Menopause is often presented as a loss: loss of fertility, loss of hormones, loss of a prior state. That is a simple and fairly common way of describing it, but scientifically, it remains too narrow and limited. Menopause is first and foremost a normal biological transition in the human life cycle. It marks the end of ovarian reproductive function, but it does not signal the end of health, the end of biological usefulness, or the end of contribution to social and family life.
If it fascinates biologists so much, it is not only because of its physiological effects. It is above all because it raises a remarkable evolutionary question. In most mammals, females remain fertile until a relatively advanced age, often not far from the end of life. In humans, by contrast, reproduction often stops while several decades of life still remain. This long post-reproductive lifespan is rare in the animal world.
Very rare, in fact.
It is relatively easy to ask: what is menopause?
However, a much more complex question is asked less often: why has evolution allowed such a long phase of life after reproduction to appear — and persist — in only a few species?
What menopause is, in a few words
From a clinical point of view, menopause corresponds to the permanent cessation of menstruation, confirmed after 12 consecutive months without a period in the absence of another cause. It occurs within a broader transition, perimenopause, during which cycles become irregular and hormonal fluctuations become more pronounced.
The literature notably describes:
- changes in the menstrual cycle;
- vasomotor symptoms such as hot flashes and night sweats;
- sleep disturbances;
- urogenital changes;
- accelerated bone loss;
- changes in metabolism and body composition.
Large longitudinal cohorts show that the hormonal transition does not begin at the time of the final menstrual period, but several years earlier. In the SWAN study, FSH rises on average about 6 years before the final menstrual period, accelerates about 2 years before it, and then stabilizes afterward. Estradiol remains more stable at first, then declines more sharply as the final menstrual period approaches.$^1$
More surprisingly, in humans menopause does not occur on the brink of death. It often opens onto a long second half of life.
The real anomaly is not menopause: it is long life after reproduction
From the perspective of evolutionary theory, producing descendants is central. One might therefore expect a female to continue reproducing for as long as her body allows. That is, in fact, what is observed in the majority of species. Although fertility declines, there is generally no long clearly post-reproductive phase.
In humans, things are different. Female fertility declines well before the organism’s general senescence. As several comparative studies have shown, human reproductive senescence is largely decoupled from somatic senescence. In chimpanzees, reproductive decline appears to be much more closely tied to mortality and to the general deterioration of bodily condition.$^2,^3$
And yet, in humans, reproduction does not cease because one is in the process of dying. It ceases while one may still live for a long time, act, produce, contribute, transmit, and help.
Why is this so?
The question deserves to be asked.
Why does menopause exist?
No single theory commands unanimous support today. Scientific debate is less about the existence of the phenomenon than about the selective forces that may have made it advantageous, which would help explain why this stage of life persists.
The main hypotheses can be grouped into two broad families:
- Non-adaptive theories, in which menopause is primarily a by-product of human biological history;
- Adaptive theories, in which the cessation of reproduction was favored because it provided an indirect evolutionary advantage.
The by-product theory: menopause as a consequence of longevity
The first explanation is the most parsimonious. Menopause would not be an adaptation in itself, but rather an age-related phenomenon — an epiphenomenon. Humans would have evolved toward greater longevity, but not toward an equivalent extension of female reproductive lifespan. The ovaries would age faster than the rest of the body, thereby producing a long post-reproductive phase.
This theory was notably discussed by Kristen Hawkes$^4$ and taken up more formally by Peccei,$^5$ who asked whether menopause was an adaptation or an epiphenomenon.
Strengths of this theory
- It is parsimonious.
- It does not require assuming that reproductive cessation was actively favored.
- It is consistent with the fact that organs do not all age at the same rate.
Weaknesses of this theory
It does a poor job of explaining why the human post-reproductive lifespan is so long, so regular, and potentially so useful socially and familially. It may explain how menopause became visible, but not as well why it would have been retained without major cost by natural selection.
The mother hypothesis: better to protect children already born
The mother hypothesis rests on a simple idea: in humans, children remain dependent for a long time. As a woman ages, initiating a new pregnancy becomes more costly and riskier, while her already-born children still need her. In that context, the best evolutionary choice may be to stop reproducing in order to increase the chances of survival of the existing offspring.
This logic was formulated in early evolutionary work on menopause and later developed in several subsequent models.$^6$ The idea is not that late reproduction is impossible or undesirable, but rather that it becomes less beneficial than parental investment in existing offspring.
Strengths of this theory
It is faithful to two human realities:
- prolonged child dependency;
- the increased costs of late pregnancies.
Weaknesses of this theory
It explains why stopping reproduction may become preferable at an advanced age, but it explains less well why that cessation is then followed by a very long useful lifespan that extends beyond the period of children’s “active” dependency. It accounts for the stopping point, but not fully for the duration that follows.
