The strongest mothers are much more likely to bear sons in times of food shortages than weaker women, suggest a new study in Ethiopia.
Animals have long been known to manipulate the sex of their offspring in response to food availability. But such a phenomenon has never before been shown in humans, says study leader Ruth Mace, an evolutionary anthropologist at University College London, UK.
Mace suspected that this lack of evidence resulted from confounding factors in developed societies. 鈥淚t gets hidden in modern human populations 鈥� other things mask it like obesity and pollution,鈥� she told New Scientist. Going to rural Ethiopia, where these factors were not present, revealed the 鈥減ronounced鈥� sex bias.
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In 2000, the researchers interviewed and examined over 300 married women in southern Ethiopia shortly after widespread crop failure had left 20 per cent of them suffering from chronic energy deficiency.
Bigger, faster
Overall, the ratio of boys to girls born dropped to 0.88. Sons require more reproductive effort than daughters. 鈥淭hey grow a bit faster, they鈥檙e bigger when they鈥檙e born,鈥� Mace says. Males are also less likely to survive if malnourished as children, and stunted males are less attractive as potential mates.
The bias against boys was most pronounced for the weakest 25 per cent of mothers in the group, as measured by the amount of muscle on the upper arm. This group had 1.7 girls for every boy.
Balancing this bias was the strongest group of the women, which produced 1.5 boys for every girl. High body mass index also correlated to an increased chance of a male birth, but this effect was less strong.
Mace says the results are unlikely to be noticeable in a Western population. 鈥淲hat鈥檚 different is these people are living on very high workloads and are very food stressed. That might be the sort of circumstances in which the body jettisons male foetuses.鈥�
Journal reference: Biology Letters: (DOI 10.1098/rsbl.2003.0031)