And we know that many people live for decades after they go through menopause.

But for a long time, researchers have wondered why that is.

If the whole goal of any organism is to pass along its genes, why live so long after you can no longer reproduce?

One of the hypotheses those researchers came up with is called the grandmother hypothesis,

which says that the reason that human females live so long is to fill the social role of a grandmother, caring for her grandchildren when she's not able to have any more children of her own.

There's just one tiny problem with this hypothesis.

It turns out humans aren't the only species that experiences menopause.

Not even close.

The idea that grandmothers can help improve survival rates of their grandchildren has been around since the 1980s.

Grandmothers can take on some of the burden of acquiring food for nursing mothers or dependent children, or provide care for those children while mothers are out foraging.

They can also help teach children the complexities of culture and be repositories of generational knowledge.

This version of the grandmother hypothesis was developed after anthropological field work with the Hadza people of northern Tanzania.

In this hunter-gatherer group, postmenopausal women spent more time collecting food than women of reproductive age did.

Which makes sense.

If you don't have kids to take care of, you have more time on your hands.

These Hadza grandmothers also gathered more tubers in addition to the berries that everybody would gather.

Those tubers were harder to harvest, so the researchers hypothesized that older women without children to take care of could devote more energy to that task than younger women could.

And that meant there'd be more food for these womens' grandchildren, so it would help them survive.

In other words, this grandmother hypothesis suggests that being a grandmother and helping to take care of your adult offspring and their offspring is a better way of ensuring your genes get passed on than continuing to reproduce yourself.

Hence, menopause.

But why are we just talking about grandmothers?

What about my papa?

Human males can retain their fertility for their whole lives, which would suggest that whatever evolutionary pressures led to grandmothering, didn't apply to grandfathering.

One potential explanation for this is paternal uncertainty.

Since the only way to be 100% sure that an offspring is yours is to give birth to it, that means the males of a species might not know for sure how many offspring they actually have.

Which means that it's in their best interest, evolutionarily speaking to keep being able to have more offspring.

Now, importantly, the data that led to the formation of the grandmother hypothesis was collected in 1989,

and a lot of the research trying to support or disprove the hypothesis has also been done in modern humans.

The evidence for grandmothers actually improving reproductive outcomes in modern human societies is mixed, but that's not a big deal, right?

After all, the evolutionary pressures acting on humans now are way different than they would have been when menopause and grandmothering first developed.

So researchers were able to sort of hand wave away the fuzziness of those results, for a while.

And while researchers have noted that there were a few exceptions to the grandmother hypothesis, they tended to be ones that prove the rule.

Take killer whales.

Killer whales can live more than 20 years after they're no longer able to reproduce, and also just so happen to live in matrilineal pods, with daughters and granddaughters staying in the same pod they were born in.

That means that these grandmothers are using their decades of knowledge to benefit their whole extended family tree, since their grandchildren live right alongside them.

These wise grandmas know the best places to scope out food, how to avoid predators, and might even know some fun games to play, like smashing boat rudders–

check out our video on the orca uprising for more on that story.

But for killer whales and humans alike, grandmothering is described as the key to survival, and sets us apart from other species who don't have these matrilineal practices, and are fertile their whole lives.

There's just one problem.

It turns out, there are more animals besides just killer whales whose females stop being fertile in their old age.

A lot more.

Oops.

Now, it's true that other mammals don't experience menopause in the way humans do, because menopause for us specifically refers to when menstruation ends, and most other species don't menstruate in the first place.

But those other animals do undergo reproductive senescence, or a decline in fertility with aging.

And this is where things get tricky.

Because it's really hard to measure menopause or ovulation in species that don't visibly menstruate.

So scientists have traditionally used demographic data based on what we know about a species's age-associated fertility and survival rates.

But this kind of data is flawed, in part because there are a lot of uncertainties around species' potential lifespans in the wild.

