Last year, news reports covered the theory that menopause is an adaptation of evolution that allows older females to help their dependents raise children. According to the theory, menopause helps aging females avoid the increased complications and risks of childbirth, freeing them to pitch in with child care.
But a new study finds no evidence for this "grandmother hypothesis." Reporting in the April 23 issue of the journal Nature, three researchers suggest that menopause carries no evolutionary benefit or cost. Menopause, they say, is simply a consequence of aging, and how long females live after menopause depends upon how long a species needs to raise last-born infants to the age of independence.
The three researchers, including Marc Tatar, assistant professor of ecology and evolutionary biology, analyzed 35 years of data on birth, death and fertility among populations of baboons and lions in Tanzania. The study's lead scientist is Craig Packer, ecology professor at the University of Minnesota. The other scientist is Anthony Collins, a research director at Gombe Stream Research Centre in Tanzania.
Menopause occurs across the spectrum of mammals. However, the researchers find no evidence among elderly baboons and lions that menopause evolved for any useful function, such as to avoid increased dangers of middle-age childbirth or to permit grandmothers to better help raise their offspring. While grandmother baboons and lions both engage in kin-raising behavior, the survival of "grand-infants" did not depend on whether the grandmothers were reproductive or not.
In the study, reported on National Public Radio and in newspapers worldwide, only 7 percent of baboons and less than 3 percent of lions live long enough to reach menopause. If these animals were to keep on producing, they would barely contribute to population rates, indicating there no evolutionary significance to older-age reproduction.
The researchers say their data fit the theory that reproductive decline begins once a mother can no longer expect to successfully carry a new infant to its reproductive maturity. Post-reproductive life expectancy, however, can continue to evolve toward the time required to raise additional offspring. So, if a woman in a pre-technological society expects to live 50 years, and her child cannot survive independently until age 10, then a mother's reproductive decline will begin at age 40.
The study indicates that female baboons don't live past age 27 and that their young require two years of maternal care. Baboon reproductive rates decline around age 21, allowing plenty of time for the youngest offspring to become independent. Among lions, reproductive rates fall at age 14, while life expectancy of that 14-year-old is 15.8 years. Lion cubs need just one year of maternal care.
According to Packer, the female reproductive system is the first to go, declining at a point where a mother will not live long enough to raise an additional baby. Because human infants have a more prolonged period of dependency compared to other species, menopause among women occurs earlier in life, he said.
Tatar said the findings propose an alternative model to the "grandmother hypothesis" for the post-reproductive life span of human females.
"If reproductive decline is purely age related and not an adaptation, then we may consider menopause as a degenerative process that can possibly be addressed by medical treatments, such as hormonal therapy, to counteract the degeneration," he said. "If menopause has no adaptive significance, hormonal therapy should not produce side effects, because it will be a matter of replacing what declined naturally."
Tatar said the findings help researchers better understand the differences between males and females relative to reproductive decline. "The study also suggests that we may use baboons to study human menopause. In particular, we can do genetic research, as baboons and humans are both primates."
The researchers have contributed to an ongoing discussion about the timing and role of menopause. An author of the grandmother hypothesis says the latest findings still do not explain why, unlike baboons and lions, up to 40 percent of women live well beyond the decade needed to ensure that their offspring survive.