The grandmother effect: implications for studies on aging and cognition

Abstract

Background: Women experience more years of vigorous life after ovulation has ceased than do females of other primate species. Is this an epiphenomenon of the greater life expectancy humans have enjoyed in the past century or so, or is long post-menopausal survival the result of an evolutionary selection process? Recent research implies the latter: Long post-menopausal survival came about through natural selection. One prominent line of thought explaining this selection process is the grandmother hypothesis.

Objective: To evaluate the implications of the hypothesis for non-human primate studies of aging and cognition.

Method: The author presents a synopsis of the hypothesis, evaluates the uniqueness of the 'grandmother effect' to humans, and discusses its implications for non-human primate models of cognitive aging.

Results: The hypothesis contends that, in past epochs, women who remained vigorous beyond their fertile years may have enhanced their reproductive success by providing care for their grandchildren. This care would have enabled their daughters to resume reproduction sooner, endowing them with greater lifetime fertility. Genes of grandmothers possessing such old-age vigor would be more likely to persist in subsequent generations. Is midlife menopause a uniquely human phenomenon, or does the chimpanzee, our closest primate relative, also display this trait? If so, we might expect a grandmother effect in this species as well. However, female chimpanzees continue to cycle until near the end of their maximum life span of about 60 years.

Conclusion: Long survival beyond fertility and a long life expectancy are distinctive human adaptations. The robustness of ancestral human grandmothers necessarily included resistance to cognitive decline through preservation of functions present in many primates but also development of processes of social cognition unique to our species. Cognitive traits such as language and social cognitive functions may function in our species in particular as mechanisms to compensate for age-related decline. This has significant implications for research in which non-human primates are considered as models of human cognitive aging; it also means that some processes can be studied only in humans.

Fig. 1

Schematic diagram depicting approximate ages at puberty, menopause, and maximum life span of female macaques (Macaca spp.), chimpanzees (Pan troglodytes), and humans (Homo sapiens). Bars are aligned to the age at menopause; shading indicates the fertile time span between onset of puberty and menopause. Approximate ages used are based upon references [15, 16, 18, 48, 49] : puberty: 3.5, 8, 12.5 years; median life span: 16, 26, 80 years; menopause: 25, 50, 50 years; life span: 35, 60, 100 years. The age of 100 years was arbitrarily chosen for the human life span because survival to this age is very rare; only about 2 years of life expectancy remain in 20th century European and North American cohorts [50].