After analysis of births and deaths in Costa Rica, a study has found that grandmothers who live longer have fewer grandchildren raised by their daughters.
For the research, Lorena Madrigal at the University of South Florida, Tampa, US, and Mauricio Melendez-Obando of the Costa Rican Academy of Genealogical Science, analyzed church and civil government record data on births and deaths in the country between 1500 and 1950.
The analysis revealed that women who died before they were 50 had an average number of 7 children, whereas women who lived to beyond 80 had an average of 8.3 children.
This in itself was unexpected, since on average all women stop reproducing at around the same time of 50.
Astonishingly, the analysis also found that women living beyond 80 had nearly 50 percent fewer grandchildren - 5.75, on average - than women who died before they were 50; these women had on average 8.9 grandchildren.
The researchers argue that that the reason behind such a trend is that when women lived into old age they increased the chances of survival for their children by being good mothers, but living too long created a strain on resources that harmed their grandchildren.
"The ones who really suffer with the presence of grandmothers are the grandchildren. With grandmothers around it could be a case of them eating food out of the grandchildrens' mouths, but we need more research to be certain exactly what it is that they are doing to reduce the reproductive success of their daughters," New Scientist magazine quoted Madrigal, as saying.
In theory, there are two likely reasons for women living long after menopause. The "grandmother hypothesis" says that menopause forces women to stop having children of their own so they can help raise grandchildren.
The "mother hypothesis" says menopause functions to force women to raise the children they already have and not run the risk of dying by having babies in old age. The new findings suggest that menopause is all about improving child rather than grandchild survival.
The study is published in The American Journal of Physical Anthropology.