The Mathematics of Menopause

Image by RickC via Flickr

Among long-lived animals, scientists have found only three species that undergo menopause: humans, short-finned pilot whales, and resident killer whales. What makes these species so special? A new study finds that it all comes down to their unique social structures.

Menopause occurs when the ovaries are depleted of eggs, marking the end of female fertility. But evolution favors the production of offspring, so why would some species abandon reproduction? Some researchers have argued for the "grandmother hypothesis." The idea is that an older woman who devotes herself to caring for her grandchildren benefits from the survival of her genes more than she would if she continued to bear children. If she continued reproducing in old age, then she would risk dying during childbirth or while young offspring are still dependent on her.

Human beings are more disposed toward menopause than are other species. That's because, according to a recent study by evolutionary biologists Rufus Johnstone of the University of Cambridge and Michael Cant of the University of Exeter, both in the United Kingdom, women have historically left their family groups after marrying. This placed them in a new family, meaning it was beneficial to care for their own children at first (because they were more genetically related to them than they were to the rest of the family) and to care for their grandchildren later (when more of their genes made it into the family group).

But this scenario can't explain why short-finned pilot whales and resident killer whales are also menopausal, as neither gender leaves the family group to join a mate. Instead, males and females return to their respective family pods after mating with whales from other pods—an arrangement unique in the mammalian kingdom. In the new study, Johnstone and Cant used a mathematical model to analyze the kinship dynamics in a population with this mating pattern. They found that this unusual mating scenario led to an increase in the number of individuals that a female is related to in her group as she ages—a trend that favors younger breeders and older child-rearing helpers. And that sets the stage for menopause to evolve, the team will report online tomorrow in the Proceedings of the Royal Society B. The researchers saw the same trend when they applied the model to the mating pattern of humans societies in which a female leaves home to join her husband's family. This is also an unusual social structure, as in most other social mammals it is the male who leaves home.

Evolutionary biologist Thomas Kirkwood of Newcastle University in the United Kingdom isn't convinced that strong conclusions about the evolution of menopause can be drawn from a mathematical model that predicts relatedness within a species. "It doesn't show how the presence or absence of the menopause affects Darwinian fitness," how successful an individual is in passing on his or her genes to future generations, "which is the all-important evolutionary yardstick."