If you think about it, you might realize that after an animal finishes reproducing, it should die, because genes that make you live on after you can no longer reproduce have no selective advantage: they are no better than genes that kill you off when you’ve had your last child. In principle, natural selection should keep you pumping out gametes and children until you die. But in some species, namely ours, some of our ape relatives, and, curiously, some toothed whales like killer whales, females continue to live for considerable periods after their reproduction ends. We call that end “menopause”. This leads to three questions:
a. Why do animals cease reproducing? That is, why don’t they continue to reproduce until they die?
b. Why in some cases do animals continue to live even after they cease reproducing?
c. Why don’t males undergo “manopause” in our species?
This new paper from PNAS (click title below to read, or find the pdf here) deals with the first two questions, but not with the third; and I’ll leave readers to ponder that one. The paper in fact, simply shows that in one population of mountain gorillas in Uganda, many females do show a form of menopause, living on for nearly a quarter of their adult lives as nonreproductives. While this phenomenon has been demonstrated in chimps, other studies of gorillas have not shown it. The authors posit that there may be different results in different wild populations of gorillas, though that’s hard to understand if you think the phenomenon involves natural selection. Why should such selection differ among populations of the same species? The hypotheses below don’t predict interpopulation variation.
Read on:
The first question above can be answered by realizing that menopause may be partly a cultural phenomenon. For the vast majority of our evolutionary history, humans probably died before the females stopped reproducing: probably between ages twenty and forty. There may have been no menopause in our species because nobody lived long enough to show it. And that may be one clue for why we show it now: any genes that cause women, at least, to lose reproductive ability when older were simply not expressed, and thus not selected against. This may also be the reason why earlier studies of chimps showed menopause: they were taken care of in zoos or reserves in a way that allowed them to live longer than they did during most of their evolutionary history.
Further, this population of gorillas, though living in a reserve, were not given special food or treatment (some were given vet care, but those were omitted from the study), and still showed not only menopause, but long lives after menopause. The “evolutionary history” phenomenon can’t easily explain that. Nor can it explain postreproductive life in toothed whales—unless that was seen only in aquaria where they lived longer than they would have during much of their evolution, including now when living in the open ocean. (One would have to look at the studies to determine that.)
But regardless of the cause, one can say that one population of mountain gorillas under natural conditions—probably similar to those that obtained during most of their evolution—often show not only a cessation of reproduction but also considerable years of life beyond that. And the behavior of gorillas makes some of the evolutionary hypotheses for menopause seem unlikely.
Results. This will be short. The authors studied 25 adult female mountain gorillas (Gorilla beringei beringei, one of two subspecies of the Eastern Gorilla) in Bwindi Impenetrable National Park in Uganda. The females came from four groups, and their life histories were known, presumably through intense observation. (Mountain gorillas hsve indeed been studied intensively, most famously by George Schaller and later by Dian Fossey (who was murdered during her studies.) The study populations are fairly easily habituated to human presence, which allows this study.
Here’s how they define “post-reproductive females” and how many of them showed menopause:
According to a commonly used definition, “postreproductive females” are those who live past the age of their last reproduction for longer than the mean plus two SD of successful interbirth intervals (2). We calculated this value as 7.7 y [5.1 (2 1.3)] in our study population, suggesting that seven out of the 25 study females qualified as postreproductive. Six of these seven females have been conservatively estimated (based on the ages of genetically identified offspring, body condition and hair loss) to be older than 35 y old, which is the maximum age of observed reproduction (Figs. 1 and 2). All the seven postreproductive females exhibited a postreproductive lifespan of at least 10 y (Fig. 1), minimizing the possibility to be “mistakenly” classified as postreproductive. These females were not observed mating for an average of 7.4 5.8 y before they exit the study
And a summary:
Our study shows that wild Bwindi mountain gorillas can exhibit long postreproductive lifespans. Given that female gorillas rarely reach 50 y of age in the wild (6), the 10 postreproductive years lived by one third of the study females represents at least 25% of their adult lifespan (adults: 10 y old). More generally, the standardized population measure of PrR suggested that females spend 10% of their adult lifespan as postreproductive. Importantly, neither of the two methods we used to derive postreproductive lifespan can distinguish menopause from other causes of sterility, such as an increased fetal loss probability in old females. Nevertheless, the extensive duration of postreproductive lifespan, the reduced or lack of mating activity, and previous endocrine analyses of old females (8, 9) suggest that menopause is a highly plausible cause for the reproductive patterns we observed. The selective pressure(s) which might have favored the evolution of this trait in gorillas remain unclear.
