Whale grandmas

March 19, 2024 4:06 AM

Menopause has evolved only once in terrestrial animals - in humans - but at least four times in toothed whales. in a paper published last week in Nature, The evolution of menopause in toothed whales, a team of researchers examined whale life histories to evaluate five different hypotheses for menopause: Live-long vs. stop-short, and the grandmother hypothesis (previously) vs. reduced reproductive conflict vs. extended male lifespan. University summary, NYT summary, NYT archive.

The outcome:

The ordinary maximum lifespan (age at which 90% of adult life years have been lived) of whales with menopause is predicted to be 40 ± 5 yr (mean ± s.d.) longer than the same-sized species without menopause. In contrast, species with menopause do not have a shorter reproductive lifespan than expected given their size (Fig. 1c; proportion of posterior less than zero is 0.20). These results are consistent with the predictions of the live-long hypothesis but not the stop-early hypothesis.

...

The results presented here provide support for a synthesis of different theories for the evolution of menopause. Our comparative analyses of toothed whales are consistent with a key role of grandmother benefits in the evolution of menopause, as predicted by the grandmother hypothesis. But unlike the expectations of the grandmother and mother hypotheses, we find no evidence that the benefits are necessary because offspring are particularly costly to raise to maturity in species with menopause or result in a faster baseline rate of reproduction in species with menopause. Rather, and consistent with the predictions of the reproductive conflict hypothesis, we find that species with menopause do not have increased reproductive overlap with their offspring, despite their extended lifespan. More generally, we find support for the hypothesis that menopause allows females to maintain separate reproductive generations while increasing overall generational overlap. In agreement with some other recent studies in humans, we find no evidence for the male-driven menopause hypothesis: in toothed whales the evolution of menopause seems to be driven by selection on female life history.

The outcome in table form.

posted by clawsoon (9 comments total)

The results presented here provide support for a synthesis of different theories for the evolution of menopause.

I love when it works out like this and most of the offered hypotheses have some partial truth or explanatory power.

Long live the grandmother hypothesis!
posted by SaltySalticid at 5:40 AM on March 19

It's making me wonder about species like meerkats and naked mole rats, where there's intergenerational reproductive conflict between the dominant breeding female and her daughters. (Only partially successful in the case of meerkats; fully successful in the case of naked mole rats.) In the very long term, might one of them become the second terrestrial mammal with menopause?
posted by clawsoon at 7:59 AM on March 19

Hmm. Maybe? Idk how universally successful the mole rats are for that matter. Bumblebees also have a lot of reproductive conflict, the queen basically beats her daughters into a non-reproductive state, and eats any eggs they may manage to lay. But they have maybe other caps that would prevent insect menopause from developing.
posted by SaltySalticid at 8:09 AM on March 19 [1 favorite]

Steven Austad has a chapter on whales in his Methuselah's Zoo.
posted by neuron at 11:08 AM on March 19

Please break the last hypothesis in the chart down for me. I've read the hypothesis and prediction about 10 times and I'm really struggling with the logic flow.

[this is very cool, btw]
posted by es_de_bah at 1:50 PM on March 19

es_de_bah: Please break the last hypothesis in the chart down for me. I've read the hypothesis and prediction about 10 times and I'm really struggling with the logic flow.

The first paper they reference in the paragraph that talks about it, The Patriarch Hypothesis: An Alternative Explanation of Menopause, states it most clearly:

The patriarch hypothesis proposes that once males became capable of maintaining high status and reproductive access beyond their peak physical condition, selection favored the extension of maximum life span in males. Because the relevant genes were not on the Y chromosome, life span increased in females as well. However, the female reproductive span was constrained by the depletion of viable oocytes, which resulted in menopause.

In other words: There's a selective advantage for high-status men who can live a long time and keep making babies. If an 80-year-old guy is still fathering children, he's going to leave more offspring behind. So mutations which cause men to live longer will be selected for.

However... men will also pass most of their long-life mutations on to their daughters, since we inherit DNA from both of our parents. Maybe one or two long-life mutations will only be on the Y chromosome and therefore only be passed on to sons, but most of the long-life mutations will be spread out all over the genome and passed on to both sons and daughters.

As a result, women will end up with long-life mutations not because it's good for them, but because it was good for their father and they just happened to inherit those genes from their father.

(And of course we should mention that hypothesis was not supported by the whale data.)

Does that make sense?
posted by clawsoon at 6:26 PM on March 19 [2 favorites]

thanks clawsoon! I got the hypothesis alright. I don't understand how the prediction was unsupported, necessarily.

(mindyou, I have a sick hobby of arguing with Redpill guys about this type of thing on reddit. I know it's not good for me. But, understanding these arguments would be suuuuper useful. So I appreciate anyone taking the time to dumb it down for me so I can dumb it down for angry 16-26 year olds)
posted by es_de_bah at 4:48 AM on March 20

es_de_bah, it looks like everything they have to say about it is in the paragraph Female lifespans and male longevity:

Were female menopause an artefact of selection on male lifespan we would expect that (1) males would live longer in species with menopause than in species without menopause and (2) in species with menopause males will have a higher probability of reaching, and live longer beyond, the age of female reproductive cessation than females (Table 1). We find no evidence to support the male-driven menopause hypothesis in toothed whales...

In other words, males in species with menopause have shorter lifespans relative to females than in species without menopause and in species with menopause males are less likely to reach the age of female reproductive cessation than females and live less long from that age if they do reach that age.

As I'm reading that - and anybody feel free to correct me if I'm wrong - if male longevity is "pulling" female longevity forward into menopause, then males in those species should live longer compared to females than males in species without menopause do, and they don't.

Although that appears(?) to be contradicted by this sentence:

In contrast to the expectations of the male-driven menopause hypothesis, in toothed whales we find that (1) the ratio of female to male lifespan is lower in species with menopause than species without menopause...

I've written the lead author to ask. We'll see if they answer...
posted by clawsoon at 5:38 AM on March 20 [1 favorite]

definitely shoot me a message if you get any idea. If it takes long enough, I might actually make it through the reading
posted by es_de_bah at 5:22 PM on March 21