A new study has been published online in Ecology Letters by Mirkka Lahdenperä and colleagues, which suggests that competition between grandmothers and their daughters-in-law may explain the evolution of menopause. The study used a 200-year dataset of births, deaths and residency patterns in pre-industrial Finland to show that competition between unrelated females of different generations was a key component of selection for menopause.
Humans are among only four species known to lose their ability to reproduce long before they die; the others being killer whales, pilot whales and one aphid species. This phenomenon of menopause poses somewhat of an evolutionary conundrum: how could the loss of the ability to reproduce increase an individual’s fitness?
One possible answer was suggested by Cant & Johnstone, based on differences in how related a mother and daughter-in-law are to each other’s offspring. Historically, females of reproductive age usually leave their family to co-habit with their spouse’s family in most human societies, while males stay near their parents. This means that elder females are typically unrelated to next generation of reproductive females in their locale. Thus, it is expected that young females should invest in competition with their mother-in-law, while the elder mothers-in-law may be selected to cease investing in reproduction and instead invest in helping to raise their related grandchildren.
The new study by Lahdenperä et al. showed that when a mother and daughter-in-law reproduce at the same time offspring survivorship is reduced by up to 66%, while simultaneous reproduction by a mother and daughter had no effect. These patterns suggest that a daughter and mother-in-law compete strongly for resources for their children, as predicted by Cant & Johnstone.
The authors also used their data to parameterise a kin selection model to show that selection should favour menopause around the age of 50 in order to reduce this conflict. This study provides an excellent example of how theory and data can be combined to tackle evolutionary problems, and provides insight into one of the great peculiarities of the human species.
1. Lahdenperä M, Gillespie DOS, Lummaa V, Russell AF (2012) Severe intergenerational reproductive conflict and the evolution of menopause. Ecology Letters. (http://onlinelibrary.wiley.com/doi/10.1111/j.1461-0248.2012.01851.x/abstract)
2. Uematsu K, Kutsukake M, Fukatsu T, Shimada M, Shibao H (2010) Altruistic colony defense by menopausal female insects. Current Biology 20: 1182-1186. (http://www.sciencedirect.com/science/article/pii/S0960982210006391)
3. Cant MA, Johnstone RA (2008) Reproductive conflict and the separation of reproductive generations in humans. Proceedings of the National Academy of Sciences 105: 5332-5336. (http://www.pnas.org/content/105/14/5332)
Luke McNally: mcnalll[at]tcd.ie