Female reproductive competition within families in rural Gambia

Proceedings of the Royal Society B: Biological Sciences, Jun 2012

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.

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Female reproductive competition within families in rural Gambia

Ruth Mace () 0 Alexandra Alvergne 0 0 Department of Anthropology, University College London , London WC1E 6BT, UK Articles on similar topics can be found in the following collections Receive free email alerts when new articles cite this article - sign up in the box at the top right-hand corner of the article or click here References Subject collections Email alerting service To subscribe to Proc. R. Soc. B go to: http://rspb.royalsocietypublishing.org/subscriptions Female reproductive competition within families in rural Gambia Ruth Mace* and Alexandra Alvergne 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 others 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 daughters 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. 1. INTRODUCTION Studies of cooperative and communal breeding in animals focus strongly on reproductive conflict between individuals [1], whereas human studies tend to focus more on the cooperative benefits of communal life, and its relevance for the evolution of menopause [2]. Human life history is characterized by a long childhood, followed by a rapid reproductive phase, and then a long postreproductive life, at least for females. Human menopause might be selected for by kin selection favouring older mothers investing in their grandchildren rather than continuing to reproduce themselves [2]. There is now considerable evidence that grandmothers enhance the reproductive success of their offspring (reviewed in Sear & Mace [3]), including evidence from our own study in rural Gambia [4 6]. Evidence that maternal grandmothers benefit the survival of their daughters offspring has been found across a wide range of societies, although kinship norms may influence which residence patterns are most favourable [7]. However, there is disagreement about whether the grandmother effect, on its own, could select for menopause. Verbal models have emphasized other factors, such as the importance of mothers survival during childhood [8], or the importance Electronic supplementary material is available at http://dx.doi.org/ 10.1098/rspb.2011.2424 or via http://rspb.royalsocietypublishing.org. of an extended lifespan when fertile span is phylogenetically constrained [9]. Some mathematical models informed by data either fail to predict any fitness benefit associated with terminating reproduction so long before death [10] or predict small fitness benefits [11]. Implicit in all the various models of the grandmother hypothesis is the notion that mothers and daughters are in reproductive competition, as it is assumed that only by becoming non-reproductive can a grandmother really help her daughters reproduction. It is striking in humans how little human female generations overlap; as a daughter reaches reproductive age, her mother reaches menopause, and as she reaches menopause, her mother dies. However, while reproductive conflict predicts that reproductive generations should reduce overlap [12], it not does address why it is the older woman who is foregoing reproduction rather than the younger one, as is generally the case in most cooperatively breeding birds or mammals [1,13]. Johnstone & Cant [14] argue that relatedness to group members, and hence the power of (...truncated)


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Ruth Mace, Alexandra Alvergne. Female reproductive competition within families in rural Gambia, Proceedings of the Royal Society B: Biological Sciences, 2012, pp. 2219-2227, 279/1736, DOI: 10.1098/rspb.2011.2424