Parental investment refers to the resources that the parent puts into raising its offspring, including time, energy, and materials. The concept was proposed by evolutionary biologist Robert Trivers to explain why in most species, one sex invests more resources in their offspring than the other sex. The sex that invests more becomes a limiting resource for which the other sex must compete. The differing levels of parental investment between the two sexes is considered the foundation for much of sexual selection theory and the evolution of different reproductive strategies.
females invest more in offspring due to eggs vs sperm
In most species, the female invests more resources into producing eggs than the male does in producing sperm. Producing eggs requires more energy and materials, and limits the number of offspring a female can have at one time. Once born, offspring are also nourished through the female body in many species via nursing or provisioning. Females therefore take on greater costs in reproducing, and are a scarcer reproductive resource. This is why males of most species must compete for access to females.
greater investment leads to choosier mating strategy
The sex that invests more will be choosier about their mates because they have more at stake in each reproductive event. Females gain reproductive success through quality of offspring rather than quantity, so they are selective and mate only with high quality, healthy males. Males focus on mating with as many females as possible to have the highest quantity of offspring. This accounts for the observation that males are typically the ones that must compete to gain access to mates.
differences in investment underlie sexual selection
The asymmetry in parental investment is considered the root of sexual selection – the evolution of traits and behaviors related to competition for mates. Because females invest more, they are a limited resource, and males must compete to mate with them, leading to the evolution of weapons, ornamentation, and competitive mating behaviors in males. Females are choosier, leading to the evolution of traits in males that can demonstrate their quality as a mate.
exceptions prove importance of concept
There are exceptions to the typical sex-based pattern of parental investment, such as seahorses where males brood young in a pouch. But these exceptions help demonstrate the importance of parental investment in determining mating systems – when male seahorses invest more in offspring, their mating behavior mirrors that typically seen in females.
Parental investment theory proposes that the sex that invests more biological resources in producing and caring for offspring will become a limiting reproductive resource for the other sex. This drives the evolution of competitive mating strategies and choosy mate selection. Differences in parental investment are considered the foundation for much of sexual selection theory.