Written By: Claudia Kowal

Introduction 

For most species, the only goal in life is to spread their genes to future generations, reproducing until death. However, females of menopausal species live almost half of their lifetime without the ability to reproduce, taking on a purpose other than reproductive fitness. For example, the average human female undergoes menopause in their mid 40’s, but has a lifespan of around 79 years. Very few species experience menopause, leading scientists to question its evolutionary benefits (Zimmer, 2024). 

To answer this question, scientists at the Department of Physiology in University of Exeter, Exeter, U.K. turned to the group dynamics of whales, the only species (aside from humans) whose females experience menopause. Specifically, these researchers studied the suborder Odontoceti, or toothed whales, in which menopause has independently evolved several times across different whale species. By studying these group dynamics and interactions between individuals in toothed whale populations, researchers were able to better understand the purpose of menopause in whales, and then apply these findings to other species, like humans (Ellis et al., 2024). 

Figure 1

A beluga and her calf, members of the toothed whale suborder.

Source: ZooBorns

The Study

To conduct the study, the researchers took lifespan and reproductive span data from 18 whale species, 4 of which experience menopause. They estimated that species with menopause, on average, do not have a shorter reproductive span than those without menopause. Instead, species with menopause have adapted to have a longer lifespan in comparison to those without. 

To make sense of this data, the researchers created a kinship model, which predicts the potential for intergenerational help in whales. In other words, the model indicates how whales help others in their family, providing one potential explanation for menopause. The model uses factors like the number of years an adult female is alive at the same time as her offspring- the number of years that adult whales could potentially help their offspring by nurturing, foraging, and hunting. The model also considers the number of years of reproductive overlap between adult females and their offspring, indicating potential competition for mates (Ellis et al., 2024). 

Using the model, the researchers found that whales that experience menopause are likely to see at least one of their grand-offspring reach maturity. On the other hand, whales that do not experience menopause, on average, see at least one of their grand-offspring reach only 36% maturity. These results indicate that whales who experience menopause have more potential for intergenerational help compared to those who do not (Ellis et al., 2024). It also explains why some menopausal whales, like orcas, are nurtured or fed by their parents well into adulthood. While this may seem costly to the mother, it also provides some benefits. Though she is sacrificing her reproductive years for her current offspring, it does help her offspring succeed in life and, hopefully, reproductive fitness. This ensures that her genes are passed on either way (Pennisi, 2012). 

Despite having a longer lifespan, the results of the study indicated that whales who experience menopause are not likely to have a longer reproductive overlap than those who do not experience menopause. Researchers believe that menopausal whales have adapted to separate reproduction spans in different generations, to reduce competition for mates (Ellis et al., 2024). This is yet another way that menopausal whale species contribute to intergenerational help. 

Figure 2

The ‘Grandmother Effect’ demonstrates how grandmother menopausal whales are able to support their grand offspring into maturity.

Source: Science in the News

Conclusion

According to the results of the study, the evolutionary purpose of menopause could be seen almost as a sort of genetic altruism. This means that individuals are helping others, even though it may be costly to themselves. While menopausal whales are supporting their offspring to help pass on their own genes, they are ultimately helping their family members thrive. In this case, altruism is not a learned behavior but an innate one, built into their DNA and passed down by their ancestors.

Though the study focused on whales, the results could also extend to other menopausal species, like chimpanzees or humans. Both of these species have been known to perform altruistic behaviors, but the possibility of these behaviors being innate has not been commonly explored; this could be an interesting endeavor for scientists in the future. 

References and Sources

Ellis, S., Franks, D. W., Lybkær, M., Weiss, M. N., & Croft, D. P. (2024). The evolution of menopause in toothed whales. Nature. https://doi.org/10.1038/s41586-024-07159-9

National Aquarium. (2020, February 21). Are Animals Altruistic? National Aquarium. https://aqua.org/stories/02-21-2020-are-animals-altruistic#:~:text=Evolutionary%20biologists%20determined%20that%20an

Pennisi, E. (2012, September 13). Adult Killer Whales Need Their Mamas. Www.science.org. https://www.science.org/content/article/adult-killer-whales-need-their-mamas

Zimmer, C. (2024, March 12). Why Do Whales Go Through Menopause? The New York Times; The New York Times. 

https://www.nytimes.com/2024/03/13/science/whale-menopause.html