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Do any animals other than humans undergo menopause?

Do any animals other than humans undergo menopause?


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Do any animals other than humans undergo menopause?

Also, is there any difference between animals in captivity and animals in the wild as regards menopause? For example, even if menopause has been observed in a captive member of a particular ape species, do individuals of that species typically live long enough in the wild to also undergo menopause?

I guess here's what I'm really getting at: is menopause a common thing in the animal kingdom, or is it only a common thing in humans?


Yes. Menopause is common for long-lived mammals. For instance, in the wild, killer whales go in a sort of menopause as reported in 2009 by Ward et al. Front Zool. 2009 Feb 3;6:4. So it is not due to captivity. According to a Nature review, reproductive cessation has also been documented in non-human primates, rodents, whales, dogs, rabbits, elephants and domestic livestock (Packer et al. Nature. 1998; 392(6678):807-11)


The ecological knowledge that older females have gained throughout the course of their lives can be extremely useful for the survival of their offspring. This is sometimes referred to as the "grandmother effect". For example, older females may remember key information about rare events, such as how to avoid/defend against predators, or how to find food/water in times of scarcity.

Check out this paper describing the grandmother effect in humans: Herndon (2009). The grandmother effect: implications for studies on aging and cognition. Gerontology, 56(1), 73-9.

And in killer whales: Brent, L. J., Franks, D. W., Foster, E. A., Balcomb, K. C., Cant, M. A., & Croft, D. P. (2015). Ecological knowledge, leadership, and the evolution of menopause in killer whales. Current Biology, 25(6), 746-750.


Only three - humans, killer whales, and pilot whales. (Reference)

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.


Animals That Go Through Metamorphosis

From birth, all animals undergo various morphological, anatomical and biochemical changes. Even after they reach adulthood, there are still some changes which occur as cells replicate and die in constant order. Many of these changes are physiological and affect body size, shape, etc. Others are hormonal, which can affect physiology as well as mental capacity and other aspects of cognition. Some of changes are minute and essentially imperceptible. In some animals, these changes are so drastic, their entire morphology changes and the adult does not even resemble the juvenile at all. This is the process of metamorphosis.

AnimalWised brings you everything you need to know about animals that go through metamorphosis. We show you the different types of animals which can go through these changes and, most importantly, why they do so.


The Origin of Menopause: Why Do Women Outlive Fertility?

New research sheds light on why women survive for decades when females in many other species die after they lose the ability to reproduce.

The origin of menopause has puzzled evolutionary biologists for the last half-century. Three new studies attempt illumination. The real question, though, is probably not: Why menopause? Rather, it is: Why do women long outlive their fertility?

Human ovaries tend to shut down by age 50 or even younger, yet women commonly live on healthily for decades. This flies in the face of evolutionary theory that losing fertility should be the end of the line, because once breeding stops, evolution can no longer select for genes that promote survival.

The most popular explanation, the "grandmother hypothesis," argues that a generous post-reproductive life span makes sense if a grandmother improves the survival and reproduction of her grandchildren, thus ensuring continuation of her own genes&mdashincluding genes that contribute to longevity. But skeptics say the math is askew. From an evolutionary perspective, it is hardly ever better for a woman to give up a chance to bear additional children of her own, and so pass on half her genes, for the sake of improving the survival of her grandchildren, who carry only a quarter of her genes.

"The problem is that these grandmother benefits aren't big enough to ever favor stopping breeding between the ages of 40 and 50," says Michael Cant, an evolutionary biologist at the University of Exeter in England and co-author of a new study on the genesis of menopause published this week in Proceedings of the National Academy of Sciences USA. "When you look at data from hunter-gatherers and other natural fertility populations, the sums just don't add up." Grandmothers do benefit their descendants, he says, but the genetic payoff is small compared with those of producing another child.

Cant and co-author Rufus Johnstone, an evolutionary biologist at the University of Cambridge in England, used game theory to argue that menopause is early cessation of reproduction that originated through reproductive conflict between generations. In most cooperatively breeding species, reproduction is suppressed in younger females, who act as helpers to older reproducing females. By contrast, they say, younger women in human social groups win the reproductive sweepstakes, because the older ones stop having babies.

"We showed that, compared to other primates that exhibit a post-reproductive life span, humans really stand out, because there is absolutely no overlap in reproduction between generations," Cant says. "Women stop breeding on average when the next generation starts to breed."

This makes evolutionary sense, Cant and Johnstone say, because, contrary to most mammals, young women tend to move to their mates' communities, where they become immigrants whose only genetic kin are their own children. There is no genetic profit in helping their mothers-in-law bear more children, because they will not share any genes with those children. But an older woman who helps her son's wife reproduce will benefit by bequeathing 25 percent of her genes to her grandchildren.

"We show that the mother-in-law's best strategy is to stop breeding, avoid competition, and allow the daughter-in-law to breed and help her," Cant says. "It's the first time anyone has taken the idea that humans evolved with this sex bias in dispersal and looked at the implications for how these conflicts will be resolved within the family."

The mother of the grandmother hypothesis, anthropologist Kirsten Hawkes of the University of Utah, says Cant and Johnstone are right to focus on intergenerational conflict. Elephants have babies in their 60s, and some whales give birth in their 80s. "It's clearly something selection can adjust," she says. "So explaining why it hasn't in us has to be part of the story." But she disputes their claim that female-bias dispersal is, in fact, the universal human/ape residence pattern, pointing out that half of the young female chimps at anthropologist Jane Goodall's Gombe Stream Research Center remain with their mothers, and that recent studies show that hunter-gatherers often live with the wife's family as well.

Another explanation for menopause is the "mother hypothesis," which holds that it occurs because older mothers might profit more, genetically speaking, by investing resources in their existing children than in giving birth to new ones. Researchers at the Max Planck Institute for Demographic Research in Rostock, Germany, make the case for this in the American Journal of Physical Anthropology (AJPA), concluding that menopause is advantageous when a woman has aged enough to face an increased risk of stillbirth, birth defects and her own death in childbirth.

