Category: reproduction

Menopause in gorillas: a new study

October 23, 2025 • 10:45 am

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!

Male seahorse gives birth

April 17, 2025 • 2:02 pm

Seahorses (Hippocampus spp.) which are fish, have an unusual reproductive system. The males get “pregnant”, meaning that they carry the eggs, which are deposited in the male’s pouch by the female and then fertilized there. (Note: this doesn’t mean that seahorse males are “females”, or that there are more than two sexes!)  We don’t really know why males gestate the eggs, but we do know that females produce eggs faster than males can gestate them. This means that, unlike most animals, females compete for the attention of males.  Here’s a birth; National Geographic says that 2,000 babies are being born. Wikipedia says that the babies can be as few as 5 or as many as 25.

That’s your biology of the day; and I am sorry that for the next few days I won’t be posting much.

 

The NYT oversells a new report of a “virgin birth” in crocodiles

June 8, 2023 • 9:30 am

This is an example of science reporting that’s misleading—not because it gets the facts wrong, but because it oversells a rather mundane finding as a potentially important insight into the life of the extinct dinosaurs.  And no, there’s nothing in the original paper—about a single “virgin birth” in a crocodile (really a “stillbirth”)—to suggest the Big Sell: that dinosaurs could have reproduced via “virgin births”, too. Dinosaur Jesus probably didn’t exist.

So here we have one new paper from the Royal Society’s Biology Letters; click on first screenshot below to read (the pdf is here) reporting the occurrence of parthenogenesis in one crocodile in one zoo. The NYT Trilobite reports about it in the second screenshot below (I found it archived here).

Parthenogenesis is a form of asexual reproduction that occurs without the fusion of a sperm and an egg. It occurs across various animal taxa, and can arise in different ways.  Here’s what Wikipedia says about its distribution (note that it doesn’t occur naturally in mammals).

Parthenogenesis occurs naturally in some plants, algae, invertebrate animal species (including nematodes, some tardigrades, water fleas, some scorpions, aphids, some mites, some bees, some Phasmatodea and parasitic wasps) and a few vertebrates (such as some fish, amphibians, reptiles and birds).

Parthenogenesis can occur in a variety of ways, often after hybridization between species that produces an individual whose chromosomes can’t pair properly during meiosis (cell division producing sexual gametes). That mispairing increases the likelihood that an unfertilized egg in a hybrid can have a full chromosome complement and develop into an offspring. But in most other cases, like this one, asexual reproduction occurs as a modification of non-hybrid meiosis.  That’s what apparently happened in this case.

The authors report that an 18-year-old American crocodile (Crocodylus acutus) in a reptile park in Costa Rica laid a clutch of 14 eggs. Candling of the eggs (holding them up to the light) revealed that seven apparently contained embryos (or dense spots), but none developed to hatching. One fetus, however, almost made it, and when the egg was dissected it contained a single female. Here’s the photo from the paper: mom on top and the fetus at the bottom. There was clearly something wrong with the baby as it failed to hatch, and I suspect it was simply screwed up.

(From paper): Figure 1. (a) Adult American crocodile, Crocodylus acutus. Photo courtesy of Q. Dwyer. (b) Stillborn fetus of American crocodile, Crocodylus acutus, Parthenogen. Photo courtesy of Q. Dwyer.

Genetic analysis of the fetal DNA compared to mom’s showed that they were pretty much genetically identical, with the fetus being, in effect, a clone of the mother.

How did this happen? If you don’t need the details, skip this next part between the lines.

DNA results from the paper suggest that the fetus came from fusion of two of the products of meiosis. Here’s a diagram of how a female produces an egg (or a male produces a sperm) during meiosis (figure from Wikipedia). It entails doubling of each of the chromosomes in a sperm or egg precursor cell followed by two bouts of cell division.  The circle on the extreme left is of the precursor gamete cell in a species having four chromosomes, with two copies of each of two different chromosomes (remember, we have two pair of every chromosome).  Then each pair doubles, which happens during normal cell division (mitosis) as well as in meiosis (second circle).

