Researchers used demographic data from pre-industrial Finland to show, irrespective of access to resources, mothers but not fathers with many sons suffered from reduced survival, a media report said.
But this association reduced, as mothers got older, according to the Royal Society journal Biology Letters.
“Our results provide evidence that Finnish mothers traded long post-reproductive lifespan for giving birth to many sons,” Dr Samuli Helle, of the University of Turku, said.
Previous studies have suggested sons are especially costly for the mother because they are, on average, born heavier and place more physical stress on the body.
“They also raise levels of testosterone in their mothers”, which can age the immune system, making it is less able to defend the body.
Moreover sons need more care after birth. On the other hand daughters prolong mothers’ lives by helping in tasks such as obtaining food and rearing younger siblings, at least in the traditional society studied.
Dr Helle’ study found that women’s post reproductive survival declined with the number of sons they gave birth to, regardless of the socio-economic status.But was age dependent. That is, the survival costs of the number of sons born decreased linearly as women aged.
However, the number of daughters born was not connected to women’s lifespan and, in men, neither the number of sons nor daughters born were related to their survival, irrespective of their socioeconomic status.
Added Dr Helle: “Our results show producing sons shortened the post reproductive lifespan of Finnish women and this association deteriorated as mothers aged.”