Parental Investment and Mate Selection: The Evolutionary Forces Shaping Reproduction

The well-being, survival, and future reproductive success of offspring depend on the efforts their parents invest in them from the moment of conception. In 1972, Robert Trivers, an American evolutionary biologist and sociobiologist at Harvard University, introduced the parental investment theory. He defined parental investment as the time, energy, resources, and risks parents devote to their offspring, noting that this investment varies across taxonomic groups and between sexes.

Trivers observed that, before birth, males typically invest minimal time and effort in reproduction—only enough to copulate. Their evolutionary advantage lies in spreading their genes and then seeking additional mates. In contrast, females invest significantly more, undergoing gestation and enduring the mental and physical demands of pregnancy. During this period, they are unable to conceive again, further emphasizing their biological commitment.

Parental investment after birth varies across species. Most aquatic invertebrates, fish, and amphibians provide little to no parental care. In birds, however, both parents often contribute to prenatal and postnatal care, including nest-building, egg protection, and brood care. Mammals—especially humans—require even greater parental investment. After a nine-month pregnancy and a prolonged nursing period, one or both parents continue to provide extensive care, sometimes for decades.

This disparity in parental investment, Trivers argued, has a profound impact on mate selection. Because females invest more in reproduction, they are generally more selective when choosing a mate. Meanwhile, males compete for mating opportunities, with success often determined by factors such as size, strength, and bright coloration, which signal health and vitality. Females tend to prefer physically fit males with superior genetic traits, high social status (alpha males), and access to resources. In species where both parents contribute to offspring care, females are more likely to choose males who demonstrate a willingness to provide support.

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.