Menopause Said to Evolve From In-Law Competition
Scientists say our in-laws may be to blame for hot flashes and hormone changes.
By Allison Takeda
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FRIDAY, August 31, 2012 —Annoyed with hot flashes and night sweats? Blame your mother-in-law.
According to a new study, menopause evolved centuries ago to prevent competition between mothers and their sons' wives.
Physiologically, menopause occurs when your ovaries cease to produce and release eggs every month. Your body's estrogen and progesterone decrease, possibly but not always causing hot flashes, night sweats, mood swings, and weight gain. This happens for most women in their late 40s or early 50s, after which they are unable to bear children — even though they may have several decades remaining of good health.
"It remains an evolutionary puzzle," the authors write in their report, published in the journalEcology Letters. "Studies have largely failed to account for diminishing selection on reproduction beyond 50 years. ... We propose that menopause evolved, in part, because of age-specific increases in opportunities for intergenerational cooperation and reproductive competition under ecological scarcity."
Translation? Women stopped having children when their daughters-in-lawstartedhaving children to avoid conflicts over the division of food, money, care, and other resources.
A Solution for In-Law Infighting?
Researchers reviewed data on birth and death rates in Finland from 1702 to 1908, before modern contraception. They found that when a woman had a baby later in life, and at the same time as her daughter-in-law, she gained no benefit from passing on her DNA. In fact, she risked hurting her genetic legacy, said researchers from the University of Turku, the University of Exeter, the University of Sheffield, and Stanford University.
Women ended up with fewer grandchildren if they bore children after age 51, the average age of menopause, explains study co-author Andrew Russell, PhD, an evolutionary biologist at the University of Exeter's Centre for Ecology and Conservation. "From an evolutionary perspective, this means that women who stopped reproducingbefore51 years were better off than those that attempted to carry on."
The kids appeared to do better, as well.
"Intergenerational competition between unrelated females over resources can have negative impacts on child survival," Dr. Russell says. Children born to a woman and her daughter-in-law around the same time, for example, were twice as likely as other children to die before age 15. The same was not true of babies born to women and their daughters around the same time. Researchers speculate that many women moved in with their husbands' families after marriage and had to share food and other supplies with their in-laws.
"The research adds weight to the argument that menopause evolved because of the vital role that grandmothers played in rearing grandchildren in traditional societies," University of Sheffield biologist Virpi Lummaa, PhD, says. "Although family roles have changed, many grandmothers still play a vital role in caring for their grandchildren, and in Western society a large number provide daycare."
The Evolution of Evolutionary Menopause Theories
Menopause is one of the great unsolved mysteries in evolutionary biology, which theorizes that species survive natural selection based on their ability to reproduce. Most animals today — mammals, especially — can breed until very shortly before they die. Bonobo apes, for example, generally ovulate for all but the last 10 years or so of their adult lives, according to the Bonobo Conservation Initiative, which estimates their longevity at around 50 years. Even human males are capable of fathering children at a relatively late age, though studies suggest that doing so may put the kids at higher risk for autism, schizophrenia, and other conditions.
Killer whales and human females are among the only known mammals that outlive their fertility by several decades. It's what the researchers call "an evolutionary enigma."
"We are so used to the fact that all women will experience menopause, that we forget it is seriously bizarre," Dr. Russell says. "Evolutionary theory expects animals to reproduce throughout their lifespan, and this is exactly what happens in almost every animal known, including human men. So why are women so different?"
Other popular theories include:
- The "grandmother hypothesis" argues that women benefit from improving their grandchildren's survival.
- The "mother hypothesis" argues that it's genetically advantageous to invest in existing children rather than producing newer ones.
- The "reproductive conflict hypothesis" uses game theory to conclude that women stop bearing new children when assisting younger females in the upcoming generation mitigates conflict.
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