Researchers of a different AJPA study, based on 400 years' worth of data on births in Costa Rica, believe that postmenopausal longevity is associated with an increased number of children but a decreased number of grandchildren&mdasha finding that supports mothers over grandmothers.

"We're not saying grandmothers do not provide benefits in some societies," says study co-author Lorena Madrigal, an anthropologist at the University of South Florida in Tampa. But, "we should not assume that one pattern fits all."

Data on great ape fertility is spotty, but what there is shows that our closest relatives&mdashchimps, bonobos, gorillas, even orangutans&mdashstop having babies about the same age that we do: the late 30s. The difference is, they generally die a short time later. "The thing that makes us different from apes is not the age of fertility decline, it's the lack of aging in other systems," says Hawkes. "I have been saying this for a long time and I don't think it's what anybody is hearing. Probably what a lot of people are prepared to listen to is the way Cant and Johnstone have framed this, that the real question is: 'Why do we stop [reproducing] so early?' I think the bottom line is that, compared with our closest living relatives, we don't."

ABOUT THE AUTHOR(S)

Tabitha M. Powledge is a long-time science and medical journalist whose work has appeared all over the place, including Scientific American. She began writing On Science Blogs for the National Association of Science Writers in 2009. On Science Blogs moved recently to the PLOS Blog Network.


Where Are All the Books About Menopause?

For women, aging is framed as a series of losses—of fertility, of sexuality, of beauty. But it can be a liberation, too.

Before my hysterectomy, when I was forty-four, I had more than enough information about what was the matter with me: peer-reviewed studies of cervical adenocarcinoma, multiple medical opinions, knowledgeable advice from friends and acquaintances. After the surgery, I found myself in a wasteland of desperate, incoherent blog posts, trying to understand my condition now that, technically, nothing was wrong with me at all. Three months after my uterus and fallopian tubes were removed, along with what was left of my cervix, I went to see the senior gynecological oncologist at a prestigious institution. I asked her about all the postoperative symptoms I hadn’t expected and couldn’t resolve. The most immediately pressing one was what the Internet had taught me to call—clinically, impersonally, at a safe distance from my mortification—anorgasmia. One out of every three times or so, a climax began, then suddenly disappeared. The doctors had mentioned pain and swelling, but not this. What, mechanically, was going on with me?

I was at a hospital ranked as one of the best in the country. The oncologist, this specialist of specialists, brought out an ancient-looking laminated book with illustrations of the pelvic nerves. She showed me the nerves that had been cut during my surgery (the ones controlling arousal), and the nerves that hadn’t (the ones controlling everything else). But she didn’t have any data, couldn’t name any studies, couldn’t answer my questions. When I asked her whether I should maybe consult a neurologist instead, she laughed.

I went to more doctors. They insisted that my problem was imaginary, or, if not imaginary, then emotional, or, if not emotional, then a symptom of perimenopause that I hadn’t noticed before—which is to say, it was natural, unavoidable. I was left with no answers other than that my problem was mine alone. My body no longer served any reproductive purpose what else was there to say? I went home convinced that I would collect real data, that I would write about it, that I would do it all myself. I am still too exhausted and angry to apply critical thinking to this problem. It may be that my anger isn’t unique.

Darcey Steinke is a writer who, through a memoir and five novels, has explored the overlap of the spiritual and the sexual, rendering female subjectivity as both a site of resistance and a simple fact of life. In her new book, “Flash Count Diary: Menopause and the Vindication of Natural Life” (Sarah Crichton Books), Steinke is doing it all over again, this time from the perspective of a postmenopausal woman—herself.

In menopause, even the blondest and the most protected of women will join the rest of us in ignominy they, too, will become, as Steinke writes, “not only invisible but also despised.” A hysterectomy isn’t the same as menopause, but it’s been, for me, a kind of preface to the story of what happens after privileged, fertile womanhood ends—the story that Steinke is committed to telling. Indeed, a hysterectomy can trigger menopause, even if the ovaries are left intact, as mine were. So far, I still have cycles, but they’re punctuated by ghost periods without blood. My primary symptom of perimenopause, which seems to have ramped up since the surgery, resembles the writer Suzanne Moore’s description of her own: “I don’t really have the mood swings that some talk about. I have just the one mood. Rage.”

Steinke, who is about twelve years older than I am, gets her first hot flashes in her mid-fifties. She suffers auras of foreboding, throbs with heat in the night, sweats through her clothes at work, strips down to an undershirt when she “flashes” on the subway. She starts a diary to log the frequency of her hot flashes, nine or ten per day. Eventually, as she writes, as she suffers and seeks to understand, the diary of tallies becomes the book we have in our hands.

In “The Change,” Germaine Greer’s wide-ranging account of menopause, from 1991, which Steinke cites, Greer posits that our aversion to menopausal women is “the result of our intolerance for the expression of female anger.” Menopausal rage is more than a symptom of disappointment at not being fertile or conventionally attractive or socially powerful—although it may be those things, too. It is brought on by the waning of estrogen, which in turn reduces serotonin production. Serotonin is believed to be the mood regulator whose sudden absence causes the anger that Steinke describes: “Irritability is a demeaning word, laughably imprecise when what I actually feel is a bright, ascendant rage.”

Steinke aptly compares the uncontrollable force of menopausal rage to the transformative anger of the Incredible Hulk and the “thorn in the flesh” of St. Paul. Unsurprisingly, the available analogies are all male women are accustomed to translating their subjectivity onto men’s bodies. Plenty of movies depict female rage, but that rage is infantilized, sexualized, or subdued by the male heroes of the film. On women, even displeasure is unbecoming. On black women, multiply that a thousandfold.

Lately, female political anger has undergone some reappraisal. (In her book “Good and Mad,” Rebecca Traister writes, “It is bananas that women’s rage has never been given its proper due,” and other recent books by or about angry activist women concur.) Such outrage will perhaps eventually become palatable we’re more willing to condone anger as long as it’s about something. But what if that anger isn’t separable from physiology? What if, as menopausal women discover more forcefully than the rest of us, the duality of body and mind is simply irrelevant?

