The Evolutionary Ecology of Human Lactation: Correlates of Duration of Breastfeeding

Humans breastfeed our infants for less time than expected for primates of our size. Additionally, human breastfeeding duration appears more variable than in nonhuman species. This early and flexible weaning pattern affects maternal fertility as well as infant health and survivorship, from both within- and across- species perspectives. So, understanding what factors enabled humans to wean relatively early and flexibly is an important goal for human evolutionary demography. Yet, what these factors are and how they influence breastfeeding behaviour remain unclear. To address this gap, I present a series of three papers that each tests several hypotheses regarding influences on breastfeeding duration. The first paper reports a study that uses secondary data from small-scale human societies to investigate the effects of energetic factors on among-population variation in breastfeeding duration. The second has similar aims to the first, but uses within-population field data from indigenous Maya women from Guatemala to evaluate the energetic hypotheses. The third study, again using field data from Maya women, assesses a different set of hypotheses: that socio-ecological change and sources of socially-transmitted information about how to feed infants influence duration of breastfeeding.The first study shows that, across populations, breastfeeding duration associates negatively with maternal body mass, positively with maternal height, and negatively with dietary quality of weaning foods. The second indicates that within-population variation in breastfeeding duration associates negatively with maternal height, negatively with maternal access to help with infant care, and positively with parity. The last study suggests that duration of exclusive breastfeeding associates negatively with conservativeness of the source from which mothers learn about infant feeding behaviour. It also indicates that full duration of breastfeeding associates positively with household modernization. Taken together, these results suggest two things. One is that reduced duration of breastfeeding relates to greater maternal access to energy. The second is that socio-cultural factors influence variation in duration of breastfeeding in humans. These findings are consistent with previous claims that increases in energy availability and/or the development of complex cultural systems for information transmission contributed to the evolution of short, flexible breastfeeding and high fertility in humans.