The Evolution of Human Parental Investment

Parental investment is any contribution that a parent gives to an offspring to increase the offspring’s chances of survival and reproduction at the expense of the parent’s ability to contribute to other aspects of its fitness (Barrett et al., 2002). Investment can be time, energy, food, protection, shelter, or other forms of care that benefit an offspring (Barrett et al., 2002). Costs to the parent are incurred on its inclusive fitness through lessening its ability to reproduce or invest in other existing offspring (Barrett et al., 2002). Human parental investment is unique in two respects, both of which are best understood from an evolutionary perspective. First, the amount of parental investment is large compared to other primates. Second, fathers share a larger proportion of parental investment in humans than in most other mammals.

The large amount of parental investment in humans results from the high energetic cost of growing and maintaining brain tissue combined with the large size of human brains and their relative immaturity at birth. A unit of brain tissue requires over 22 times more metabolic energy than an equivalent unit of muscle tissue (Aiello, 1997). Additionally, human brains are six times larger than would be expected for a primate of their body size, and primates already have large brains compared to other mammals of similar size (Barrett et al., 2002). Furthermore, human brains are one-fourth of their full size at birth, compared to those of Great Apes, which are one-half their full size at birth (Barrett et al., 2002). Thus, post-natal human brains start off comparatively underdeveloped and have a larger amount of growing to do, a process that has high metabolic costs. This relationship is the bas...

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