Rites of passage are gateways to inclusion in an inner circle, but the milestones of female life are chiefly represented from the point of view of outside observers. Menarche, the gateway to womanhood, is seen as proof that women are viable objects of desire for men. Pregnancy, the gateway to motherhood, is seen as proof that women are viable sources of children. That the birth process also produces a mother is a mere footnote to everyone but the mother herself. The features of menopause are commonly described as losses—of fertility, beauty, sexuality, attention-worthiness. So would they appear to an outside observer.

In Amsterdam, at the eleventh European Congress on Menopause and Andropause (age-related hormonal shifts in men), Steinke finds exactly this sort of subjective erasure. A man on a panel “talks of shrinkage, lack of pliability, dryness. All his descriptions explain how the vagina might feel to an incoming penis. The vagina as a viable penis holder. Not how a vagina might feel to the woman it belongs to.” In her own book, Greer raises the possibility that hormone-replacement therapy for menopause is a male conspiracy to neutralize and contain women’s wisdom and rage.

We are culturally prepared to perceive women’s natural aging as uninteresting at best, pathological at worst—deserving of dismissal or disgust or both. Steinke makes a neat collection of five centuries’ worth of vile examples of the “male bafflement and repugnance” and “boilerplate misogyny” through which menopause has been perceived. Demonologists in the sixteenth and seventeenth centuries considered signs of aging to be proof of sorcery: “Chin hairs. Witch. Wrinkles. Witch. Warts. Witch. If, in the presence of others, a woman grew red and perspired heavily, then she was a witch. . . . If she was quarrelsome, angry, spoke loudly, and moved, at times, in quick bursts of chaotic energy, to open a window or get a ladle of water, then she was definitely a witch.” Quoting TV shows and movies, Steinke suggests that we’re not past condemning women for the inevitabilities of time and nature: men joke about women’s tantrums, women’s varicose veins, about all the visible proof that women are, strictly speaking, no longer of use.

Failing to find common ground in the human world, Steinke turns to the natural one. She takes an immediate interest in pilot whales and orcas, the only other animals known to undergo menopause. She visits Lolita, an orca who has lived at the Miami Seaquarium for nearly half a century, since she was six years old. Her pool is “less than four lengths of her body, its depth less than one length.” Steinke describes her swimming “frenetically from wall to wall, like an agitated soul trapped inside a concrete body.” If she were free, Lolita would likely be entering middle age in her pod, part of the Salish Sea whale clan, in the Pacific Northwest. The clan is led by two matriarchs—Ocean Sun, the eighty-five-year-old whale believed to be Lolita’s mother, and Granny, who is a hundred and four.

Steinke gets interested in these whales, and then obsessed. For her, Lolita becomes an amalgam of scientific proof and spiritual symbol. “I am restricted, stuck in the box the greater culture uses to enclose and reduce older women,” she writes. In the same way, Lolita is “a prisoner who must be grateful to her captors, a female who does tricks in order to be fed.” Steinke can’t save herself, but that imprisoned orca—she might yet be saved.

When Steinke travels to the San Juan Islands, in Washington, for a whale-watching tour, she finds a balm for the loneliness of menopause. The whales give her something like a peer group. Steinke is at her best when she writes searchingly, before the moment of understanding, as in the narrative of her encounter with the geriatric Granny. She calls friends back home on the East Coast to tell them she’s seen the whale in the wild, but, each time she recounts the story, she worries about “selling out the experience, making the encounter sound like yet another entertainment.” She is wary of becoming merely an outside observer: “When I describe the whales as vibrant, muscular, huge, the whales become visual objects separate from myself. But what I actually felt was a dilation.” This is not an encounter between two species, but a visceral connection between two bodies. Seeing Granny, she writes, “was like having my daughter, an event outside human evaluation.” When Granny disappears, a few months after this sighting, Steinke feels a quiet, eerie sense of communion. She imagines how the great whale must have experienced death, losing the strength—or perhaps the will—to rise to the surface for air, then sinking to the ocean floor for the last time.

There may be comfort in affinity with the natural world, but there’s a limit to what insight it can provide. Steinke needs to understand her body’s passage not just as a fact of nature but as a lived experience that half the human population is destined to undergo. “I knew so much more going into both menstruation and pregnancy than I did going into menopause,” she writes. Where is the literature about the menopausal transition? Situating herself in the literary world, it turns out, is a more vexed project than situating herself in the natural one. In an attempt to find companion texts to her own experience, she reads memoirs by gender-transitioning men and women, which “posit hormonal change as both arduous and interesting.” They don’t exactly offer a community of peers, but engaging with them is more helpful than studying whales.

Steinke partakes in the current trend of cross-genre memoir—stories that are heavily decorated with quotations, part autobiography and part commonplace book. Sometimes authors get the blend right, but usually the quoted texts are unsurprising, and they stand in for the textured analysis of real life. In Steinke’s case, the standardness is perhaps the point. By starting a dialogue with a rote lineage of writers—Simone de Beauvoir, Judith Butler, Kathy Acker, Audre Lorde—Steinke isn’t just saying, I’m in the group. She is saying, There is no established group yet, but this is where we have to start.

The group of women writers is one I belong to now, though its membership wasn’t always what I had in mind. So few badges of femininity are badges of honor. Before I was a mother, I was pretty sure what it entailed. It was boring, trivial, sentimental, stultifying, gross. Most of all, it didn’t have anything to do with being an artist. I wasn’t entirely wrong: it didn’t have anything to do with being a certain sort of celebrated male artist, which was the kind of artist I already knew I wanted to be—the kind with the most badges.

The actual experience of motherhood scuttled my arrogant assumptions. When my son was born, I crossed the divide and was permanently humbled. Now I look ahead to another rite of passage, and wonder what further education is coming. It is hard for me, still perimenopausal, to imagine a life in which sexual release is no longer a pressing need, in which that thunderclap isn’t part of the general weather.


