Female Whales That Experience Menopause Live 40 Years Longer
The evolutionary framework developed to explain the evolution of menopause in humans could also apply to toothed whales.
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Toothed whales and humans might not seem to have all that much in common; after all, our last shared ancestor lived ~90 million years ago. But a new study from scientists at the University of Exeter suggests that the pathway leading to menopause in humans might also explain the evolution of menopause in toothed whales.
“I think one of the striking features of this work is the fact that we find this really incredible and rare life history strategy, which we see in human societies, in the ocean, but not elsewhere in modern societies,” said lead study author Professor Darren Croft, a behavioral ecologist at the University of Exeter.
The research is published in Nature.
Menopause is an unusual trait
Half the global population experiences menopause, but in the context of the wider animal kingdom, it’s an unusual trait.
Humans are the only terrestrial mammals that have evolved an extended post-reproductive lifespan; that is, we typically live for a considerable period post-menopause while continuing to play incredibly important roles in society.
Why menopause exists is a question that has both puzzled and fascinated scientists. There are two dominant theories surrounding the evolution of menopause: the live-long hypothesis and the stop-early hypothesis.
The live-long and stop-early hypotheses
The live-long hypothesis argues that menopause evolved by increasing a woman’s total lifespan – without changing the reproductive lifespan – compared to her non-menopausal ancestors. Based on this theory, we would expect species with menopause to have a longer total lifespan, but the same reproductive lifespan, as comparable species that do not experience menopause.
The stop-early hypothesis, in contrast, suggests that menopause evolved via a shortening of the reproductive lifespan, with no change in total lifespan. This would mean that species with menopause have the same total lifespan, but a shorter reproductive lifespan, compared to species without menopause.
Research comparing humans and other primates has lent support to the live-long hypothesis. Women have longer total lifespans than chimpanzees, but the same reproductive lifespan. This suggests that long post-menopausal survival is the outcome of an evolutionary selection process, which might be explained by the “grandmother hypothesis”.
The grandmother hypothesis proposes that women in their post-reproductive years contribute to the survival of future generations by aiding in the care of their grandchildren, ensuring the successful transmission of genes to future generations.
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Given that very few species experience menopause, attempts to explore its evolution haven’t been straightforward. But in 2018, Croft and colleagues discovered evidence for several evolutions of post-reproductive lifespan in toothed whales, including beluga and narwhals.
Their latest study leverages this evidence to explore whether menopause in toothed whales evolved through the live-long or stop-early hypothesis. “What we've been able to do with this work is test the evolutionary framework that has been developed to explain the evolution of menopause in humans,” Croft said.
The grandmother hypothesis and reproductive competition
The researchers created a multispecies database of toothed whale life history. Applying mathematical models, they calculated the total lifespan of 32 species of female toothed whales. Reproductive lifespan was estimated using age-linked ovarian corpora counts from postmortem whales, which was used as an indicator of ovarian activity.
Five species included in the data analysis experience menopause – the narwhal, short-finned pilot whale, false killer whale, killer whale and the beluga whale. These whales lived 40 years longer than their non-menopausal counterparts, on average. Their reproductive lifespan, however, was not shorter than expected, given their size. Collectively, this data aligns with the live-long hypothesis we see in humans.
“We need to try and understand why selection has acted to extend lifespan beyond reproductive lifespan,” said Dr. Samuel Ellis, lecturer in psychology at the University of Exeter, and the study’s first author.
“In the second half of the paper, we dug into this question by comparing patterns of health and harm over the life span. We showed that species with menopause have a much longer time spent alive with their grand offspring, giving them many more opportunities for intergenerational health, but because they have stopped reproducing, they do not have any more reproductive competition with their daughters.”
Competition between mothers and daughters can be costly. The researchers’ data on killer whales found that, when mothers and daughters try to breed at the same time, the calves of the older females have lower survival rates – there’s competition over reproduction.
“It’s absolutely striking that we can draw these comparisons with a group of animals that are so different. What’s similar is the dynamics of the social structure,” Croft concluded.
Reference: Ellis S, Franks D, Nielsen M, et al. The evolution of menopause in toothed whales. Nature. 2024. doi: 10.1038/s41586-024-07159-9
