Peter B. Gray Ph.D.

The Evolving Father

Grandfathers and Their Brood

What happens when fathers shape the next generations?

Posted Feb 24, 2016

Robert Trivers recounts at the end of his autobiography, “Wild Life”: “I have five children and nine grandchildren, and they occupy an increasing fraction of my time. So there are perhaps two sources of joy in my life—my children and grandchildren and the continuing ability of science to generate new and exciting facts and vistas.” (p. 220) What happens when we combine an evolutionary perspective with reflections on grandfathers?

Degree of relatedness matters, we and Robert Trivers would think. We might anticipate that stepgrandfathers would feel less attached and invested in grandchildren than biological grandfathers. A few U.S. studies suggest something like this is happening, but there’s no clear or definitive word yet. Some scholars recognize that paternity uncertainty suggests, all else equal, that maternal grandfathers should invest more than paternal grandfathers; those paternal grandfathers may have an inkling of doubt, unlike maternal grandfathers, whether their efforts reach their genetic descendants. There is some support for that idea in the U.S. and Germany, for example. However, the cultural context of fathering and grandfathering suggests that other factors can trump that concern. In parts of rural Greece where grandchildren were more likely to reside with paternal grandparents, paternal grandfathers were more invested than were maternal grandfathers. So consider cultural context and you must also consider residence and proximity in understanding the patterns of grandfathering.

As men face direct reproductive senescence, their relative contributions to reproductive effort can tilt toward more parental (and grandparental) investment and away from mating effort. The age-related declines in men’s ability to physically compete, court new reproductive-aged partners, and maintain erections arguably reflect diminished adaptive maintenance of direct reproductive function. Indeed, most sexual and reproductive partners for older men are longstanding partners who themselves trace an arc of reproductive life. A married man’s age-specific reproductive output takes a similar shape as that of his wife, even if older men are more likely to repartner than women and are capable of having children at advancing ages. Among !Kung foragers of southern Africa, widowed men were more likely to repartner and invest in a new partner’s children than were widowed women. In some parts of India, when one’s kids begin reproducing it is expected that you will curtail your own sexual and reproductive life. If you aren’t going to devote your spare time and effort to making more babies, why not tend to the children and grandchildren you already have?

Older men, including grandfathers, can serve a variety of roles. The cross-cultural literature tells us that many older men are esteemed for the resources they may control, their political abilities, their involvement in arranging the marriages of descendants, and the specialized knowledge they hold about the natural and economic world. Tsimane elder men are valued for their knowledge of the natural world and forest spirits, and for their abilities to voice opinions and resolve conflicts. Celebrate the patriarchs of past and present. But the lot of grandfathers is varied.

Just as human males tend to exhibit higher reproductive variance than females, there are parallels in the diversity of older male outcomes. Some men never became fathers, much less grandfathers. Many older grandfathers die well before their spouses, reflections of being older at the time of marriage and having shorter lives. The social and political inequality among men leaves some grandfathers holding valuable resources and political standing, but many others struggling to be of value to their descendant kin. Grandfathers have variable relationships with their own children, which can shape the access they have to their grandchildren. As men’s physical capacity diminishes with age, they often shift their economic activities, providing different kinds of foraged foods, helping maintain their social utility. Those same once-useful men are treated more poorly by community members and family when their valued roles wane. In the face of cultural disruption by Western influences among Asmat hunter-gatherers in New Guinea, older men lost the influence they had wielded over ritual regulations.

If having and caring for a man’s grandchildren offers purpose to his activities, a lack of descendants can have its own impact. Among the Tsimane, “Living alone without kin support and visits by children is a haunting prospect for many older people and characterizes the essential fear of growing old among many Tsimane." (p. 61) If you’re a renowned evolutionary biologist (Trivers), taking stock of evolutionary theory and your life, here’s the moment to celebrate your biological descendants and write the tales that have made it all a wild life.


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