Department of Anthropology, University College London, London WC1E 6BT, UK.
Proc Biol Sci. 2012 Jun 7;279(1736):2219-27. doi: 10.1098/rspb.2011.2424. Epub 2012 Jan 18.
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.
许多研究表明，扩展的人类家庭在养育后代方面可能会有所帮助，尤其是祖母，她可以提高后代的存活率。然而，人们对女性亲属之间的竞争和共同居住者之间的竞争关注较少。有人认为，代际之间的生殖冲突可以解释那些具有合作繁殖特征的物种中更年期的进化，在这些物种中，雌性会分散，而较年长的雌性通过儿子与年轻雌性的后代有关，而年轻的、新来的雌性与较年长的雌性无关。这意味着帮助的模式是不对称的，因此年长的雌性在生殖冲突中失败，成为“不育的帮手”。在这里，我们使用来自冈比亚农村人口的纵向人口数据来寻找女性生殖竞争的证据，并研究女性何时会帮助或损害彼此的生殖成功。我们发现，年长的女性会从与她们同住在一个大院的女性的生殖竞争中受益，而年轻的女性则会受到损失。但对于母亲和女儿来说则相反；如果母亲和女儿的生育期重叠，如果年轻的女性（女儿）生育，年长的女性会减少她的生育，而女儿的生育则不受母亲生育的影响。已婚女儿通常不与母亲共同居住，因此我们不仅发现了与同居住女性的竞争效应，也发现了与已经离散的女儿的竞争效应。离散在不同的人类社会中有所不同，但我们的研究结果表明，无论离散模式如何，生殖冲突都可能影响生殖计划。在这个人群中，男性晚婚的文化规范使祖父/孙媳的生殖重叠几乎降至零。我们认为，围绕居住和婚姻的文化规范本身就是一种文化适应，以减少人类家庭中代际之间的生殖冲突。
