Female reproductive competition within families in rural Gambia

Abstract

Many studies show that the extended human family can be helpful in raising offspring, with maternal grandmothers, in particular, improving offspring survival. However, less attention has been given to competition between female kin and co-residents. It has been argued that reproductive conflict between generations explains the evolution of menopause in cooperatively breeding species where females disperse, and that older females are related to the offspring of younger females through their sons, whereas younger, incoming females are unrelated to older females. This means the pattern of help will be asymmetric, so older females lose in reproductive conflict and become 'sterile helpers'. Here, we seek evidence for female reproductive competition using longitudinal demographic data from a rural Gambian population, and examine when women are helping or harming each other's reproductive success. We find that older women benefit and younger women suffer costs of reproductive competition with women in their compound. But the opposite is found for mothers and daughters; if mother and daughter's reproductive spans overlap, the older woman reduces her reproduction if the younger woman (daughter) reproduces, whereas daughters' fertility is unaffected by their mothers' reproduction. Married daughters are not generally co-resident with their mothers, so we find not only competition effects with co-resident females, but also with daughters who have dispersed. Dispersal varies across human societies, but our results suggest reproductive conflict could be influencing reproductive scheduling whatever the dispersal pattern. A cultural norm of late male marriage reduces paternal grandmother/ daughter-in-law reproductive overlap almost to zero in this population. We argue that cultural norms surrounding residence and marriage are themselves cultural adaptations to reduce reproductive conflict between generations in human families.

The great human expansion

“…This custom tends to limit the number of children born per family to six or seven, and approximately two thirds of these die before reproduction, so only approximately two children per family survive to reproduce, and population size remains constant. Mace and Alvergne (83) show that a similar phenomenon occurs among Gambian agriculturalists; as reproductive spans overlap between mothers and daughters, mothers reduce their fertility when their daughters begin have their own children, even if the daughters are dispersed outside their natal home. This custom of premature menopause also has the property of being elastic and responding to changes in death rates of the young.…”

Community differentiation and kinship among Europe’s first farmers

“…Status and wealth differences, which correlate with reproductive advantages (12,13,15,16), could well be crucial for modeling and understanding the genetic consequences of prehistoric human dispersals (17). Because genetic modeling approaches also begin to incorporate sex-biased mobility differences (18,19), there is increasing need for explicit evidence concerning these phenomena more directly.…”

When not to have another baby: An evolutionary approach to low fertility

“…Relatedness to the co-resident group will gradually increase with age as the woman's own offspring (particularly sons who do not disperse) are born and then grow up to reproduce themselves. We have shown that this pattern in relatedness to co-residents applies in the case of patrilocal compounds (where fathers, sons, and brothers, together their families, co-reside) in rural Gambia (Mace and Alvergne 2012). When an older woman finds herself in competition with her son's wife for reproductive resources, there is an essential asymmetry in that the older woman is related to her son's offspring (and will thus suffer a fitness cost if she harms the reproduction of her son's wife); but as the son's wife is not related to the older woman's offspring, natural selection does not favour her helping her husband's mother to reproduce.…”

“…This social arrangement is known as patrilocality or virilocality. It may therefore be the case that kinship, residence, and marriage norms in human societies are themselves cultural adaptations for reducing reproductive conflict in human groups (Ji, Xu, and Mace 2014;Mace and Alvergne 2012). Fertility schedules and rates could be co-evolving with human kinship systems, with an earlier female age at the first birth (and possibly a later male age at the first birth) associated with patrilocal residence.…”