Thursday, August 19, 2010

Sacrifice on the Serengeti: Life History, Genetic Relatedness, and the Evolution of Menopause

The latest stop in the #PDEx tour is being hosted by Dr. Carin Bondar at her website

Imagine you’re on the Serengeti Plateau and your children are hungry. For miles in every direction there’s nothing but dry scrub grass with the occasional flat-topped acacia tree marking the landscape. Your oldest has found a spot to dig for tubers but he and your daughter aren’t strong enough to scrape away the hard, baked earth by themselves. Your husband is tracking a wounded gazelle and could be gone for days. Meanwhile, the infant slung to your hip has started screaming and the distinctive sound triggers a release of oxytocin that causes your breasts to swell and leak. You have to feed her but you can’t do that and make sure your other children get enough to eat. There’s a very real chance that some of them will be too weak to survive the next time fever breaks out unless you can get help. You simply can’t be everywhere at once. It’s a desperate feeling but these are the daily realities among the East African Hadza. If it wasn’t for your mother, already kneeled on the ground and using a stick to claw through several layers of tough sediment, it might have been your reality as well. While your baby makes soft cooing sounds as she suckles you can only feel grateful that you were the youngest child in your family, or else your mother might well have had an infant of her own to care for.

This scenario provides the backdrop behind a perplexing question about human evolution: the advent of female reproductive senescence. Between the ages of 45-50 all women undergo physiological changes commonly known as menopause that result in the cessation of ovarian function. Since most women live longer than 50, even in preindustrial and hunter-gatherer societies, this raises a profound evolutionary question: Why would a species “choose” to forego one-third (and sometimes as much as one-half) of their reproductive potential?

Read the rest of the post here and stay tuned for the conclusion of The Scientist and the Anarchist tomorrow (see here for Part I and Part II) hosted by Deborah Blum at Speakeasy Science.


Hawkes, K. (2010). How grandmother effects plus individual variation in frailty shape fertility and mortality: Guidance from human-chimpanzee comparisons Proceedings of the National Academy of Sciences, 107 (Supp. 2), 8977-8984 DOI: 10.1073/pnas.0914627107

Fox, M., Sear, R., Beise, J., Ragsdale, G., Voland, E., & Knapp, L. (2010). Grandma plays favourites: X-chromosome relatedness and sex-specific childhood mortality Proceedings of the Royal Society B: Biological Sciences, 277 (1681), 567-573 DOI: 10.1098/rspb.2009.1660