“Ghost of reproductive competition past”.

Cant and Johnstones complementing hypothesis that early reproductive cessation reflects “the ghost of reproductive competition past” (Hrdy 2009 and Cant and Johnstone 2008) predicts that there will be obvious costs to females that are breeding alongside a reproductive grandmother. This hypothesis does not imply that older females who have experienced menopause should not assist daughters if the dispersal system becomes less female-biased or mothers are able to maintain kin ties to their daughters.

Variation amongst individuals or populations in factors that change the intensity or timing of reproductive competition from the next generation is predicted to associate with the age at last reproduction as well as reproductive overlap. Social system variation shown by modern humans also demonstrates the physiological processes underlying the species-wide trait of rapid reproductive senescence (a cell that is no longer capable of dividing but still alive and metabolically active) compared with the possible flexibility of behavior that leads to menopause.

The decision to help daughters outside the family increases the relatedness asymmetry which favours younger females in within-family conflict, as older females are able to help produce grand offspring that are related by ¼.

However, assuming that parenthood uncertainty is extensively accepted as a factor favouring maternal over paternal grand mothering, it is helpful to compare the magnitude of this effect with the magnitude of the relatedness asymmetry between older and younger females.

The kin-selected benefits of helping can explain post reproductive survival, but not why women cease reproduction so early in the first place. A model incorporating reproductive competition can help to account for this trait and for the particular timing of reproductive cessation in human females. Cant and Johnstone suggest that a combined model that takes into account both the inclusive fitness benefits of helping and the potential inclusive fitness costs of reproduction suggestions an improved understanding of the evolution of menopause.

References:

Hrdy, S.B. (2009). Will the Real Pleistocene Family Please Step Forward? Anon, Mothers and Others: The Evolutionary Origins of Mutual Understanding (pp. 207). United States of America: Anon. 

Bereczkei, T., & Dunbar, R.I.M. (1997). Female-biased reproductive strategies in a Hungarian Gypsy population. Proc R Soc London Ser B 264:17–22

Johnstone, R.A., & Cant, M.A. (2008). Reproductive conflict and the separation of reproductive generations in humans, 105(14):5332-5336. Doi:10.1073/;pnas.0711911105.

Sear R., Mace R., & McGregor I.A. (2000). Maternal grandmothers improve nutritional status and survival of children in rural Gambia. Proc R Soc London Ser B 267:1641–1647

Voland E., & Beise J. (2002). Opposite effects of maternal and paternal grandmothers on infant survival in rural Krummhörn. Behav Ecol Sociobiol 52:435–443.