One of the curiosities about humans is that females go through menopause, ending their capacity to reproduce, years and often decades before death.
Menopause is relatively rare, documented only in humans, orcas and a species of pilot whale. Otherwise, female fertility ends with death. Consider Grizzly 399, a 27-year-old bear who lives in and around Grand Teton National Park. When she emerged from her den this spring she had a single cub in tow. This year’s cub, nicknamed Spirit, made 399 the oldest known wild mama griz of all time.
A single cub was a letdown only because just three years ago, 399 gave birth to quadruplets. But birthing any cub at 27, for a species that rarely makes it to its 30s, is remarkable.
Female grizzlies don’t do menopause. Why female humans and orcas do is the subject of considerable research and debate in the scientific community. The most popular theory is the “grandmother hypothesis” which suggests the most long-lived females were too valuable to the lives of their grandchildren to risk continued childbirth.
Childbirth, for humans at least, is fairly dangerous due to the size of an infant’s head compared to the width and shape of the mother’s pelvis.
Losing their fertility meant those grandmothers lived longer, allowing more time to share their wisdom for food gathering and survival. Also, these grandmothers spent their later years helping their daughters raise their offspring. And there’s good evidence that grandmothers in hunter-gatherer societies do help their grandchildren live longer.
I’ve developed my own theory — call it the “memaw hypothesis.” I never called either of my grandmothers “memaw” as that’s a Southern thing, but dang it, both were amazing. There’s no doubt I’ve lived a longer, healthier life thanks to their love and care when I was young.
The grandmother hypothesis has its doubters, however. These are people who never gathered in my Gramma B’s den on the holidays, scraping the remnants from a bowl of shrimp dip with a shard retrieved from an otherwise empty bag of Ruffles.
Lacking my essential, albeit anecdotal data, they’re forced to study the rare menopausal wild species.
And there the evidence isn’t quite so clear as it was my grandmother’s den. A recent study confirmed for the first time that some chimpanzees also go through menopause. One chimp, Garbo, who was a star of the Netflix Series “Chimp Empire” is 67. She was 38 the last time she was pregnant.
Chimps are of course the species most closely related to humans, so maybe sharing traits like menopause isn’t all that surprising. Chimps also share with humans a complex social structure, living in large groups where cooperation and the nurturing of youngsters would likely pay off.
What makes sense for humans from an evolutionary standpoint might not make sense for chimps, however, because chimp females tend to disperse when they reach child-bearing age, joining new groups rather than staying with the group of their youth, according to a story in Scientific American magazine.
There goes the grandmother hypothesis, right? Not exactly, because baby chimps have fathers too, and their fathers are the offspring of grandmother matriarchs like Garbo. So maybe the grandmother hypothesis creates a competitive advantage for some species, but doesn’t explain every example of menopausal species.
Remember, never trust a biologist who tells you they’ve figured out exactly why nature does what it does. They’re likely trying to sell a book project.
Orcas, the other big, social, menopausal mammal, adds another complication. Matriarchal orcas live in pods with their daughters, and often both are pregnant at the same time. The offspring of daughters are more likely to survive to adulthood, however.
Does that mean orca grandmothers aren’t all that when it comes to helping their young whales survive? Maybe, but I’m not betting on some magic-bullet theory making it all make sense.
Complexity is the only certainty in nature.