Granny really does play favourites
Grandmothers play favourites among their grandchildren to preserve their genetic legacy, according to new research into the 'grandmother effect'.
The 'grandmother effect' or 'grandmother hypothesis' is the name for the widely accepted explanation for why women evolved to live so long beyond menopause.
It suggests that grandmothers who lived longer played a vital role nurturing their grandchildren and in doing so ensured their genetic line continued.
But a study published in today's Proceedings of the Royal Society B suggests that 'grannies' may be a little pickier than previously thought.
It points to grandmothers playing a greater role in ensuring the survival of those particular grandchildren to which they are more closely genetically related.
The international research team led by Molly Fox at the University of Cambridge based their work on the fact that grandmothers are not equally related to all of their grandchildren.
In particular, grandsons and granddaughters share different proportions of the highly influential X-chromosomes, with each of their grandmothers.
Paternal grandmothers share the greatest proportion of genes with their granddaughters and the least proportion of genes with their grandsons, but maternal grandmothers share a similar percentage of genes with grandchildren of both sexes.
The researchers then looked at the grandmother relationship in seven populations ranging from seventeenth century Japan to modern Ethiopia.
They found that grandchild mortality could be linked to the proportion of genes shared with grandmothers who were nearby.
In theory this means that thanks to evolution, it may be more dangerous for boys to grow up with their paternal grandmother than their maternal grandmother.
Biological anthropologist at the Australian National University Dr Robert Attenborough says the theory is feasible, but some significant holes remain.
"It's not a seamless story in which every aspect is cleared up. They are not very clear on how grandmothers would favour boys or girls."
The paper speculates that physical resemblance, smells or pheromones secreted by grandchildren may prompt favouring by one of their grandmothers, although it says that there is little evidence that it would be conscious behaviour.
Attenborough's ANU colleague and fellow anthropological biologist, Professor Simon Easteal agrees that much more information is needed about the cultural habits of the populations studied before anybody starts avoiding grandma.
"A lot of childcare goes on in human societies by people with all kinds of relationships - including none at all - to the children," says Eastel.
"The explanation only works if the genes that encode post-menopausal survival are X-linked, for which there is no evidence."