Cooperative breeding in South American hunter-gatherers

Abstract

Evolutionary researchers have recently suggested that pre-modern human societies habitually practised cooperative breeding and that this feature helps explain human prosocial tendencies. Despite circumstantial evidence that post-reproductive females and extra-pair males both provide resources required for successful reproduction by mated pairs, no study has yet provided details about the flow of food resources by different age and sex categories to breeders and offspring, nor documented the ratio of helpers to breeders. Here, we show in two hunter-gatherer societies of South America that each breeding pair with dependent offspring on average obtained help from approximately 1.3 non-reproductive adults. Young married males and unmarried males of all ages were the main food providers, accounting for 93-100% of all excess food production available to breeding pairs and their offspring. Thus, each breeding pair with dependants was provisioned on average by 0.8 adult male helpers. The data provide no support for the hypothesis that post-reproductive females are the main provisioners of younger reproductive-aged kin in hunter-gatherer societies. Demographic and food acquisition data show that most breeding pairs can expect food deficits owing to foraging luck, health disabilities and accumulating dependency ratio of offspring in middle age, and that extra-pair provisioning may be essential to the evolved human life history.

Figure 1.

Tri-monthly total meat acquisition for the top three Ache hunters from 1996 to 2003 at the Arroyo Bandera settlement. Each point represents the 90-day total meat acquisition. The dotted horizontal line represents approximately 10 per cent of the mean 90-day meat acquisition. See electronic supplementary material for more hunters.

Figure 2.

Measured daily energy acquisition and consumption for (a) Ache and (b) Hiwi hunter–gatherers based on observed foraging success, and weight and height of potential consumers. Male consumption, dotted line; female consumption, solid line; male production, triangles; female production, circles.

Figure 3.

Expected food acquisition and consumption for Ache nuclear families when the reproductive female of the family is of specified age. Parental consumption, solid line; total family consumption, dotted line; total family production, circles.

Figure 4.

(a) Estimated mean net daily energy production for 119 monogamously or polygynously married Ache women, their mates and their dependent offspring (15 years or less), based on reported family composition in 1970 (Hill & Hurtado 1996) and age–sex-specific food production patterns measured between 1980 and 2007. Triangles, monogamous pairs; circles, polygynous pairs. (b) Mean net daily energy production for 19 Hiwi monogamously married women, their mates and their dependent offspring from 1985 to 1988 based on actual family composition and measured individual food acquisition rates.