Inclusive fitness and differential productivity across the life course determine intergenerational transfers in a small-scale human society

Abstract

Transfers of resources between generations are an essential element in current models of human life-history evolution accounting for prolonged development, extended lifespan and menopause. Integrating these models with Hamilton's theory of inclusive fitness, we predict that the interaction of biological kinship with the age-schedule of resource production should be a key driver of intergenerational transfers. In the empirical case of Tsimane' forager-horticulturalists in Bolivian Amazonia, we provide a detailed characterization of net transfers of food according to age, sex, kinship and the net need of donors and recipients. We show that parents, grandparents and siblings provide significant net downward transfers of food across generations. We demonstrate that the extent of provisioning responds facultatively to variation in the productivity and demographic composition of families, as predicted by the theory. We hypothesize that the motivation to provide these critical transfers is a fundamental force that binds together human nuclear and extended families. The ubiquity of three-generational families in human societies may thus be a direct reflection of fundamental evolutionary constraints on an organism's life-history and social organization.

Keywords: food sharing; grandparental investment; human life history; inclusive fitness; intergenerational transfers; parental investment.

Figure 1.

Net production of Tsimane’ (a) individuals and (b) nuclear families.

Figure 2.

Net transfers of food as a function of donor and recipient age and sex. Means and standard errors are showed in green for females and purple for males. (a) Net from a focal mother/father to all children. (b) Net to a focal daughter/son from all parents. (c) Net from a focal grandmother/grandfather to all grandchildren. (d) Net to a focal granddaughter/grandson from all grandparents. (e) Net from a focal wife/husband to her/his spouse (or spouses). (f) Net from a focal mother/father-in-law to all children-in-law. (g) Net to a focal daughter/son-in-law from all parents-in-law. Values are derived from electronic supplementary material, tables E1–E7. Note that the y-axis varies depending on the scale of the net transfer values, and that transfers from mothers to offspring do not include contributions through lactation.