Comments

After reading your post, I thought that if Menopause had to be explained in terms of survival of species, these animals should live long enough to help in the process. I was typing a question, “how long do these other animals survive after menopause”. I decided to do a search my self and came across the same article which you had linked in your post. I found these observations rather interesting.

However, one swallow does not make a summer, nor does one sterile female constitute menopause. Establishing the existence of menopause as a biologically significant phenomenon in the wild requires far more than just coming upon the occasional sterile elderly individual in the wild or observing regular sterility in caged animals with artificially extended life spans. It requires finding a wild animal population in which a substantial proportion of females become sterile and spend a significant fraction of their life spans after the end of their fertility.

But human female menopause remains sufficiently unusual in the animal world that its evolution requires explanation. We certainly did not inherit it from pilot whales, from whose ancestors our own ancestors parted company over 50 million years ago. In fact, we must have evolved it after we separated from the apes just 7 million to 5 million years ago, because we undergo menopause whereas chimps and gorillas appear not to (or at least not regularly).

There were few other links which gave some interesting insights

Evolution of the human menopause.

A hypothesis for the origin and evolution of menopause.

“There may be little advantage for an older mother in running the increased risk of a further pregnancy when existing offspring depend critically on her survival.”

“Results of a mathematical model are presented which show that reproductive senescence can be advantageous even when maximum potential lifespan is only 50 years, if the premature cessation of reproduction allows females to moderately increase the survival and fertility of their existing subadult offspring.”

That is right Archana. As i pointed out, jared diamond does make a powerful argument. But then as i also pointed out, what explains Guppies and other animals listed showing menopause. These are not the occasional sterile animals that Jared Diamond dismissively says but clear cases of animal menopause. I thought i was bought into Diamond’s argument but then this dismissiveness on sterile animals drove me to research this further and i chanced upon the Guppy study report. So I can only reasonably conclude that human menopause is just another animal menopause that some animals have for no apparent reason other than longevity of life or some other reason to be figured out. It is not some special human thing.

I understand that Sukumar. I was wondering about the length of time they survive after Menopause. Jared Diamond also makes the point they need to be alive for “significant fraction of their life spans after the end of their fertility”. If these other animals underwent menopause and lived long enough to be noticeable, how come most scientist are discovering it only now?

That is right Archana. Post-menopausal longevity is an important one. But the crux of the theory is what is popularly called “grandmother hypothesis” that is menopause is their for women to take care of their children and grand children. This also means that women will hang around with the family post menopause to take care of the family. The problem is – Gorillas which also have menopause and have sufficient post-menopausal longevity actually leave their family groups. So then the grandmother hypothesis doesn’t apply to Gorillas.

As to why scientists didn’t observe it, there are a variety of reasons including funding for research and hot research interests. This may not have been a hot field for focused research. Also, you could ask the same question of so many XYZ findings that come every day – why are the scientists finding this XYZ thing now? Don’t know how you answer that except that we don’t know a lot of stuff about the universe and we will find new stuff every day for the foreseeable future and still no develop a complete understanding.

Animals living past their reproductive age would be a fairly obvious thing to observe, I thought.

Definitely agree, we are yet discover so many things, we can’t be asking why we did not discover them yet.

Thanks Archana. Don’t know why it is difficult, but my guess is that to track animals in the wild over a long period of time must not be easy.


Why Women Change

Most wild animals remain fertile until they die. So do human males: although some may eventually become less fertile, men in general experience no shutdown of fertility, and indeed there are innumerable well- attested cases of old men, including a 94-year-old, fathering children.

But for women the situation is different. Human females undergo a steep decline in fertility from around the age of 40 and within a decade or so can no longer produce children. While some women continue to have regular menstrual cycles up to the age of 54 or 55, conception after the age of 50 was almost unknown until the recent advent of hormone therapy and artificial fertilization.

Human female menopause thus appears to be an inevitable fact of life, albeit sometimes a painful one. But to an evolutionary biologist, it is a paradoxical aberration in the animal world. The essence of natural selection is that it promotes genes for traits that increase one’s number of descendants bearing those genes. How could natural selection possibly result in every female member of a species carrying genes that throttle her ability to leave more descendants? Of course, evolutionary biologists (including me) are not implying that a woman’s only proper role is to stay home and care for babies and to forget about other fulfilling experiences. Instead I am using standard evolutionary reasoning to try to understand how men’s and women’s bodies came to be the way they are. That reasoning tends to regard menopause as among the most bizarre features of human sexuality. But it is also among the most important. Along with the big brains and upright posture that every text of human evolution emphasizes, I consider menopause to be among the biological traits essential for making us distinctively human--something qualitatively different from, and more than, an ape.

Not everyone agrees with me about the evolutionary importance of human female menopause. Many biologists see no need to discuss it further, since they don’t think it poses an unsolved problem. Their objections are of three types. First, some dismiss it as a result of a recent increase in human expected life span. That increase stems not just from public health measures developed within the last century but possibly also from the rise of agriculture 10,000 years ago, and even more likely from evolutionary changes leading to increased human survival skills within the last 40,000 years.

According to proponents of this view, menopause could not have been a frequent occurrence for most of the several million years of human evolution, because (supposedly) almost no women or men used to survive past the age of 45 or 50. Of course the female reproductive tract was programmed to shut down by age 50, since it would not have had the opportunity to operate thereafter anyway. The increase in human life span, these critics believe, has occurred much too recently in our evolutionary history for the female reproductive tract to have had time to adjust.

What this view overlooks, however, is that the human male reproductive tract and every other biological function of both women and men continue to function in most people for decades after age 50. If all other biological functions adjusted quickly to our new long life span, why was female reproduction uniquely incapable of doing so?

