The next time you find a blue eggshell lying empty beneath a tree, consider the work of Donna Holmes, whose research shows that birds are exceptionally long-lived for their size. Under good conditions, female birds may experience up to a third of their life in post-reproductive years. In other words, says Holmes, birds, like humans, can experience menopause – as can guppies, lab rats and mice, opossums and primates.
Holmes, associate research professor in the School of Biological Sciences and Center for Reproductive Biology, is involved in a number of studies that ultimately seek to compare the biology of female reproductive aging across the animal kingdom. Asking the questions of why some animals age more slowly than others and why some have longer reproductive spans, she confronts the conventional wisdom that says the ovary is the primary timekeeper in aging females.
Specifically, Holmes is interested in discovering whether or not a finite pool of oocytes (eggs in the ovary) plays a central role in reproductive aging. Female mammals are born with a limited number of eggs, which mature as needed after puberty. Fish, amphibians and reptiles, on the other hand, produce new eggs with every breeding cycle throughout their lives.
In theory, these “cold-blooded” animals should not experience menopause. Yet, in a study co-authored by Holmes, it was shown for the first time that guppies could indeed have reproductive “senescence” – a polite word for aging – and extended post-reproductive life spans. In fact, what researchers observed in the fish looked a lot like reproductive aging in women.
Grandmother hypothesis discredited
Which brings us to the grandmother hypothesis. Popular among anthropologists, the theory suggests that ancestral grandmothers played a role in the survival and evolution of the human species by helping feed and care for grandchildren.
“The grandmother hypothesis makes the case that human menopause is a special Darwinian adaptation,” said Holmes. “By stopping reproduction in midlife, it is speculated that women could invest that energy into ensuring the survival of daughters and grandchildren.”
Stirring up a bit of controversy among feminists, Holmes’ data on comparative reproductive aging challenges that assumption.
“The hypothesis has been a popular idea because it gives a supportive role to older women in society,” she said. “But it is very important not to let science be driven by what we’d like to see.”
She explained that if menopause were truly an adaptation, we should see a certain statistical leveling off of mortality rates with loss of fertility. Instead, her data shows that infertility rates in women are associated with the same reproductive aging (and mortality) patterns as seen in guppies, birds and rodents.
“Reproductive aging in humans looks a lot like reproductive aging in animals that have no real social lives or extended family networks that would benefit from midlife care on the part of grandmothers,” said Holmes. “When animals – including humans – are in a stable, protected environment, they are more likely to live long enough to experience reproductive aging or menopause.”
Our bodies, ourselves
Her conclusions have not been popular with feminists, although she also considers herself to be in this group.
“Some feminists say I have betrayed the cause,” said Holmes. “But feminism and science have long been uneasy bedfellows- in general women are up against a bewildering array of contradictory information about their bodies.”
She said it can make a big difference whether a woman views menopause as an adaptive mechanism or simply as a result of living longer.
“Women who view it as part of natural selection tend to want to ride out the distress and physical changes of menopause without medical intervention. Those who see it as a part of aging may be more willing to accept treatment for hot flashes, heavy periods or osteoporosis,” said Holmes.
“It is a very complicated set of medical issues,” she said. “The more we come to understand about reproductive aging in women within the evolutionary context, the better we will be able to handle the medical issues of menopause.”