The grandmother hypothesis: helping grandchildren rather than having one last child
This is the hypothesis that comes up most often. It proposes that a menopausal woman may increase her selective value no longer by directly producing children, but by helping her descendants raise others. In other words, rather than adding one more child to her own offspring, she increases the reproductive success of her children and grandchildren.$^7$
This idea was popularized by Hawkes, O’Connell, Blurton Jones, Alvarez, and Charnov.$^4$ The argument is compelling: if a grandmother helps feed children, reduces her daughter’s interbirth interval, or improves the survival of grandchildren, then the cessation of her own reproduction may become a favorable evolutionary trade-off.
Strengths of this theory
- it highlights the value of a long post-reproductive lifespan;
- it emphasizes the role of intergenerational transfers;
- it is supported by some ethnographic data showing that grandmothers can increase the survival or well-being of descendants, in humans and in other socially complex mammals.
Weaknesses of this theory
Grandmother effects are not universal, nor are they identical across societies. They vary with ecology, residence patterns, family organization, and local economy. The hypothesis is therefore convincing, but probably incomplete if treated as the sole answer.
The intergenerational reproductive conflict hypothesis
This theory has become especially important because it helps explain why menopause is so rare. In some social structures, when younger and older females reproduce at the same time within the same group, this creates reproductive conflict — a form of reproductive competition. They compete for help, resources, group energy, and even access to care for their young. If that conflict is more costly to the older generation than to the younger one, then natural selection could favor the cessation of reproduction in older females.
Menopause could thus be the evolutionary solution to a competitive question: who has the greater interest in continuing to reproduce when several generations coexist?
Strengths of this theory
It helps explain why menopause is not found everywhere. A long lifespan is not enough; a particular social organization is also needed, one in which relatedness changes across the life course and competition between generations becomes asymmetrical.
Weaknesses of this theory
It more readily explains why older females would stop reproducing when they enter into competition with younger females, but it does not fully justify why they would then live for a long time after the end of fertility. It also depends on very particular social structures that are difficult to generalize to all humans. Finally, it places most of the emphasis on the costs of reproductive competition, whereas other theories do a better job of explaining the positive benefits of post-reproductive life, such as helping descendants, transferring resources, and transmitting knowledge.
The theory of learning, embodied capital, and transfers
In humans, survival depends heavily on slowly acquired skills:$^8,^9$ subsistence techniques, ecological memory, cooperation, sharing, and cultural transmission. The value of an adult therefore cannot be reduced to the ability to bear children. It also includes the capacity to produce, transmit, and support the young.
From this perspective, menopause becomes an important component of a life strategy in which the benefits of late reproduction decrease while the benefits of investing in descendants increase with age.
This idea has been summarized clearly in recent work: fertility costs, economic production in later life, and kin investment may combine to favor a relatively early cessation of reproduction compared with total lifespan. This approach draws on the broader theory of “embodied capital” developed by Kaplan and colleagues.
Strengths of this theory
It connects menopause to the whole of the human life strategy:
- long childhood;
- heavy juvenile dependence;
- the importance of knowledge;
- late productivity;
- transfers of resources from older generations to younger ones.
Weaknesses of this theory
It is rich but difficult to test in full. It requires extensive comparative and ethnographic data, and it can sometimes seem to combine several ideas at once rather than isolating a single cause.
Why does menopause exist in only a few species?
If this transition is advantageous, why do only a handful of species benefit from it?
For a long time, human menopause was treated as unique. Today, the strongest evidence for a long post-reproductive phase exists in a few odontocete cetaceans, notably:
- killer whales;
- pilot whales;
- and, depending on the studies and methods, certain other odontocetes such as narwhals or belugas.
Although these species are long-lived, many other long-lived species do not show a long post-reproductive lifespan. Some toothed whales show clear evidence of menopause and of long post-menopausal survival, which opens the way to comparative tests with humans.
The case of killer whales: the best-known example
Killer whales have become the major comparative model for thinking about menopause outside the human species.
Why?
Because they combine several traits that echo certain human features:
- long social lives;
- stable kin-based groups;
- important social learning;
- significant roles for older females;
- the coexistence of several generations;
- potential conflicts between reproduction and helping relatives.
In resident killer whales, older females appear to play an important social role, particularly in guiding movement and accessing food during difficult periods. An older female may still improve the survival of her descendants even without producing additional calves, thereby generating positive evolutionary effects. This is precisely the kind of context in which both the grandmother hypothesis and the reproductive conflict hypothesis become plausible.
The human–killer whale parallel: similarities and differences
Similarities
In both humans and killer whales:
- females can live long after the end of reproduction;
- older individuals retain social value;
- groups are structured by durable kin relationships;
- information and experience may have adaptive value;
- helping descendants may become more beneficial than producing one last offspring.