Furthermore, the claim that in the past few women survived until the age of menopause is based solely on paleodemography, which attempts to estimate age at time of death in ancient skeletons. Those estimates rest on unproven, implausible assumptions, such as that the recovered skeletons represent an unbiased sample of an entire ancient population, or that ancient adult skeletons’ age of death can accurately be determined. While there’s no question that paleodemographers can distinguish an ancient skeleton of a 10-year-old from that of a 25-year-old, they have never demonstrated that they can distinguish an ancient 40-year-old from a 55- year-old. One can hardly reason by comparison with skeletons of modern people, whose bones surely age at different rates from bones of ancients with different life-styles, diets, and diseases.

A second objection acknowledges that human female menopause may be an ancient phenomenon but denies that it is unique to humans. Many wild animals undergo a decline in fertility with age. Some elderly individuals of many wild mammal and bird species are found to be infertile. Among animals in laboratory cages or zoos, with their lives considerably extended over expected spans in the wild by a gourmet diet, superb medical care, and protection from enemies, many elderly female rhesus monkeys and individuals of several strains of laboratory mice do become infertile. Hence some biologists object that human female menopause is merely part of a widespread phenomenon of animal menopause, not something peculiar to humans.

However, one swallow does not make a summer, nor does one sterile female constitute menopause. Establishing the existence of menopause as a biologically significant phenomenon in the wild requires far more than just coming upon the occasional sterile elderly individual in the wild or observing regular sterility in caged animals with artificially extended life spans. It requires finding a wild animal population in which a substantial proportion of females become sterile and spend a significant fraction of their life spans after the end of their fertility.

The human species does fulfill that definition, but only one wild animal species is known to do so: the short-finned pilot whale. One-quarter of all adult females killed by whalers prove to be postmenopausal, as judged by the condition of their ovaries. Female pilot whales enter menopause at the age of 30 or 40 years, have a mean survival of at least 14 years after menopause, and may live for over 60 years. Menopause as a biologically significant phenomenon is thus not strictly unique to humans, being shared at least with that one species of whale.

But human female menopause remains sufficiently unusual in the animal world that its evolution requires explanation. We certainly did not inherit it from pilot whales, from whose ancestors our own ancestors parted company over 50 million years ago. In fact, we must have evolved it after we separated from the apes just 7 million to 5 million years ago, because we undergo menopause whereas chimps and gorillas appear not to (or at least not regularly).

The third and last objection acknowledges human menopause as an ancient phenomenon that is indeed unusual among animals. But these critics say that we need not seek an explanation for menopause, because the puzzle has already been solved. The solution, they say, is the physiological mechanism of menopause: the senescence and exhaustion of a woman’s egg supply, fixed at birth and not added to after birth. An egg is lost at each menstrual cycle. By the time a woman is 50 years old, most of that original egg supply has been depleted. The remaining eggs are half a century old and increasingly unresponsive to hormones.

But there is a fatal counterobjection to this objection. While the objection is not wrong, it is incomplete. Yes, exhaustion and aging of the egg supply are the immediate cause of human menopause, but why did natural selection program women so that their eggs become exhausted or aged in their forties? There is no obvious reason we had to evolve eggs that degenerate by the end of half a century. Eggs of elephants, baleen whales, and tortoises remain viable for at least 60 years. A mutation only slightly altering how eggs degenerate might have sufficed for women to remain fertile until age 60 or 75.

The easy part of the menopause puzzle is identifying the physiological mechanism by which a woman’s egg supply becomes depleted or impaired by the time she is around 50 years old. The challenging problem is understanding why we evolved that seemingly self-defeating detail of reproductive physiology. Apparently there was nothing physiologically inevitable about human female menopause, and there was nothing evolutionarily inevitable about it from the perspective of mammals in general. Instead the human female, but not the human male, was programmed by natural selection, at some time within the last few million years, to shut down reproduction prematurely. That premature senescence is all the more surprising because it goes against an overwhelming trend: in other respects, we humans have evolved to age more slowly, not more rapidly, than most other animals.

Any theory of menopause evolution must explain how a woman’s apparently counterproductive evolutionary strategy of making fewer babies could actually result in her making more. Evidently, as a woman ages, she can do more to increase the number of people bearing her genes by devoting herself to her existing children, her potential grandchildren, and her other relatives than by producing yet another child.

That evolutionary chain of reasoning rests on several cruel facts. One is that the human child depends on its parents for an extraordinarily long time, longer than in any other animal species. A baby chimpanzee, as soon as it starts to be weaned, begins gathering its own food, mostly with its own hands. (Chimpanzee use of tools, such as fishing for termites with blades of grass or cracking nuts with stones, is of great interest to human scientists but of only limited dietary significance to chimpanzees.) The baby chimpanzee also prepares its food with its own hands. But human hunter-gatherers acquire most food with tools (digging sticks, nets, spears), prepare it with other tools (knives, pounders, huskers), and then cook it in a fire made by still other tools. Furthermore, they use tools to protect themselves against dangerous predators, unlike other prey animals, which use teeth and strong muscles. Making and wielding all those tools are completely beyond the manual dexterity and mental ability of young children. Tool use and toolmaking are transmitted not just by imitation but also by language, which takes over a decade for a child to master.

As a result, human children in most societies do not become capable of economic independence until their teens or twenties. Before that, they remain dependent on their parents, especially on the mother, because mothers tend to provide more child care than do fathers. Parents not only bring food and teach toolmaking but also provide protection and status within the tribe. In traditional societies, early death of either parent endangers a child’s life even if the surviving parent remarries, because of possible conflicts with the stepparent’s genetic interests. A young orphan who is not adopted has even worse chances of surviving.

Hence a hunter-gatherer mother who already has several children risks losing her genetic investment in them if she does not survive until the youngest is at least a teenager. That’s one cruel fact underlying human female menopause. Another is that the birth of each successive child immediately jeopardizes a mother’s previous children because the mother risks dying in childbirth. In most other animal species that risk is very low. For example, in one study of 401 rhesus monkey pregnancies, only three mothers died in childbirth. For humans in traditional societies, the risk is much higher and increases with age. Even in affluent twentieth-century Western societies, the risk of dying in childbirth is seven times higher for a mother over the age of 40 than for a 20-year-old. But each new child puts the mother’s life at risk not only because of the immediate risk of death in childbirth but also because of the delayed risk of death related to exhaustion by lactation, carrying a young child, and working harder to feed more mouths.

