Primates Reveal the Value of Grandmothers
Julio Torres, Intern.
Ancient Bodies, Modern Lives: How Evolution Has Shaped Women’s Health written by Wenda Trevathan, Ph.D., a Regents Professor of Anthropology at New Mexico State University, we learn about a range of women’s health issues. Trevathan’s hypothesis is that many of the health challenges faced by women today result from a mismatch between how our bodies have evolved and the contemporary environments in which we live. In the following excerpt, Trevethan draws from Jane Goodall’s observations of primates to illuminate how grandmothers, by virtue of being present in the family, contribute to the growth of prosperity of the grandchildren and the family unit as a whole.
Grandmothers and Reproductive Success
Most long-lived, group-living mammals have in their social groups as many as three generations present at any one time. Examples include elephants, whales and many primates. For primates who live in matrilocal groups, that usually means three generations of females: Infants, their mothers, and their grandmothers. A famous example comes from Jane Goodall’s studies of a Tanzanian chimpanzee social group in which Flo, her adult sons Faben and Figan, and her daughter Fifi lived together. Flo was a high-ranking female and her presence had a number of positive effects on her offspring. For example, Fifi was able to stay in the troop into which she was born, whereas the more typical pattern among chimpanzees appears to be for young females to leave their birth troops at maturity. By staying with her mother, Fifi was also able to rise to a high status. She began reproducing much earlier than most chimpanzee females and not only set the record for reproductive success at Gombe, but one of her sons became the largest male ever recorded at Gombe. Two of Fifi’s sons rose to high status in the dominance hierarchy and her daughter began reproducing much earlier than Fifi did. There is little doubt that grandmother Flo’s status had an effect on her daughter’s (and thus her own) reproductive success. There is no evidence, however, that Flo contributed directly to the care and feeding of her grandchildren, although it is true that she was not in good health at the time Fifi’s first infant was born in 1971.
Anthropologist Sarah Hrdy notes that despite her reproductive success, Flo serves as a good example of why having offspring at later ages may not be a good way to achieve this success or why “stopping early” might be selectively advantageous. Flo reproduced for the last time when she was very old and in poor health, but that infant did not live long. Goodall proposes that this last pregnancy was so draining for her that she was unable to mother her other young offspring, Flint, and when Flo died, Flint died also, even though he as at an age when he should have been able to survive on his own. In fact, if Flo had stopped reproducing after Flint, he probably would have lived, perhaps going on to sire another offspring and increasing Flo’s reproductive fitness through her grandchildren.
Similar evidence that the presence of grandmothers has positive effects on reproductive success comes from observations of a number of other primate species. Again, it is not usually resources and direct care that older female grandmothers provide; rather, they help to defend the infants from other troop members (including infanticidal males) whose behaviors endanger them. In fact, observers report that grandmothers will often act even more vigorously in defense of infants than younger kin. Grandmother Japanese macaques make a significant difference in survival of their grandchildren through the first year of life. Furthermore, females have much greater reproductive success if they have living mothers, even when those older females are still reproducing. Similar reports have come from studies of vervets, langurs and rhesus monkeys, as well as elephants. On the other hand, African Lions and olive baboons, while showing extensive caretaking by adults other than the mother (known as allomaternal care), do not seem to have their reproductive success influenced by the presence of grandmothers.
These descriptions of primate social groups with three generations of females are not very different from what is seen in traditional human societies and even in extended family households in health-rich nations like the United States. What is different, however, is that in most cases the grandmother is not only helping her own older children but she also provides care and resources to her grandchildren.
Another view of menopause focuses not on the mother and early termination of reproduction (the “long-lived mother hypothesis”) but on the grandmother who maintains health long after ceasing to reproduce. Known as the “grandmother hypothesis,” this proposal assumes that termination of fertility at about age 50 is a given, but that natural selection favored a long postreproductive period in women’s lives because by ceasing to bear and raise their own children, postmenopausal women would be freed to provide high-quality care for their grandchildren. In this scenario, older women “trade” their diminished chances of successfully raising an infant for enhanced opportunities to help raise their grandchildren. This is simply the continuation of a behavior that women have practices for most of their adult lives: providing food and care for children who have been weaned but who are not yet capable of getting their own foods in sufficient quantity and quality to survive. This continuity-of-care hypothesis also explains why so much of the focus on older people as alloparents is on grandmothers.
When the grandmother hypothesis was first proposed by Kristen Hawkes and her colleagues, it included supporting evidence based on their studies of the Hadza, a foraging population in Tanzania. Among these people, when a woman gives birth, her time providing food for her older children is severely curtailed and remains lower than usual for several months. During this time, the grandmother increases her foraging to make up for the reduction by the new mother. Certainly her success is increased if she is still in good health and able to travel widely gathering food. Thus, the argument is that natural selection not only favors termination of reproducing at about ago 50 and provisioning of older infants by grandmothers, but it also favors continued vigor and good health in the grandmothers until their own daughters cease reproducing and become provisioning grandmothers themselves. Notably, this argument also proposes that matrilineal proximity would be favored as well, calling into question the assumption that early human social groups were patrilocal and that females dispersed at maturity. Older women who provision their sons’ children would also increase their fitness, although certainty of kinship is higher through matrilines than through patrilines.