Parental Investment and Sexual Selection

The well-being of offspring, indeed their survival and future reproductive success, depends upon the effort their parents make for their benefit from the time of copulation. In 1972, Robert Trivers, an American evolutionary biologist and sociobiologist then at Harvard University, proposed a parental investment theory. The time, energy, resources, and risk that parents expend for the benefit of their offspring constitutes their investment, with the nature of that investment varying among taxonomic groups and between sexes.

Trivers noted that prior to the offspring’s birth, the male only invests a small amount of time and effort to achieve reproductive success—just sufficient to copulate. His evolutionary return is great, spreading his genes, after which he can move on and seek another mate(s). By contrast, female members of the species invest in the gestation of their offspring, undergo the extended mental and physical costs that accompany pregnancy, and during pregnancy cannot reproduce. The postnatal parental investment varies among taxonomic groups. With few exceptions, the offspring of aquatic invertebrates, fish, and amphibians receive little or no postnatal care by either parent. Prenatal and postnatal parental investment by one and commonly both bird parents involves preparing the nest, guarding the eggs, and caring for the brood. With mammals, and humans in particular, after a ninemonth-long pregnancy and period of nursing, the investment by both, but sometimes by only one parent, is extensive and may persist for decades.

While female lions hunt in groups of three to eight for food, males are the protectors of the cubs. The cubs are vulnerable to attacks by hyenas and leopards; however, the greatest threat comes from other male lions.

Such a relative difference in the investment by each parent, Trivers argues, plays a profound influence on selection of a mate, with the female far more discriminating in her choice. Males compete with one another for the opportunity to mate, with success determined by such factors as size, strength, and bright coloration, an indicator of health and vitality. Females prefer males to be physically fit, with superior physical traits (i.e., good genes to pass to their offspring), high status (alpha males), and resources. For species in which both parents participate in the care of the offspring, females will select a male perceived to be interested in assisting.