Infants of older mothers are themselves increasingly unlikely to survive or be healthy, because the risks of abortion, stillbirth, low birth weight, and genetic defects rise as the mother grows older. For instance, the risk of a fetus’s carrying the genetic condition known as Down syndrome increases from one in 2,000 births for a mother under 30, one in 300 for a mother between the ages of 35 and 39, and one in 50 for a 43-year-old mother to the grim odds of one in 10 for a mother in her late forties.

Thus, as a woman gets older, she is likely to have accumulated more children, and she has been caring for them longer, so she is putting a bigger investment at risk with each successive pregnancy. But her chances of dying in or after childbirth, and the chances that the infant will die, also increase. In effect, the older mother is risking more for less potential gain. That’s one set of factors that would tend to favor human female menopause and that would paradoxically result in a woman’s having more surviving children by giving birth to fewer children.

But a hypothetical nonmenopausal older woman who died in childbirth, or while caring for an infant, would thereby be throwing away even more than her investment in her previous children. That is because a woman’s children eventually begin producing children of their own, and those children count as part of the woman’s prior investment. Especially in traditional societies, a woman’s survival is important not only to her children but also to her grandchildren.

That extended role of postmenopausal women has been explored by anthropologists Kristen Hawkes, James O’Connell, and Nicholas Blurton Jones, who studied foraging by women of different ages among the Hadza hunter-gatherers of Tanzania. The women who devoted the most time to gathering food (especially roots, honey, and fruit) were postmenopausal women. Those hardworking Hadza grandmothers put in an impressive seven hours per day, compared with a mere three hours for girls not yet pregnant and four and a half hours for women of childbearing age. As one might expect, foraging returns (measured in pounds of food gathered per hour) increased with age and experience, so that mature women achieved higher returns than teenagers. Interestingly, the grandmothers’ returns were still as high as women in their prime. The combination of putting in more foraging hours and maintaining an unchanged foraging efficiency meant that the postmenopausal grandmothers brought in more food per day than women of any of the younger groups, even though their large harvests were greatly in excess of their own personal needs and they no longer had dependent young children of their own to feed.

Observations indicated that the Hadza grandmothers were sharing their excess food harvest with close relatives, such as their grandchildren and grown children. As a strategy for transforming food calories into pounds of baby, it’s more efficient for an older woman to donate the calories to grandchildren and grown children than to infants of her own, because her fertility decreases with age anyway, while her children are young adults at peak fertility. Naturally, menopausal grandmothers in traditional societies contribute more to their offspring than just food. They also act as baby-sitters for grandchildren, thereby helping their adult children churn out more babies bearing Grandma’s genes. And though they work hard for their grandchildren, they’re less likely to die as a result of exhaustion than if they were nursing infants as well as caring for them.

But menopause has another virtue, one that has received little attention. That is the importance of old people to their entire tribe in preliterate societies, which means every human society in the world from the time of human origins until the rise of writing in Mesopotamia around 3300 B.C.

A common genetics argument is that natural selection cannot weed out mutations that do not damage people until they are old, because old people are supposedly postreproductive. I believe that such statements overlook an essential fact distinguishing humans from most animal species. No humans, except hermits, are ever truly postreproductive, in the sense of being unable to aid in the survival and reproduction of other people bearing their genes. Yes, I grant that if any orangutans lived long enough in the wild to become sterile, they would count as postreproductive, since orangutans (other than mothers with one young offspring) tend to be solitary. I also grant that the contributions of very old people to modern literate societies tend to decrease with age. That new phenomenon of modern societies is at the root of the enormous problems that old age now poses, both for the elderly themselves and for the rest of society. But we moderns get most of our information through writing, television, or radio. We find it impossible to conceive of the overwhelming importance of elderly people in preliterate societies as repositories of information and experience.

Here is an example of that role. During my field studies of bird ecology on New Guinea and adjacent southwestern Pacific islands, I live among people who traditionally were without writing, depended on stone tools, and subsisted by farming and fishing supplemented by hunting and gathering. I am constantly asking villagers to tell me the names of local birds, animals, and plants in their language, and to tell me what they know about each species. New Guineans and Pacific islanders possess an enormous fund of biological knowledge, including names for a thousand or more species, plus information about where each species occurs, its behavior, its ecology, and its usefulness to humans. All that information is important because wild plants and animals furnish much of the people’s food and all their building materials, medicines, and decorations.

Again and again, when I ask about some rare bird, only the older hunters know the answer, and eventually I ask a question that stumps even them. The hunters reply, We have to ask the old man [or the old woman]. They take me to a hut where we find an old man or woman, blind with cataracts and toothless, able to eat food only after someone else has chewed it. But that old person is the tribe’s library. Because the society traditionally lacked writing, that old person knows more about the local environment than anyone else and is the sole person with accurate knowledge of events that happened long ago. Out comes the rare bird’s name, and a description of it.

The accumulated experience that the elderly remember is important for the whole tribe’s survival. In 1976, for instance, I visited Rennell Island, one of the Solomon Islands, lying in the southwestern Pacific’s cyclone belt. When I asked about wild fruits and seeds that birds ate, my Rennellese informants named dozens of plant species by Rennell language names, named for each plant species all the bird and bat species that eat its fruit, and said whether the fruit is edible for people. They ranked fruits in three categories: those that people never eat those that people regularly eat and those that people eat only in famine times, such as after--and here I kept hearing a Rennell term initially unfamiliar to me-- the hungi kengi.

Those words proved to be the Rennell name for the most destructive cyclone to have hit the island in living memory--apparently around 1910, based on people’s references to datable events of the European colonial administration. The hungi kengi blew down most of Rennell’s forest, destroyed gardens, and drove people to the brink of starvation. Islanders survived by eating fruits of wild plant species that were normally not eaten. But doing so required detailed knowledge about which plants are poisonous, which are not poisonous, and whether and how the poison can be removed by some technique of food preparation.

