during famine, more baby boys for strong moms

When times are tough, the strongest women bear more sons, reported British researchers in the journal Proceedings of the Royal Society last week. University of London evolutionary anthropologists Mhairi Gibson and Ruth Mace studied mothers in a rural Ethiopian community undergoing serious food shortages. Overall, fewer boys than girls were born. It’s known that sons take more of a reproductive toll than daughters: boys grow faster and are bigger at birth. Malnourished baby boys also are more likely to die than malnourished baby girls. So, from an evolutionary standpoint, it makes sense that malnourished women would give birth to more daughters. But the researchers also found a strong link between a child’s gender and its mother’s muscle mass, a marker of how well-fed she is. Women who were in the upper 25th percentile of arm muscle mass were more than twice as likely to have had a son than those in the lowest 25th percentile. A well-nourished mother can most afford the physical cost of bearing a son. Gibson and Mace’s study suggests that mothers’ bodies can somehow manipulate the sex of their children in response to food availability. While this has been seen in wild animals, this is the first time it has been observed in humans.

This news brief appeared in the Random Data column of the Boston Globe’s Health/Science section on 5/27/2003.