Variation in the durations of exclusive breastfeeding (exBF) and any breastfeeding (anyBF) is associated with socioecological factors. This plasticity in breastfeeding behavior appears adaptive, but the mechanisms involved are unclear. With this concept in mind, we investigated whether durations of exBF and anyBF in a rural Maya population covary with markers of a form of socioecological change—market integration—and whether individual factors (individual learning, physiological plasticity) and/or learning from others in the community (social learning, norm adherence) mediate these changes. Using data (...) from 419 mother-child pairs from two Guatemalan Maya villages, we fit a bivariate linear mixed model. The model compared exBF and anyBF among children from households of varying degrees of market integration whose mothers follow what we inferred to be local infant-feeding norms. It controlled for other factors expected to affect breastfeeding durations. We found evidence that exBF is associated with whether mothers follow their population’s infant feeding norms, but no evidence that exBF is associated with the household’s level of market integration. Conversely, anyBF is significantly associated with the household’s market integration, but not with the villages’ inferred norms. Because deviations from exBF norms are likely to result in infant mortality and reduced fitness, we hypothesize that the incentive to conform is relatively strong. Relatively greater individual plasticity in anyBF allows mother-child pairs to tailor it to socioecological conditions. Deviations from anyBF norms may be tolerated because they may provide later-life health/fitness payoffs, while posing few risks to infant survival. (shrink)

Cross-cultural studies have revealed broad quantitative associations between subsistence practice and demographic parameters for preindustrial populations. One explanation is that variationin the availability of suitable weaning foods influenced the frequency and duration of breastfeeding and thus the length of interbirth intervals and the probability of child survival (the “weaning food availability” hypothesis). We examine the available data on weaning age variation in preindustrial populations and report results of a cross-cultural test of the predictions that weaning occurred earlier in agricultural and (...) pastoral populations because dairy and cereal production increased the availability of easily digestible, nutrientrich foods appropriate for weaning. We found that, contrary to predictions, supplementation with liquid foods other than breast milk was delayed in agricultural populations relative to less agriculturally dependent ones and complementary feeding with solid foods was delayed in pastoral populations relative to those less dependent on herding. Although the duration of breastfeeding was longer in populations dependent on hunting, there was no qualitative evidence that such populations lacked foods appropriate for weaning. The patterns observed suggest that the relationships between demography and subsistence observed among preindustrial societies cannot be explained by the “weaning food availability” hypothesis. We discuss the implications for understanding the mechanisms underlying prehistoric human demography, subsistence shifts, allocation to parenting and mating effort, and the evolutionary implications of tradeoffs between diet and disease. (shrink)