Differences
In humans:
- transmission is also cultural, technical, and economic;
- children remain dependent for an extraordinarily long time;
- older adults can provide not only social help, but also net production and material transfers;
- according to some authors, men may also show reduced direct reproductive effort linked to family strategies, even without any strict equivalent of female menopause.
In menopausal cetaceans:
- there is no exact equivalent of the human economy;
- social mechanisms are different;
- males generally do not show a comparable documented phenomenon of prolonged reproductive cessation, which is also emphasized in the literature.
Menopause in humans and in killer whales is probably not the same story repeated twice, but rather two variants of a broader evolutionary logic. When the help provided by older females to their relatives yields more than continued reproduction, a long post-reproductive lifespan can become selectable.
Why is this phenomenon so rare?
The rarity of menopause is itself a clue. It suggests that menopause does not emerge simply whenever a species lives a long time. A particular combination of conditions is needed:
A long adult lifespan
First, individuals must be able to live a long time after reproductive age.
A strong social structure
Individuals must remain embedded in a sufficiently stable kin network for helping relatives to pay off.
Real benefits of late-life help
Older females must still be able to improve the survival or reproduction of their descendants.
Rising costs of late reproduction
At some point, continuing to reproduce must become less advantageous than transferring time, energy, or knowledge.
An asymmetry between generations
The coexistence of reproductive mothers and daughters, or of several generations of females, must create a context in which cessation of reproduction in the older females becomes beneficial.
It is probably this rare combination, rather than any single cause, that explains why menopause is so exceptional in the living world.
So which theory is best?
After my reading, probably none on its own… and perhaps that is precisely why choosing just one is unhelpful.
Contemporary literature instead suggests that human menopause most likely results from several converging forces:
- the differential aging of the ovaries;
- the risks and costs of late reproduction;
- prolonged child dependency;
- the benefits of intergenerational help;
- reproductive conflicts between generations;
- the productive and cultural value of older adults.
Menopause is probably neither a simple accident nor the product of a single selective pressure. It looks more like an evolutionary solution that emerged within a very particular way of life, one characteristic of long-lived social species. This phase of life, together with the social organization in which older females continue to matter biologically after the end of fertility, may have proved useful for the survival of the species.
Another way of thinking about menopause
Menopause ceases to be merely the end of something. It also becomes the expression of a broader biological reality. In some species, the value of an individual is not measured solely by the ability to produce descendants, but also by the ability to support those who already exist.
Perhaps that is what makes menopause so fascinating. It reminds us that in evolution, direct reproduction is not the whole story. There is also transmission, help, experience, transfers, social knowledge, and kinship lived out across an extended timescale.
And that is probably why menopause can only be fully understood when taken out of the strictly medical register. Menopause is not only a hormonal transition. It is also one of the most striking clues that, in some species, growing old can remain biologically useful.
References
- Randolph, J.F., Jr., et al. Change in Follicle-Stimulating Hormone and Estradiol Across the Menopausal Transition: Effect of Age at the Final Menstrual Period. The Journal of Clinical Endocrinology & Metabolism 96, 746–754 (2011).
- Hill, K. & Hurtado, A.M. The evolution of premature reproductive senescence and menopause in human females: an evaluation of the “grandmother hypothesis”. Human Nature 2, 313–350 (1991).
- Gould, K.G., Flint, M. & Graham, C.E. Chimpanzee reproductive senescence: A possible model for evolution of the menopause. Maturitas 3, 157–166 (1981).
- Hawkes, K., O’Connell, J.F., Jones, N.B., Alvarez, H. & Charnov, E.L. Grandmothering, menopause, and the evolution of human life histories. Proceedings of the National Academy of Sciences 95, 1336–1339 (1998).
- Peccei, J.S. Menopause: Adaptation or epiphenomenon? Evolutionary Anthropology: Issues, News, and Reviews 10, 43–57 (2001).
- Rogers, A.R. Why menopause? Evolutionary Ecology 7, 406–420 (1993).
- Hawkes, K., O’Connell, J.F., Jones, N.G.B., Alvarez, H. & Charnov, E.L. Grandmothering, menopause, and the evolution of human life histories. Proceedings of the National Academy of Sciences 95, 1336–1339 (1998).
- Kaplan, H. The evolution of the human life course. In Between Zeus and the Salmon: The Biodemography of Longevity, 175–211 (1997).
- Kaplan, H., Hill, K., Lancaster, J. & Hurtado, A.M. A theory of human life history evolution: Diet, intelligence, and longevity. Evolutionary Anthropology: Issues, News, and Reviews 9, 156–185 (2000).