When I began pestering my middle-aged Rennellese informants with questions about fruit edibility, I was brought into a hut. There, once my eyes had become accustomed to the dim light, I saw the inevitable frail old woman. She was the last living person with direct experience of which plants were found safe and nutritious to eat after the hungi kengi, until people’s gardens began producing again. The old woman explained that she had been a child not quite of marriageable age at the time of the hungi kengi. Since my visit to Rennell was in 1976, and since the cyclone had struck 66 years before, the woman was probably in her early eighties. Her survival after the 1910 cyclone had depended on information remembered by aged survivors of the last big cyclone before the hungi kengi. Now her people’s ability to survive another cyclone would depend on her own memories, which were fortunately very detailed.

Such anecdotes could be multiplied indefinitely. Traditional human societies face frequent minor risks that threaten a few individuals, and also face rare natural catastrophes or intertribal wars that threaten the lives of everybody in the society. But virtually everyone in a small traditional society is related to one another. Hence old people in a traditional society are essential to the survival not only of their children and grandchildren but also of hundreds of other people who share their genes. In preliterate societies, no one is ever postreproductive.

Any preliterate human societies that included individuals old enough to remember the last hungi kengi had a much better chance of surviving the next one than did societies without such old people. The old men were not at risk from childbirth or from exhausting responsibilities of lactation and child care, so they did not evolve protection by menopause. But old women who did not undergo menopause tended to be eliminated from the human gene pool because they remained exposed to the risk of childbirth and the burden of child care. At times of crises, such as a hungi kengi, the prior death of such an older woman also tended to eliminate all the woman’s relatives from the gene pool--a huge genetic price to pay just for the dubious privilege of continuing to produce another baby or two against lengthening odds. That’s what I see as a major driving force behind the evolution of human female menopause. Similar considerations may have led to the evolution of menopause in female pilot whales. Like us, whales are long-lived, involved in complex social relationships and lifelong family ties, and capable of sophisticated communication and learning.

If one were playing God and deciding whether to make older women undergo menopause, one would do a balance sheet, adding up the benefits of menopause in one column for comparison with its costs in another column. The costs of menopause are the potential children of a woman’s old age that she forgoes. The potential benefits include avoiding the increased risk of death due to childbirth and parenting at an advanced age, and thereby gaining the benefit of improved survival for one’s grandchildren, prior children, and more distant relatives. The sizes of those benefits depend on many details: for example, how large the risk of death is in and after childbirth, how much that risk increases with age, how rapidly fertility decreases with age before menopause, and how rapidly it would continue to decrease in an aging woman who did not undergo menopause. All those factors are bound to differ between societies and are not easy for anthropologists to estimate. But natural selection is a more skilled mathematician because it has had millions of years in which to do the calculation. It concluded that menopause’s benefits outweigh its costs, and that women can make more by making less.


Chimps Grieve Over Dead Relatives

In November of 2008, a chimpanzee in her 50s known as Pansy became lethargic and obviously ill at the Blair Drummond Safari and Adventure Park in the United Kingdom. Three other adults, including Pansy's 20-year-old daughter Rosie, began tending to her, grooming her, and sleeping nearby instead of in their own nesting areas. Pansy continued to deteriorate over the next few weeks, until one day her breathing suddenly became erratic. During the 10 minutes before Pansy's death, the others groomed and caressed her constantly, and Rosie remained near her mother during the night. Keepers removed Pansy's body the next day, and the adult chimps remained unusually subdued for nearly a week.

In two new studies, published online today in Current Biology, researchers cite this and a case of chimp mothers carrying around their dead infants to support the idea that our closest evolutionary cousins share many humanlike responses to death, including mourning. The work offers some of the first clear evidence of grieving in animals other than humans, though we may consider some of the behaviors bizarre.

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Most animals show few signs of being disturbed when one of their fellows dies. For example, red deer used to hearing rifle shots will remain relatively impassive when another deer is shot dead by a hunter. One notable exception is the African elephant, which has often been observed engaging in empathetic and caring behavior toward dying members of its species. (In one such episode, observed by researchers in Kenya in 2003, an elderly elephant named Eleanor collapsed and was helped to her feet several times by other elephants before she died the following day. Her fellow elephants then kept an apparent vigil over her body for about a week, occasionally poking at her and lifting her legs and trunk.)

Researchers have long suspected that chimps and other apes might engage in similar behavior, but they very rarely observe the death of a chimp in the wild, says Richard Byrne, a psychologist at the University of St. Andrews in the United Kingdom. Zoos usually separate dying individuals from fellow chimps, he adds, making observations of their responses very difficult. In Pansy's case, zookeepers decided to allow the other chimps to stay with her as she died, while a research team, led by psychologist James Anderson of the nearby University of Stirling , observed their reactions.

In a second example of chimpanzee grieving, a research group led by Dora Biro, a zoologist at the University of Oxford in the U.K., observed two chimp mothers carrying the remains of their dead infants for weeks. The observations were made in the forests of Bossou, Guinea, where primatologists have been studying wild chimps for 3 decades. In 2003, an epidemic of respiratory disease broke out at Bossou, killing five chimps. Two were infants, 1-year-old Jimato and 2-year-old Veve. The mothers of the infants carried their dead bodies around on their backs for 68 and 19 days, respectively, even as they dried out and became mummified. They brushed flies away from the babies, groomed them regularly, and allowed other chimps—including other young animals—to poke at the bodies, lift their limbs, and even carry them around for short distances.

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In the case of Pansy, Anderson and his colleagues conclude that the other chimps reacted to her death in humanlike ways, experiencing grief and mourning. "Some of the behaviors appear strikingly similar to aspects of human responses to death and dying," Anderson says, adding that many researchers have considered such reactions to be unique to humans. Christophe Boesch, a primatologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, says that his team has observed similar behavior among chimps at Tai Forest in the Côte d'Ivoire and agrees that "we have certainly underestimated awareness of death in chimpanzees."

