Parental Investment

Parental investment (PI) is defined within Life History Theory as the allocation of resources, such as time or energy, to offspring that incurs some cost to the parent. That cost is usually measured in terms of the benefits to a particular offspring which curtail the parent’s ability to invest in and produce other offspring. Some level of PI is found in all reproducing organisms, but PI is most costly in those species providing parental care behavior at pre-natal or post-natal stages of life, including egg guarding, preparation of nest, brood carrying, incubation, placental nourishment, and post-natal care including food provisioning, lactation, and protection of offspring. Parental care is found in species of invertebrates, fish, amphibians, reptiles, birds and mammals. PI for most primate species, including chimpanzees, bonobos, and orangutans, is typical for the mammals, in that female primates invest heavily both pre-natally and post-natally in the care and feeding of infants. In comparison, males of most primate species invest relatively little in offspring. Exceptions are the considerable male PI provided in species of marmosets and tamarins, Aotus (South American owl monkey), some macaques (e.g., Macaca sylvanus, the Barbary macaque), gorillas, and humans. In general, there is greater paternal PI with greater paternal certainty, e.g., paternal certainty is relatively low for chimpanzees but relatively high for gorillas. In contrast, the marmosets and tamarins practice polyandrous breeding (low paternal certainty) and the paternal PI may be a behavior to enhance mating opportunities. Humans in some cultures have, perhaps, the highest levels of paternal PI of all primate species. Humans also practice a unique form of biocultural cooperative breeding. Rules of marriage and kinship, as well as paternal certainty and enhancement of mating opportunities have been proposed to explain the human difference. The human style of PI may have begun to evolve with the appearance of Homo ergaster (Homo erectus) in the fossil record.