Indeed; menopause remains a mystery in all species that show it. We have hypotheses but no substantive answers.
So the question arises of what, if any, selective pressures could have promoted female longevity beyond reproduction. This assumes—which we don’t know—that postreproductive survival was an adaptation. If it was, and not just a “spandral” here are a few hypotheses. The bold headings are mine, and indented text is from the paper:
a. Reproductive conflict:
The “reproductive conflict hypothesis,” posits that old females cease reproduction to avoid competition for limited reproductive opportunities with young (related) individuals (12); e.g., their daughters or the mates of their sons]. Female gorillas disperse from their natal groups and often disperse again from groups where they have reproduced (13), meaning that they have low relatedness to their groupmates. Hence, the benefits of reproduction for female gorillas at an old age may be greater than that for chimpanzees or humans, where female local relatedness increases with age and females reproduce simultaneously with their offspring (12, 14).
Avoiding conflict with individuals is advantageous only if they’re related, for this would be a form of “kin selection”. Since gorillas’ dispersal take them away from their kin, that makes this hypothesis less likely but not completely unlikely.
b. Intergenerational help, one form of which is the “grandmother hypothesis”.
Another relevant set of hypotheses, also relatively unlikely to apply to gorillas, posit that intergenerational help, and its positive influence in grandoffspring fitness, may drive the evolution of postreproductive lifespan through two not mutually exclusive evolutionary pathways [see also “grandmother hypothesis”; (1)]: by selecting for longer female lifespan to allow females overlap with grandoffspring and help them increase their fitness (e.g., by offering their ecological knowledge, or by defending them). . .
. . . The associated “mother hypothesis” (15) might have greater predictive power in gorillas. This hypothesis posits that old females cease reproduction to minimize energy expenditure or other reproductive costs, and maximize investment to existing offspring and their fitness. Consistent with this hypothesis, maternal presence, care, and support is critical even for adults in gorillas and other hominids (16).
This too is a form of kin selection (as is parental care), for genes that help you take care of your grand-apes, or your offspring when you’re old, will still be helping copies of those genes in their still-reproductive descendants. This is feasible for taking care of offspring, but given the dispersal of female gorillas, the “grand-ape” hypothesis is less likely.
And here’s a nonadaptive hypothesis, but one that is popular:
c. Menopause is a nonadaptive byproduct of gorillas’ life history.
A final hypothesis posits that postreproductive lifespan is a nonadaptive by-product of life-history patterns. Given that many wild animals die from predation, disease, or starvation, genes whose deleterious effects appear only in advanced ages, may not be purged (15). When “favorable” conditions allow individuals to survive at these ages, deleterious effects that prevent reproduction may appear (4, 11). Accordingly, greater food abundance and potentially lower predation pressure in comparison to the evolutionary history of chimpanzees, may allow Ngogo chimpanzees to live longer and exhibit menopause (4). Similarly, Bwindi gorillas currently do not face any predation risk from leopards, their main potential nonhuman predators,
A version of this hypothesis is that some genes have the effect of promoting reproduction early in life, but at the price of inhibiting reproduction later in life. Under many conditions, such “early reproducing genes” will be more adaptive than genes promoting later reproduction, because the former leave more copies of themselves earlier. (Those genes, for example, would be heavily favored in a growing population.). Thus senescence and menopause could simply be the result of the accumulation of adaptive “early-reproducing genes.”
Which, if any, of these hypotheses are right? We don’t really know for primates or toothed whales, and though there may be evidence for “senescing” genes in some laboratory species, I’m not aware of it.
The question remains why don’t male chimps, gorillas, and humans show “manopause”. Some human males, for example, can father offspring even at the age of 80, but you’ll never find a woman reproducing at that age And we have no data from chimps or gorillas on males, at least as far as I know.
So, as always, “more work needs to be done”. But at least we now know that gorillas and chimps have menopause in females, which might make you a big hit if you bring it up at a cocktail party. And don’t forget to mention those toothed whales!