The Bossou study is more difficult to interpret, researchers say, because it is not clear whether the mothers realized that their infants were truly dead. "It's extremely difficult to make any claims either way," Biro says. "We only have access to behavior, not to internal mental states." But Byrne, who once observed a gorilla carry her dead baby around for 3 days in the mountains of Rwanda, points out that hospitals and doctors are increasingly giving the parents of a deceased infant the option of remaining with the body of their child for hours or even days before giving it up for burial, as a way of aiding the grieving process. Anderson hopes the Pansy episode will encourage zoos to do the same.


ACKNOWLEDGMENTS

We would like to acknowledge the efforts of scientists who collected the raw data used in this paper. In particular we thank Dave Ellifrit, Erin Heydenrich, Astrid van Ginneken and other staff at the Center for Whale Research for killer whale demographic data, and, we thank the Banded Mongoose Research Project for access to unpublished data. We also thank colleagues in the Centre for Research in Animal Behaviour at the University of Exeter for useful discussions and input. We would also like the three anonymous reviewers for their constructive comments which improved the manuscript.


Innate Behavior

Behaviors that are closely controlled by genes with little or no environmental influence are called innate behaviors. These are behaviors that occur naturally in all members of a species whenever they are exposed to a certain stimulus. Innate behaviors do not have to be learned or practiced. They are also called instinctive behaviors. An instinct is the ability of an animal to perform a behavior the first time it is exposed to the proper stimulus. For example, a dog will drool the first time&mdashand every time&mdashit is exposed to food.

Significance of Innate Behavior

Innate behaviors are rigid and predictable. All members of the species perform the behaviors in the same way. Innate behaviors usually involve basic life functions, such as finding food or caring for offspring. Several examples are shown in Figure below. If an animal were to perform such important behaviors incorrectly, it would be less likely to survive or reproduce.

Examples of Innate Behavior. These innate behaviors are necessary for survival or reproduction. Can you explain why each behavior is important?

Intelligence and Innate Behavior

Innate behaviors occur in all animals. However, they are less common in species with higher levels of intelligence. Humans are the most intelligent species, and they have very few innate behaviors. The only innate behaviors in humans are reflexes. A reflex is a response that always occurs when a certain stimulus is present. For example, a human infant will grasp an object, such as a finger, that is placed in its palm. The infant has no control over this reaction because it is innate. Other than reflexes such as this, human behaviors are learned&ndashor at least influenced by experience&mdashrather than being innate.

Innate Behavior in Human Beings

All animals have innate behaviors, even human beings. Can you think of human behaviors that do not have to be learned? Chances are, you will have a hard time thinking of any. The only truly innate behaviors in humans are called reflex behaviors. They occur mainly in babies. Like innate behaviors in other animals, reflex behaviors in human babies may help them survive.

An example of a reflex behavior in babies is the sucking reflex. Newborns instinctively suck on a nipple that is placed in their mouth. It is easy to see how this behavior evolved. It increases the chances of a baby feeding and surviving. Another example of a reflex behavior in babies is the grasp reflex (Figure below). Babies instinctively grasp an object placed in the palm of their hand. Their grip may be surprisingly strong. How do you think this behavior might increase a baby&rsquos chances of surviving?

One of the few innate behaviors in human beings is the grasp reflex. It occurs only in babies.


Hermaphroditism

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Hermaphroditism, the condition of having both male and female reproductive organs. Hermaphroditic plants—most flowering plants, or angiosperms—are called monoecious, or bisexual. Hermaphroditic animals—mostly invertebrates such as worms, bryozoans (moss animals), trematodes (flukes), snails, slugs, and barnacles—are usually parasitic, slow-moving, or permanently attached to another animal or plant.

In humans, conditions that involve discrepancies between external genitalia and internal reproductive organs are described by the term intersex. Intersex conditions are sometimes also referred to as disorders of sexual development (DSDs). Such conditions are extremely rare in humans. In true gonadal intersex (or true hermaphroditism), an individual has both ovarian and testicular tissue. The ovarian and testicular tissue may be separate, or the two may be combined in what is called an ovotestis. Affected individuals have sex chromosomes showing male-female mosaicism (where one individual possesses both the male XY and female XX chromosome pairs). Most often, but not always, the chromosome complement is 46,XX, and in every such individual there also exists evidence of Y chromosomal material on one of the autosomes (any of the 22 pairs of chromosomes other than the sex chromosomes). Individuals with a 46,XX chromosome complement usually have ambiguous external genitalia with a sizable phallus and are therefore often reared as males. However, they develop breasts during puberty and menstruate and in only rare cases actually produce sperm. In 46,XX intersex (female pseudohermaphroditism), individuals have male external genitalia but the chromosomal constitution and reproductive organs of a female. In 46,XY (male pseudohermaphroditism), individuals have ambiguous or female external genitalia but the chromosomal constitution and reproductive organs of a male, though the testes may be malformed or absent.

Treatment of intersex in humans depends upon the age at which the diagnosis is made. Historically, if diagnosed at birth, the choice of sex was made (typically by parents) based on the condition of the external genitalia (i.e., which sex organs predominate), after which so-called intersex surgery was performed to remove the gonads of the opposite sex. The remaining genitalia were then reconstructed to resemble those of the chosen sex. The reconstruction of female genitalia was more readily performed than the reconstruction of male genitalia, so ambiguous individuals often were made to be female. However, intersex surgery has long-term consequences for affected individuals. Later in life, for example, the person may not be satisfied with the results of surgery and may not identify with the assigned gender. Thus, patient consent has become an increasingly important part of decisions about intersex surgery, such that surgery may be delayed until adolescence or adulthood, after patients have had sufficient time to consider their gender and are able to make informed decisions about treatment. In older individuals the accepted gender may be reinforced by the appropriate surgical procedures and by hormonal therapy.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Kara Rogers, Senior Editor.


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