Why would an otherwise healthy animal stop reproducing? Natural selection usually favours genes that elevate reproductive success because the very act of reproduction is how genes proliferate. So adaptations that involve self-limited reproduction call for unusual evolutionary explanations.
Sterile worker ants and honey bees present one such unusual adaptation. These females forego any chance to reproduce so that they can help their sisters become queens. Insect sociobiologists have shown just how special the evolution of sterile workers is. The massive genetic lottery win that comes when a sister ascends to head a hive of her own more than compensates the worker for her life of sterile devotion.
But human women also stop reproducing around their late 40s. Menopause remains among the most hotly debated products of human evolution. I have written here before about the competing theories. And earlier this month Dyani Lewis wrote a very clear overview of the ideas surrounding the evolution of menopause.
But today I’d like to consider a computer simulation model published last Friday in PLOS Computational Biology. In the breathless style so beloved of journal publicity departments, the press release quotes author Rama Singh as saying:
Menopause is believed to be unique to humans, but no one had yet been able to offer a satisfactory explanation for why it occurs.
Singh, together with McMaster University colleagues Richard A. Morton and Jonathan R. Stone argue that the reason menopause evolved was that older men preferred young women as mates. Their logic resembles that behind successful theories for the evolution of ageing: natural selection grows weaker on older cohorts, allowing late-acting mutations to accumulate. Likewise, when older women get left on the shelf, late-acting mutations that rob them of their fertility are allowed to accumulate.
The paper builds on an intriguing earlier model in which men’s ability to sire offspring late in life, and the fact that many wealthy and powerful men have done just that for millenia, can have the incidental consequence of prolonging female lifespan as well, by weeding out some of the late-acting mutations that would otherwise act after women have ceased reproducing. That idea has some merit for explaining why women live a long time after menopause, but it says nothing about why they stop reproducing in the first place.
The new model shows that genes that lower the fertility of older women (but not men) can accumulate. But they do so in a rather artificial situation: the genes that alter age-dependent survival affect both males and females the same. I will wage that if the researchers had allowed mutations to affect male and female survival independently, female survival would have waned at least as much as female reproduction did. I think the assumptions of this model were artificially disposed to getting the observed outcome.
That’s not to say the idea lacks merit. I’m just not convinced that the study lives up to the press-release hype. Hype, I might add, that has seen more cut-and-paste action in the print and electronic media than a Year Three school project. Complete with stock images of ageing male celebrities stepping out with twenty-something “latest” girlfriends. Check out the Sunlight Foundation’s Churnalism, analysis of the Guardian’s version of the story.
In fact, my go-to source for garbled science news, The Daily Mail did a slightly better job than the Guardian and several other sites. They quote Oxford post-doc, Dr Maxwell Burton-Chellew, who didn’t mince his words, calling the study just ‘plain wrong’.
It seems to me infinitely more plausible and more consistent with decades of evidence that men’s preference for younger women evolved as a response to the declining fertility of older women, and not the other way around. That said, however, science doesn’t work by rejecting ideas on plausibility grounds alone. This new idea should be developed and considered more thoroughly, including the possibility of feedback loops between reproductive ageing and preferences for younger mates.