But then meiosis begins in gonadal cells: instead of each doubled chromosome splitting and going to a new cell, producing two genetically identical cells (this is normal cell division), the chromosome twins of each pair, now doubled (and having reshuffled bits from meiotic “recombination”), go into separate cells (third circle from left).  This is the first phase of meiosis, or “meiosis I”.  Now we have two daughter cells, each containing two chromosomes that are doubled but connected by the centromere. The chromosome number is really two, not four, because these doubled chromosomes (called “chromatids”) are going to go to different cells.

During “meiosis II”, the doubled chromosomes of each pair split, with each going to a new cell, so instead of two pair of joined chromosomes, we simply get two single chromosomes.  The chromosome halves can sort out independently of each other, which is called “segregation”, and is one way to recombine existing genes. In the end, a cell with four chromosomes has, though doubling and then two  cell divisions, produced a “haploid” cell with only two chromosomes. These final cells have only one instead of two copies of each chromosome pair. They are the gametes: eggs and sperm, and are shown on the extreme right.  This happens in eggs and sperm, and when they fuse, the normal chromosome number is restored. You can see that when this happens in both sexes, you get novel combinations of parental genes—one of the likely reasons why sex evolved.

Now, how did this produce the parthenogenic crocodile fetus?  In females, usually only one of the four products of meiosis goes on to form the haploid egg, with the other three cells being segregated into “polar bodies” that eventually die. But sometimes a polar body will fuse with the egg cell, restoring the normal chromosome complement and producing a diploid individual having two copies of each chromosome.  It’s a bit more complicated than this , but the result is that a female can produce a normal diploid offspring without being fertilized. This is simply because two of the halved cells that would normally become egg cells fuse with each other.

Using DNA sequencing, the authors confirmed that this is what happened (the formal process is called “automictic parthenogensis with terminal fusion”). But the diploid fetus, which would normally go on to form a crocodile that would hatch, was somehow inviable.

Note that this is the first report of parthenogenesis in a crocodile. (Since the mother was 18 and had been alone since she was two, it was extremely unlikely that the fetus came from her storing sperm after being inseminated sixteen years before, as some animals can store sperm for a substantial period.)

So far so good. And the NYT article below gets the details and facts right. Where it goes off the rails is instantiated in the sub-headline.  (The big headline is wrong, too: this is NOT a “virgin birth” but a virgin stillbirth. No new croc entered the world. 

Because crocodiles are related to the extinct dinosaurs, author Greenwood (and the authors of the original paper), suggest that this is a case of “facultative parthenogesis that could also have occurred in dinosaurs! Dinosaurs could have reproduced asexually! T. rex could have produced baby T. rex copies without having to mate! Note the subheadline above, mentioning “Jurassic Park”.)

The article also says this:

So, did dinosaurs do it, as the discovery of parthenogenesis in crocodiles suggests? Parthenogenesis is best confirmed with DNA analysis, a process that has allowed scientists to tell it apart from delayed conception, where a female stores sperm for as long as six years before using it to fertilize eggs. Without the ability to retrieve dinosaurs’ and pterosaurs’ DNA, which does not persist in fossils, certainty is not available.

“We’ll never be able to prove they could do it,” Dr. Booth said. “But it suggests they had the ability.”

My response is “no, it doesn’t.”

But at least both the authors and the journalist say we can’t prove that there were Jesus-saurs, but I’d go further and argue that this is speculation far beyond what’s warranted from the data—speculation prompted by the desire to jazz up what is a decent but not terribly exciting result. In fact, I’d say that because the single dead offspring represents a bug and not a feature of croc reproduction, it says absolutely nothing about the likelihood that dinosaurs reproduced asexually, much less that they did so “facultatively”—as a regular evolved feature of their lifestyle.

First of all, this isn’t really “facultative parthenogesis,” which refers to species that can produce normal offspring sometimes via sexual reproduction and sometimes asexually. (That might be an evolved property, allowing an individual to pass on its genes when populations are sparse and there is nobody around to mate with.)

This isn’t what’s going on with crocs.  What we see here is probably a rare developmental screwup (it hasn’t been reported before, despite there being many captive crocs), an anomaly that is an evolutionary dead end.  Rare parthenogensis has been seen in other species too, including birds (see below), but in those species results in largely inviable or infertile offspring.  True “facultative parthenogenesis” isn’t common, but does occur in some creatures like lizards. And it’s not a bug, but an evolved feature.

