KeywordsSexual Selection Brain Size Parental Investment Male Care Bipedal Locomotion
Biparental care is the situation whereby male and female parents share responsibility for raising their mutual offspring.
Across the animal kingdom, patterns of parental care vary from no care through uniparental female or male care to biparental care where both parents invest in the offspring. Among the vertebrates, biparental care is highest in avian species where it is estimated to be the case for 81% of species (Cockburn 2006) and lowest in mammals where estimates are below 10% (Kleiman 1977). This observed variation raises the question as to what are the evolutionary driving forces for the emergence of biparental care.
The Biparental Care Hypothesis and the Evolution of Monogamy
According to the biparental care hypothesis, this pattern of exclusive cooperation in care of mutual offspring evolves when each parent achieves higher reproductive success than either can through polygamy (Wittenberger and Tilson 1980). Hence biparental care is seen as a driving force for monogamy. Support for this hypothesis has come from a number of studies involving parental removal in species of birds, fish, and amphibians (see, e.g., Møller 2000; Tumulty et al. 2014). In such studies the male of the pair is generally removed leading to a reduction in offspring survival. This has been taken as evidence that, if one parent were to perish, the remaining widowed parent would not be able to provide sufficient care for the offspring alone. The unusually high incidence of biparental care in birds is generally regarded to be a result of the extensive resource requirements for the development of flight-capable fledglings. Cleary outside of bats, this is not an issue for mammalian species.
This mammalian pattern of little or no male parental care is also the case for the majority of primate species. While some species of lesser apes such as the agile gibbon do engage in biparental care, among the great apes, male care of offspring is virtually nonexistent. In contrast, although human mating patterns vary across cultures, monogamy with biparental care is commonly observed (even in cultures where polygyny is accepted). Hence humans are, as a species, high in what Robert Trivers has described as “male parental investment” (MPI, Trivers 1972). This raises two important questions: why is biparental care so common in our species, and what effect has high MPI had on human evolution?
Bipedalism and Biparental Care in Human Evolution
Over the last four million years, human ancestors have exhibited a rapid increase in brain volume accompanied by a narrowing of the pelvic girdle to enhance bipedal locomotion. These two, arguably conflicting, changes led to an increasingly large brain having to fit through an increasingly narrow pelvis. The solution to this problem was to give birth to a developmentally younger fetus. This, in turn, led to hominin babies being born at increasingly immature and dependent stages of development. Moreover, it also led to an increase in the length of infancy. According to the provisioning hypothesis, due to the dependency of human neonates, it paid human males to shift from polygynous mating patterns to pair-bonded ones which thereby boosted reproductive success for both parties (Lovejoy 1981). It also increased selection pressures for improved bipedal locomotion since this freeing up of the hands allowed hominins to gather and bring back a wide range of foods. Note that in this model of human evolution, bipedal locomotion and pair bonding are inextricably linked. Although the provisioning hypothesis has been criticized for being male centered, features of it are widely accepted as an explanation for the evolution of biparental care in our species. In particular, with the division of labor that human biparental care allows for (broadly speaking men hunt and women forage), the range of nutritional items increased for all in the proto-family unit. It may also be that the increase in meat in our ancestral diet allowed for the expansion of the human brain (the brain demands 20 times as much energy as muscle tissue and consists largely of protein). To some, the formation of biparental care and the resultant increase in protein and energy-rich foods available to the growing child were a watershed moment in human evolution (Wrangham 2009).
Biparental Care and Human Psychological Dispositions
Evolutionary psychologists consider that biparental care has helped to shape human psychological dispositions. Following the evolution of biparental care, the introduction of meat into the ancestral diet may be seen as a necessary precursor to an increase in brain size, but it was not sufficient. In addition to an appropriate change in diet, an increase in brain size (and the resultant increase in intelligence) can only arise if there are selective pressures for this development. An increase in MPI is known to have a number of effects on both male and female reproductive behaviors. Due to their increased effort in offspring production, males become more choosey about whom they will mate with, and females become more competitive over potential male partners (Stewart-Williams and Thomas 2013; Stewart-Williams 2017). This means that, once biparental care is established in a population, both sexes then demonstrate a degree of choosiness, and both exhibit a degree of same-sex competitiveness (it should be noted, however, that females are likely to remain the choosier sex due to maintaining a higher level of parental investment than males). Same-sex competitiveness and opposite-sex choosiness are the main components of sexual selection (i.e., the opposite sex does the “selecting” rather than the environment as in natural selection).
In 2000 American psychologist Geoffrey Miller began to champion the notion that, once biparental care had evolved, the main driving force for increasing brain size and intelligence became sexual rather than natural selection. Drawing on Darwin’s (1871) work on the relationship between sexual selection and the evolution of human abilities, he called this the mating mind hypothesis. Miller proposed that, with an increasingly dependent offspring being born into a challenging savannah environment, it paid both sexes to recombine their genes with caring and intelligent individuals (Miller 2000). Given that sexual selection can act more rapidly than natural selection, it can be argued that the emergence of biparental care was a major factor in driving both human intellectual and emotional abilities toward the advanced levels we see today (Workman and Reader 2014).
Where biparental care is observed in the animal kingdom, it is seen as a driving force for monogamy. This also appears to be the case for our own species. For our hominin ancestors, the evolution of biparental care is likely to have had a profound effect on our behavior including increases both in choosiness in males and in levels of female/female competition. It may also have provided a driving force, through sexual selection, for expansion of human intelligence and cooperative behavior.
- Cockburn, A. (2006). Prevalence of different modes of parental care in birds. Proceedings of the Royal Society B 273, 1375–1383.Google Scholar
- Miller, G. F. (2000). The mating mind: How sexual choice shaped the evolution of human nature. London: Heinemann/Doubleday.Google Scholar
- Stewart-Williams, S. (2017). Are we peacocks or robins? In L. Workman, W. Reader, & J. H. Barkow (Eds.), Cambridge handbook of evolutionary perspectives on human behavior. Cambridge: Cambridge University Press.Google Scholar
- Trivers, R. L. (1972). Parental investment and sexual selection. In B. Campbell (Ed.), Sexual selection and the descent of man 1871–1971 (pp. 136–179). Chicago: Aldine Press.Google Scholar
- Workman, L., & Reader, W. (2014). Evolutionary psychology (3rd ed.). Cambridge: Cambridge University Press.Google Scholar
- Wrangham, R. W. (2009). Catching fire: How cooking made us human. New York: Basic Books.Google Scholar