Reproductive conflict between successive generations.

A new model proposed by Michael Cant and Rufus Johnstone in 2008, suggests that reproductive conflicts between successive generations of women is reduced due to the help of fertility loss. The grandmother hypothesis also helps to support this and according to Hamilton’s rule of ‘kin selection’, you are always more closely related to your own offspring than your grand offspring. So a question that was posed to me that I am wanting to answer is, is there any evidence that reproductive conflict between successive generations is actually high in humans?

Despite enormous differences in social systems as well as access to resources, the universality of menopause in modern humans illustrates the flexibility of behavior compared to the physiological process underling reproductive sequences.

The model of reproductive conflict does not suggest that older females should not help daughters or younger females if the social system becomes less female-biased, or maintaining kin ties between mothers and their daughters. But, if and when given a choice of helping either daughter or son, mothers should direct there help towards the daughters since grandchildren through sons may have been fathered by extra-pair males (for example (of an extra-pair male), a female may mate with one male, but rely on another male for resources).

Bereczkei & Dunbar (1997) and Sear et al. (2000, 2002) reported that the presences of a paternal grandmother has relatively little effect on offspring survival compared to that of a maternal grandmother from a number of studies.

For example: data obtained from the eighteenth and nineteenth centuries in the Krummhorn region of Germany show that the chance for a daughter-in-law to have a stillbirth was increased by 35% if the paternal grandmother was present in the household, and at the start of the daughter-in-laws marriage, these mortality costs are predominantly high (Beise & Voland, 2002, Voland and Beise, 2005).

Figure 1. Map of Krummhorn in Germany. Anon (2015).

This is data shows evidence that supports there is reproductive conflicts between successive generations in human families, as the assumption of a female-biased dispersal system has a positive impact on maternal grandmother.

References:

Anonymous. (2015) Map of Krummhorn in Germany. http://www.krummhoern.de/uploads/images/zoom/krummhoern_karte.jpg; Retrieved 29/05/2015.

Beise, J. & Voland, E. (2002). A multilevel event history analysis of the effects of grandmothers on child mortality in a historical German population (Krummhorn, Ostfriesland, 1720-1874). Demographic Research, 7, 469-497.

Bereczkei, T. & Dunbar, R.I.M. (1997). Female-biased reproductive stratigies in a Hungarian Gypsy population. Proceedings of the Royal Society of London 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. Proceedings of the Royal Society of LondonB, 267, 1641-1647.

Sear, R., Steel, F., & McGregor,I.A. (2002).  The effects of kin on child mortality in rural Gambia. Demography, 39, 43-63.