A lot of animals die far before their max possible lifespan, because the natural world is dangerous and scary.

So if we don't know exactly when these animals stop ovulating, and we don't necessarily know how long they can live, we can't actually say that females of most species die while still being fertile.

That's why newer research is turning to the physiological signs of ovulation as a better metric for whether or not a female can still have offspring.

Researchers can check if the ovaries have recently released an egg by measuring hormone concentrations in urine, or by looking at the different types of cells that are in an animal's vagina.

While these methods are more reliable than demographics, they can also be pretty… invasive.

And they work better when studying animals in captivity than animals in the wild.

That said, there are ways to study physiological changes in wild animals.

You can still get urine samples, for instance, as long as you're quick enough.

So as our analytical methods have gotten better, researchers wanted to take a look at our closest relatives, chimpanzees.

And it turns out that we're not so special after all, because research indicates that chimpanzees stop ovulating too.

A longitudinal study in the Ngogo chimpanzee community in Uganda used both demographic and physiological data and found that none of the chimpanzees gave birth after the age of fifty.

And their hormone changes looked a lot like what happens when humans go through menopause.

But those females kept on living, spending a full fifth of their lifespan unable to have more offspring.

And there is no way that the grandmother hypothesis could explain any of this.

For one thing, chimpanzee daughters leave the groups that they were born in when they reach adulthood, so older females don't live in the same groups as the offspring they're totally sure they're related to.

And while they do continue to live with their sons, remember, there's that whole parental uncertainty thing.

Grandma might not know for certain which babies in the group are really her descendants.

And anyway, older chimps don't really babysit for anyone in their groups, even though this isn't uncommon in other primate species.

Chimpanzees are just kind of jerks.

But that's just the chimps.

What if menopause is still really rare in the natural world?

Surely that would mean our grandmas are special!

Well, no.

It turns out that there are a lot more species that have some form of reproductive senescence, too.

In studies that using physiological and demographic data, it turns out that basically all mammals experience life after ovulation.

One study compiled the results of a whole bunch of these physiological tests and demographic surveys, and estimated that across mammalian species, females spend 28% or more of their lifespan physically unable to reproduce.

And while some had complex social structures, they didn't align with any observations of a particular importance on a grandmother in helping to care for young.

So once again, we see an end in fertility, but no significant relationship with grandmas.

Which means that grandmothering can't be the evolutionary reason behind menopause.

Or at least, it can't be the only reason.

The authors of the chimp study suggest that reproductive senescence might be related to competition with younger females for opportunities to reproduce.

But there's another, even simpler hypothesis around menopause that might explain a lot of this, called the extended mothering hypothesis.

The extended mothering hypothesis basically says that as a female gets older, it becomes a better strategy to invest energy into the current offspring she's raising, rather than having more new ones.

Pregnancy takes a ton of energy, which could be spent rearing the current crop of offspring instead of growing more.

Plus, dying before all of your children are able to take care of themselves is not great for the survival rates of any offspring who haven't reached independence.

Since baby mammals survive on milk, they're totally dependent on their mothers for at least a little while after birth, and have next to zero odds of survival without her around.

So being able to reproduce for the entirety of your lifespan would require a huge resource investment, which would all become moot if you die before your kids are all grown up.

This hypothesis fits with reproductive senescence across all these other species too, not just ours.

Of course, it's also possible that there's no evolutionary reason for menopause at all.

It could be that ovulation stops because our biological systems just have a tendency to wear down with age, just like how we don't die when our hair turns gray or our skin gets wrinkly.

So it turns out that this decades-old hypothesis was built on a house of cards, and it may have come crashing down.

We now know that females no longer being able to reproduce in their twilight years is super common, thanks to these new techniques and studies.

But hey, just because natural selection might not have made all of our grandmas for some special reason, that doesn't make your grandma any less special.

If you liked this video, you might want to listen to a few of us at SciShow ramble about some other science topics that tickle our fancy.

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