Old vs. new - menopause

As older and younger women experience reproductive encounters, why has the older generation become infertile and not the younger one?  After all, it is the other way around for most mammals that collaborate to raise offspring – the older generation continues to breed and repress the fertility of the younger generation. It is believed that it all comes down to how our ancestors mingled with one another in their social groups (Johnstone & Cant, 2008). 

A Layman's Study of Human Evolution. M.McClintock (2014).

Members are switched within social groups to some extents and it is mostly the males that strike out. However, for ancestral humans, evidence suggests that females were most likely to leave and find new groups. Supporting this idea is the evolutionary behaviour of hunter-gatherers as well as our neighbouring relatives, the chimps and bonobos, along with hereditary evidence.

This changes the behaviour of how associated females are towards other members of their group, thus shifting the stability of conflict in favour of the young new group members. A young female entering a group is better off raising her own offspring as she is completely unrelated to the existing members of the group and obtains no indirect benefits from helping to raise their offspring. However, older females benefits by either obtaining more offspring herself or assisting to raise any offspring that her sons father with the young new group members.

Unequal benefits were established, by Cant and Johnstone, which skew the results of competition towards the younger females by using a simple model to mimic these interactions (Johnstone & Cant, 2008). If reproduction is ceased in older females as the younger ones become sexually mature, then the competition becomes stably resolved.

Cant and Johnstone declare further testing is needed for their hypothesis and they have suggest ways of executing this. They have stated that if their theory is accurate, scientists ought to be able to provide that young mothers experience disadvantages if they raise offspring in conjunction with older females who are still doing the same, as is the case in some polygamous societies.

To conclude, Cant and Johnstone announce that their new hypothesis is not an alternative to existing ones, but a complement to them. In hope that it will help to gain a full understanding in the origins of menopause if it is viewed as a reflection of the “ghost of reproductive competition past”.

Finally, Cant and Johnstone note that their new hypothesis is not meant to be an alternative to existing ones, but a complement to them. They hope that it will help us to more fully understand the evolution behind the origins of menopause if we view it as a reflection of the “ghost of reproductive competition past” (Hrdy, 2009).

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.

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.

McClintock, M. (2014) A Layman's Study of Human Evolution. http://www.frontrangeforum.org/courses/a-laymans-study-of-human-evolution/; Retrieved: 07/05/2015.