#202 The Grandmother Hypothesis

Ageing Families

Why did evolution allow Grandmothers to live so long after losing their fertility

A Five-Minute Read

Humans are unique in the length of time women live after they lose their fertility. Most of the higher apes die before or soon after their menopause. Evolutionary anthropologists and biologists developed an answer. They called it the “Grandmother Hypothesis”.

Fertility as a Competitive Advantage

A human mother can have children at a much faster rate than the other apes. In natural evolutionary settings, they had a child on average every 3 years. This compares to chimpanzees at 5 years and orangutans at 8 years. This gave our species a huge competitive advantage. The scientists argue that it is only possible because we operate as a community. As the African proverb says, “It takes a village to raise a child”.

Human females take longer to be reach reproductive age. They have an extended period post menopause. Many other apes die before the menopause or soon after it. Some rhesus monkeys may live to the age of 35 but the average is much lower. Menopause occurs sometime between the age of 25 and 30. Most die before the menopause. Many chimpanzees remain fertile until they die at an average age of up to 60.

Evolutionary biologists explain why older women survive well beyond their menopause. It evolved to enable homo sapiens children to be supported by a larger group of adults. At a some point in evolutionary time the environment changed. Homo sapiens were suddenly faced with a world less able to support them. They evolved new ways of improving their fertility success rate.

Evidence for the Grandmother Effect

Putting together support for this theory is not easy. It means using data from current or recent populations. That must be used to infer something that happened in evolutionary times. Modern “hunter-gatherers” offer some clues.(Newsletter #002) . Grandparents do continue to forage for food until the end of their lives. In this way they support their children and grandchildren. Grandmothers are particularly productive in harvesting nutritious tubers.

Other studies look for pre-industrial societies. They must be isolated enough to assume that the fertility numbers are natural. The dependent measure usually taken is child mortality. The success of a mother in raising a child and keeping it alive. They need good records of births, deaths and families. 5 years of the childs life is usually taken as success. A Finnish study provides a good example. It showed that women living near their mothers reproduced at an earlier age. Overall they had a greater lifetime reproductive success. This was compared to women whose mother had died or lived in a different village.

The samples used are very varied. They include the island of St Barts in the Caribbean in the 18th and 19th century. Today it is a luxurious resort island but then it was barren. It has no natural source of water. There were isolated communities in Quebec communities in Nepal in the 1990’s and South American tribes. A recent study summarized over 45 such analyses. It looked for patterns of kinship support.

Kinship Support for Fertility

The core infant relationship is with their mother. In these more “primitive” societies, the death of a mother when giving birth is a catastrophe. Only 1.6% of children survived. This improved with the age of the child when the mother died. The death of the father had less of an impact. In two thirds of studies, it had no impact at all. In the hunter gather societies the men are responsible for providing meat. The “gathering” was the females’ role. Often it provided a much bigger proportion of the community calories. The results of the hunt were usually shared amongst the whole group. It was not just the family.

The studies do support the grandmother’s hypothesis. In 70% of the cases the presence of the maternal grandmother improved the survival of the grandchildren. This dropped to 60% for the paternal grandmother. Grandfathers seem to have no impact on the survival of their grandchildren. This measure is for up to the age of 5 years old. Beyond that there may be other effects. Authors have suggested that their role is to pass down knowledge and expertise. Also to use their wisdom to settle disputes. Pre-reproductive age siblings improved the survival rates but results are mixed. Older sisters seem to help survival.

The Grandmother Hypothesis Today.

In a recent Newsletter, I described the level of support that grandparents now provide to their offspring. They are still caring for grandchildren ( Newsletter #191 £3.5Bn of Family Childcare). The objective seems to have changed. It is no longer about the survival of the grandchildren. Instead, it is about the ability of the mother to go out to work. It is the grandmothers that are still taking the lead. Kin support in raising offspring still appears to be human universal.

The corollary of the hypothesis is that grandmothers have an extended life. They maintain their health and vitality in order to fulfill this role. Their cognitive ability must be maintained. As the age at which mothers have their first child has increased, grandmothers are living healthier longer. To help their offspring and the next generation.

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