You only see aged polar bears at the zoo: in the wild, when a polar bear reaches an age when he can no longer cope with the brutality of his existence, he will die.
It’s different for humans, and a few other creatures, such as whales, that display social behaviour. In these societies, animals go on living long after they have become infertile following menopause. The reason remains a mystery to biologists, as the theory of evolution teaches them that the creatures that produce the most offspring ultimately win the Survival of the Fittest race. Barren senior citizens consume food that could provide sustenance for the younger generation, so why don’t people simply die when they turn fifty?
In the past, there were two answers to this question. Firstly, our old age is a misrepresentation: because we can drastically alter our surroundings, we have become just like the polar bear, living in zoos we have constructed for ourselves. However, this isn´t the right answer, because you find veterans in hunter-gatherer societies, and every so often, archaeologists find the remains of elderly Neanderthal people. Evidently, people live to old age even in the wild, in contrast to polar bears.
The second answer was presented by the grandmother hypothesis, which claimed that people live to old age because the elderly can help to bring up their grandchildren. This gives the grandchildren a better chance of survival and natural selection provides for you in old age.
“Everyone supports this theory”, explains doctoral student David van Bodegom. “After all, it ties in completely with what you see: grandmas looking after their grandchildren. It gives grandmothers a raison d’être too, of course. In a Scandinavian family, with a husband, wife and fifteen children, the presence of a granny makes a huge difference.”
Nevertheless, there is a problem with the grandma hypothesis: this type of monogamous family is, in evolutionary terms, quite a recent invention. People did not live like that in ancient times. Elderly grandmothers may be useful in modern circumstances, but it does not explain why they live to old age. People are good at computer games, but that’s not a consequence of natural selection on the grounds of our scoring capabilities.
Archaeological and anthropological research has revealed that people, by nature, live in polygamous societies, in which wealthy men marry several women. Laughing, Van Bodegom continues: “Deep down inside, men know that they are polygamous by nature.” Does the grandmother hypothesis apply to this type of society as well? Van Bodegom decided to find out. He visited the Garu-Tempane district in the eastern part of Ghana, an arid and scorched region. The people are mostly self-sufficient farmers who live in extended families averaging fifteen people: one man with one to four wives, their children and perhaps the grannies. Would the presence of the grannies make any difference?
Van Bodegom counted the children in families with, and without, grandmothers, weighed them and studied which families had the highest child-mortality rate. “People sometimes ask me if that’s ethical and whether, because I’m a doctor, I shouldn’t be helping them rather than tallying how many die. But we ask this kind of research question in Leiden too – why shouldn’t I be allowed to do it in Ghana? Luckily, the government over there agrees with me.”
“We were known as the people with the papers”, recalls Van Bodegom. “Not much happens in those villages, so there was quite a stir when we arrived. I was primarily interested in families and family trees, and the people there just love talking about their families, so I was always welcome.” The researcher managed a whole team of local staff who collected data of approximately thirty thousand people in total.
Their conclusion was that children in families with grannies do not weigh more than children in families without grannies. Their chances of survival, if you allow for the fact that both the children and the grandmothers are healthier in more affluent, are equal. However, on average, families with grandmas have more children: two point three per cent more, to be precise. In other words: grandmothers in Ghana are not contributing very much. There are so many other women and older children to help care for the little ones, so having a granny around does not make that much difference.
But Van Bodegom has more to report: “What did matter was where people acquired their water. In Ghana, there are traditional wells, and boreholes that were dug recently, the latter are wells that are thirty metres deep. The first type of well is often polluted and children who drank water from them had thirty per cent more chance of dying during our study”
He continues: “Obviously, I felt compelled to share this information with the people over there. Some households had access to both sources of water, so it’s useful to know that the water from the boreholes is safer, isn’t it?” He runs his hand through his hair. “But they just went on drinking from the traditional wells, as they explained that the water from the boreholes didn’t have any taste. Unsafe? Their father always drank it and did I know how strong he was?” The cultural differences made a considerable impression on Van Bodegom: he has incorporated them in his debut novel Nood breekt wet [Necessity knows no law] which is to be published in January.
He did not find any evidence for the grandmother hypothesis, so why do people live to old age?
Van Bodegom suspects that it has more to do with the fathers: men are fertile for much longer than women, and can father children up to a great age – Charlie Chaplin apparently fathered a child at 73. So, women don’t live to old age because they are handy to have around as grandmas, but because they share the same genes that ensure senior citizens can still become fathers. Van Bodegom suggests an elderly-father hypothesis to replace grandmother hypothesis.
Van Bodegom will stay on in Leiden after receiving his PhD degree, working at the Leyden Academy on Vitality and Aging, a knowledge centre for vitality and ageing founded by his supervisor, Professor Rudi Westendorp.
Do they have any use for the evolutionary biological insights into Ghanaian families? Van Bodegom replies: “It was very basic research into ageing theories. We want to understand why we can grow old, or why we only get certain illnesses after we’ve turned 50 and we can only explain it if we think in evolutionary terms.”
David van Bodegom
Post-reproductive survival in a polygamous society in rural Africa
The PhD ceremony was held on 2 November