Dr. Yoav Ram of Tel Aviv University proposes a new model for understanding one of the greatest mysteries in evolution: how a trait that stops reproduction in midlife can actually be preserved, and perhaps even evolved, in species in which males remain in the family group and especially benefit from the help of the elderly mother.
In research supported by the Israel Science Foundation, Dr. Yoav Ram of Tel Aviv University examines one of the great mysteries of evolution: how menopause, a rare trait that reduces reproduction – and yet “wins” in evolution – evolved.
“The only species we know of that have menopause are humans and five species of whales,” says Dr. Yoav Ram, of the School of Zoology in the Faculty of Life Sciences at Tel Aviv University. The phenomenon itself is well known in women: cessation of reproduction around the age of 45–50, followed by decades of life.
The big question is why evolution “allows” this to happen. After all, seemingly, those who stop producing offspring lose in the evolutionary race.
Ram doesn't go out into the field with binoculars and a research boat. His lab is theoretical. It builds mathematical and statistical models, as well as advanced computational tools, to understand phenomena in ecology and evolution – from viruses and yeast to humans and whales.
And here comes the clue: It is possible that the answer to the question of menopause lies not only in the biology of the body, but also in the social structure in which we live, gender, and family.
The question is: Why are there species in nature where females stop reproducing in midlife—and how might their sons hold the key to this mystery?
Unusual social structure
The extreme example that Ram likes is the killer whale (orca). Killer whales live in family groups where “all offspring stay with their mother for life.” They mate outside the group, but do not form “pairs” and do not leave the family.
Simply put: it's a home that the male children never leave. Among the killers, according to Ram, the decision-making side also rests with the females – "the matriarch decides everything."
This structure is not common in nature. In elephants or lions, for example, young males leave the group. But in killer whales – not. And this exception may be exactly where evolution can “agree” to a strange feature like menopause.
What have they thought so far?
Several explanations have been proposed in the scientific literature over the years. One approach is to avoid reproductive risks in old age. Another well-known approach is the “grandmother hypothesis.” It is better for a female to stop reproducing and invest energy in caring for offspring that have already been born, especially grandchildren.
The problem, according to Ram, is that the mechanisms have not always been formulated explicitly enough, nor in a way that allows them to be tested systematically. This is where the new study comes in, which attempts to turn the discussion into a question that can be put into a model, run, and tested for what “wins” under different conditions.
The model: a gene that stops reproduction and provides help
In Ram's lab, a detailed mathematical model of a social population was built: females and males living together in groups, of different ages, and with a certain degree of departure or transition between groups.
The model included the possibility of a new gene emerging – one that stops reproduction in females at a certain age (for example, around age 40), and instead “allows them to provide help” to younger individuals in the group.
The model was not built “out of thin air.” Doctoral student Tami Yosef collected numerical data from the literature to calibrate the model, and it was applied to a variety of social species – including humans, chimpanzees, killer whales, elephants and baboons. The goal was to see under what conditions a gene that shortens reproduction in females could actually spread in a population.
According to the proposal, menopause may develop in species where males remain in the group in which they were born, there is substantial overlap in daily life between the adult female and her male offspring, and males are "expensive" in an evolutionary sense - for example, larger, requiring more investment, but also able to gain a greater competitive advantage in reproduction if they receive help.
Mother's child hypothesis
Here comes the key finding: According to the model, the most important factor in the development of menopause is the ability of an older female to influence the survival of young males who are genetically close to her. And this only works if a critical social condition is met: that the males do not leave the house, so that they remain close to her over time.
Ram also explains the intuition. If a female helps young females, their ability to reproduce is limited anyway: a female has to invest in pregnancy, nursing, and caring for offspring, so her reproductive rate is low. In contrast, “a male can reproduce as much as he can.” He is not limited by pregnancy or caring for offspring.
That is, if a grandmother improves the survival, nutrition, ability to fight, or find mates of close males, the evolutionary benefit may be particularly great.
From this, the team proposes a new formulation: not just the “grandmother hypothesis,” but a hypothesis that emphasizes the role of sons and male grandchildren – Ram calls it with a smile, the “mamma's boy hypothesis.”
According to the proposal, menopause may develop in species where males remain in the group in which they were born, there is substantial overlap in daily life between the adult female and her male offspring, and males are “expensive” in an evolutionary sense – for example, larger, requiring more investment, but also able to gain a greater competitive advantage in reproduction if they receive help.
Forecasts for testing
The model has a clear prediction: Females who have stopped reproducing are likely to invest more in helping males than females within their family group. He says there is already good evidence for this in white-tailed deer.
In humans, the picture is more complex, because social behavior is also intertwined with culture, religion, and modern social structures. But the comparison with whales can actually be an advantage: They are evolutionarily distant from us, so if you see a similar pattern there, it is difficult to explain it as a product of the same culture or human history.
The next step, according to Ram, rests on the shoulders of the field researchers: "more data" is not always needed; sometimes it is necessary to reorganize data that has already been collected and ask the right questions of it.
If the predictions are correct, we may get a new picture of menopause: not as a biological “glitch,” and not just as an evolutionary choice to avoid risks, but as part of a long-term family strategy – based on a social structure in which sons stay close, and their help generates a high “evolutionary interest.”
More of the topic in Hayadan:
One response
You can also watch chimpanzee mothers helping their male offspring find a mate (there's an article on this site about it, worth a look), which ties in nicely with the article. It seems that anything that helps spread genes becomes fixed and spreads in the population in which it appears.