortality during the early stages of development. And then, at birth, the mammalian female doesn't even take a recuperative break, for we mammals have invented another major maternal investment - milk.
Internal fertilization, gestation, placentation, lactation: Each of these evolutionary developments has had the effect of concentrating female investment in a decreasing number of offspring. In the evolutionary sequence leading to modern mammals, each advance in the effective nurture of young has amplified the sex difference in parental contributions. Among vertebrate animals with external fertilization (most fish and amphibia), things are very different: If there is any postmating parental care at all, it is at least as likely to be performed by males as by females. In birds, internal fertilization is universal, but so is an early externalization of the developing embryo, with the result that both parents can care for it; biparental investment is the rule. But in mammals, and indeed in most animals with internal fertilization, parental nurture is overwhelmingly female-dominated.