Below I show the family tree of some vertebrates, including the dinosaurs, birds, lizards (one of the “squamates” along with snakes), and the crocs. (This slide was created by my friend Phil Ward, who teaches evolution at UC Davis.) You see that the crocs are related to dinosaurs, but not as closely related as are living birds, which are a group (“clade”) phylogenetically embedded within the now-extinct dinosaurs.

The upshot is that a rare occurrence of asexual reproduction, especially when it leads to a dead or sterile offspring, says nothing about the likelihood of facultative parthenogenesis in a relative. After all, birds are more closely related to dinosaurs than are crocs, and a couple of birds have reproduced asexually (turkeys can produce fertile offspring this way, though rarely, and California condors have also done it). But nobody goes shouting from the rooftops “dinosaurs could have reproduced asexually” because of a few rare cases in birds.  Quite a few lizards reproduce normally by parthenogenesis, but you don’t hear people extrapolating from lizards to dinosaurs, either.  Even some fish reproduce parthenogenetically, so why not write that “because fish are related to dinosaurs, dinos could also have reproduced parthenogenically.”?  It’s all pilpul.

The facts in the NYT report are accurate, including the caveat that we can’t test the asexual-dino hypothesis, but the author couldn’t resist bringing in the dinosaur angle—without any good reason to do so.  What will happen is that people will ignore the fact that the single parthenogenic crocodile fetus was stillborn (or “stillhatched”), was probably just a rare developmental anomaly, and go away instead with the lesson that “DINOSAURS COULD HAVE REPRODUCED BY CLONING THEMSELVES!!!”.

Readers’ wildlife photos

December 10, 2022 • 8:15 am

Today’s batch comes from reader Bruce Cohrane, whose photos and captions are indented. Click on them to enlarge them. I’ve added a map of Seneca Lake below, one of New York’s glacial “finger lakes”:

I am fortunate enough to have inherited (along with my late sister) a cottage on Seneca Lake in the Finger Lakes region, built by my grandfather in 1939.  I first went there when I was less than one year old, and I’ve been returning yearly since then (and I’m well into my Medicare years).

First, the setting:

We are on the east side of the lake, so sunsets are often spectacular.

In addition to scenic beauty, the area is increasingly recognized as one of the outstanding wine-producing regions in America. These vines are across the road from our place:

A bit of background on the next two photos. The Seneca Army Depot was built during World War Two, and remained in operation until 2000.  It was a storage and disposal site for explosives  (including nuclear material) and thus was secured tightly.  Contained within its 10,000 acres is a deer (Odocoileus virginianus) population that contains a high frequency of leucistic (white) deer, most likely due to a combination of founder effect and subsequent genetic drift.  Note that these animals are not albinos and actually have brown eyes.

White and brown deer:

Young white deer:

The Erie Canal runs north of the lakes and, while no longer used much for commerce, is a great recreational resource.  In 2010, we rented a house boat and spent three days on the canal, including a swing through Montezuma National Wildlife Refuge, situated in the wetlands north of Cayuga Lake.  The Canal and Refuge are great places for birding.

Great Blue Heron (Ardea herodias):

Osprey (Pandion haliaetus):

Bald Eagle (Haliaeetus leucocephalus):

On our cruise, we had to pass through multiple locks, some of which were home to invasive zebra mussels (Dreissena polymorpha), a scourge of the Great Lakes area:

Returning to our cottage, we often have visitors.

Eastern Cottontail Rabbit (Sylvilagus floridanus):

Common Watersnake (Nerodia sipodon) off our dock:

Finally, when traveling between my Ohio and New York homes, I often drive through the Alleghenies in Western Pennsylvania.  The town of Benezette, in Elk County, is at the heart of the range of a herd of 1400 elk (Cervus canadensis).  While eastern elk were driven to extinction in the 1800’s, they were reintroduced to Pennsylvania (from Yellowstone) in 1912, and despite poaching and farmers’ wrath, have persisted until the present.  Viewing is best in the breeding season (September and October).

Female:

Male: