Even though a paper on guppy senescence by evolutionary biologist Donna Holmes and her colleagues has circulated for several years, the “grandmother hypothesis” still persists.
And understandably so. One of those rare feel-good stories from evolutionary theory, the grandmother hypothesis attempts to explain menopause in humans as an evolutionary adaptation. Menopause is adaptive, the argument goes, in the sense that women’s reproductive capacity is cut short barely two-thirds of the way through their lives so that the grandmother can help raise the grandchildren, thereby improving the survival of her lineage.
In spite of its appeal, however, “I’ve always thought that was a dumb theory,” says Holmes.
Much of the impetus for the grandmother hypothesis stems from the assumption that menopause is unique to humans.
It’s not, says Holmes. Guppies, for example, also experience menopause and then have a post-reproductive lifespan. Something like menopause occurs in many mammals, some birds, fishes, “animals where you wouldn’t expect care of your grandchildren to play any part in reproductive aging.”
A colleague of Holmes, David Reznik of University of California, Riverside, had long studied the evolution of aging in guppies.
They were having lunch one day at a professional meeting, “And I said, ‘Well you know, selection by predation should shape reproductive lifespan, but it shouldn’t shape the post-reproductive lifespan,’ ” says Holmes.
The post-reproductive lifespan is “evolutionarily irrelevant,” she proposed.
“Evolutionary theory predicts that if animals evolve under certain mortality pressure, it will shape the evolution of their reproductive lifespan,” says Holmes.
Reznik selected two populations of guppies to test that idea, one that had evolved under heavy predation pressure and another that had evolved under light predation pressure.
Once they analyzed their data, the researchers found that the two regimes did have different impacts on the guppies’ lifespans: “But it wasn’t quite the direction you’d expect,” says Holmes.
If an animal like a guppy lives in a stream with lots of other fish that want to eat it, says Holmes, one might assume that the population is selected to mature more quickly and have a shorter lifespan.
For some reason, the guppies that evolved under the heavy predation pressure not only started reproducing earlier, but actually lived longer.
However, as Holmes predicted, the mortality selection did not have an impact on their post-reproductive lifespan.
In other words, post-reproductive lifespan “seems to be a random add-on at the end of the life history.”
Although the appeal of the grandmother hypothesis is strong, the impulse to care for one’s grandchildren apparently is simply cultural rather than evolutionary in scale. “From an evolutionary standpoint,” says Holmes, “it doesn’t make any sense that you give up your own reproduction to promote the reproduction of your kin. The selection is too weak.
“It was good to get that paper published,” she says, “because a lot of people claim that menopause is unique to humans, and it’s not…
“The grandmother hypothesis is popular because feminists like it, it’s very woman friendly. When evolutionary biologists critiqued it, they were accused of being sexist. So it’s good to have my name on it.”