Who Needs a Grandpa?

The immense benefits of having a helpful grandmother has been touted as the reason that women live so long past the age of reproduction. But where, exactly, does that leave grandfathers? Off playing shuffleboard, apparently.

Grandmothers are a powerful evolutionary force. Studies of present day communities in Africa and historical communities in Finland and Quebec have found that having a living grandmother around to protect, care and collect food for grandchildren can significantly increase the survival of grandchildren and, subsequently the number of grandchildren that a woman leaves behind.  Anthropologists suggest this grandmother effect explains why women evolved to live so long after menopause, and even why women quit reproducing in the first place - an idea called the Grandmother Hypothesis.

When it comes to explanations for male longevity, however, biologists usually assume it stems from the fact that men can keep sowing their seed well into old age; men in many traditional human societies put less biological effort into child rearing (by not having to give birth, for one), and can continue having children by marrying younger, fertile women. But that explanation only works in societies where men have multiple wives; life-long monogamy means that once the grandmother stops having children, grandpa has no means to increase his fitness by merely fathering more children. So grandfathers in monogamous societies might boost the number of their genes left to future generation by being helpful childminders like their wives.

To test this hypothesis, Mirkka Lahdenpera at the University of Turku in Finland (who had previously investigated grandmothering in Finland) and her colleague examined extensive church records from 1719-1839 in three separate Finnish communities. They used life-history traits that they argued would make for a more accurate assessment of grandfatherly benefits: the age when a son or daughter gives birth to the first grandchild; the time between births of grandchildren, total number of grandchildren or son/daughter reproductive success. 

The results showed no real benefit of having living grandfathers, write the authors. “ Overall, in contrast to our results for women in the same population, men do not gain extra fitness (i.e. more grandchildren) through grandfathering. Our results suggest that if evidence for a ‘grandfather’ hypothesis is lacking in a monogamous society, then its general importance in shaping male lifespan during our more promiscuous evolutionary past is likely to be negligible.”

So if grandfathering doesn’t keep men alive so long, what does? Must be all that shuffleboard.