Why Your Grandma is Different to a Chimp

Art: Annie Hamilton

Chimps get grandmas, but they don’t get menopause. I mean, technically, a chimp could live long enough to experience menopause. It’s possible. But that’s a rare thing, even in captivity. On the whole, human women go through menopause and chimps don’t.

And it’s not the chimp that’s weird: it’s us. There are just three animals that naturally experience menopause: human beings, short-finned pilot whales and killer whales. On land, we're it.

In the animal world menopause is really weird. Why is menopause a thing?

Chimp mother & baby. Source: stevehdc

This didn’t used to be a question I asked myself. And it is a big question in biology. Because, on the face of it, menopause wouldn’t seem to fit with how evolution works.

Animals, like us, tend to do things that make it more likely for us to have kids and pass on our DNA. Living to a point where you can’t have kids any more seems, by definition, to be something that makes it harder to pass on your DNA. Not easier.

Why would evolution give us a pass on this?

Five killer whales swim in McMurdo Sound, Ross Sea, Antarctica. Source: NOAA via Wikipedia

We got a theory.

According to the Grandmother Hypothesis, human women get menopause around 45, instead of just dying (the chimp option). Because, without their own kids to look after, our ancestral women could then help their daughters look after their young babies, help the child to eat while mum focused on other things.

Anthropologist Kirsten Hawkes explains it like this:

Kristen in Grandmas on Not What You Think.

As far as human evolution was concerned, this created a virtuous loop: not having to care for older kids, mothers were free to have babies more frequently. Which, in turn, passed on a lot of the grandmothers’ DNA. And that DNA, in turn, often included those bits that made women live on past menopause. Bam! Lots of grandmas.

(You can also study the effect that grandads, sisters, friends and community all have as well. All those other ‘alloparents’. That’s just not the focus of this particular hypothesis.)

The Grandmother Hypothesis was first proposed in a a paper by William Hamilton in 1966 and became a big thing the 80s after Kristen thought she’d observed it happening for real during her field work. Kristen was our guest on Not What You Think this week.

And, although this hypothesis seems to focus on why grandmas are a thing, it also delves into what we get out of having grandmas. It’s not just the occasional Go nanas! feeling and getting the chance to hear embarrassing stories about your folks when they were young. It could also be that grandmas made us smarter, or more social as a species.

So, if you’ve always told people how awesome your grandma is: it’s not just you. Grandmas have always been pretty awesome but this hypothesis gives you some science to argue the point.

Click through to our show page to let Kristen explain it to you.

Not What You Think is broadcast 10:30am Saturdays in September and October, on Sydney’s FBi Radio. Listen live on 94.5 FM or the website, or subscribe to the podcast via our show page.

Kristen was in Sydney to talk at the Sydney Science Festival. You can hear her full public lecture here.

Think that “Go Nanas!” really undersells life as an older woman? We also covered that, last season.