What do middle-aged women and whales have in common? Perhaps the unexpected answer is … menopause. Although they don’t have to suffer through hot flashes and mood swings, several whale species stop getting pregnant midway through life. So-called post-reproductive lifespans have puzzled biologists for years. After all, evolutionary theory taught us that the whole purpose of existing is to pass on genes. So, scientists are now wondering: what is the meaning of life if one can no longer have babies? Let’s find out why whales survive after menopause.
Why do whales go through menopause?
Interestingly, whales like orcas, false killer whales, short-finned pilot whales, belugas, narwhals, and humans are the only known mammals to live long past their fertile age. Why these species? Part of the explanation may relate to their social structure; many whales live in female-led groups where offspring stay with their mothers for life.
Hypothesis 1: The Attentive Mother
Matrilineal society provides an excellent platform for motherly care. The “attentive mother” hypothesis suggests that older mothers benefit from investing time and energy in existing sons and daughters, rather than making new babies. In support of this, Foster et al. (2012) found that orcas are less likely to survive when their mother is absent. Looks like killer whale kids are true mommy’s boys and girls.
Hypothesis 2: The Caring Grandmother
The “caring grandmother” hypothesis also highlights the benefits of stopping direct reproduction in exchange for improved survival and reproduction of relatives. By caring for their grand-offspring, old females ensure that genes are passed on to future generations. Grandmothers can 1) increase the survival chances of their grandsons and daughters and 2) relieve daughters of the struggles of single-motherhood.
Hypothesis 3: Repository of Knowledge
“The older, the wiser” perfectly captures the idea behind the “repository of knowledge” hypothesis. Being a whale isn’t easy – many species rely on scarce resources and need to learn complicated hunting strategies to survive. Therefore, having an old leader to transfer key knowledge and skills is a huge advantage. For example, Brent et al. (2015) showed that orcas led by old females were more likely to find salmon than those with younger leaders.
Hypothesis 4: Avoid Family Conflict
Pregnancies do not come lightly (literally), and weaning mothers require 42% more resources than normal. Eating for two means that less food is available for the rest of the group. As a result, there is a trade-off between personal fitness and group survival. As older females are likely to have many relatives in a group, this trade-off becomes costlier with age. Consequently, older females try to compete less, often at the expense of their babies’ survival chances. Indeed, Croft et al. (2017) found that offspring of old orca mothers have lower survival than those raised by younger mothers (check out the graph below). When you combine this with the higher pregnancy risk that comes with age, not having kids at old age makes a lot of sense.
So which hypothesis is true?
We are still unsure about why post-reproductive lifespans evolved in the five whale species. Probably, the matrilineal structure is somehow connected to the answer. The opportunity for multigenerational care, but also the costs of family conflict makes menopause useful in a society where mothers stay with their offspring for life.
Unfortunately, we know little about post-reproductive life spans for the majority of whale species. We have evidence that female orcas, false killer whales, and short-finned pilot whales spend a long time being reproductively inactive. Only recently did we find out that belugas and narwhals are also part of the grandmother-club. Which other species are out there living post-reproductive lives? Why did this illogical life strategy evolve? What else don’t we know about these whales? A lifetime of study may still be necessary before we unravel these questions. Hopefully, with more research and ever-expanding databases, we will get to know more and more about these mysterious menopausal whales.
If you want to read more about whales menopause, check out:
- Brent, L.J., Franks, D.W., Foster, E.A., Balcomb, K.C., Cant, M.A. and Croft, D.P., 2015. Ecological knowledge, leadership, and the evolution of menopause in killer whales. Current Biology, 25(6), pp.746-750.
- Croft, D.P., Johnstone, R.A., Ellis, S., Nattrass, S., Franks, D.W., Brent, L.J., Mazzi, S., Balcomb, K.C., Ford, J.K. and Cant, M.A., 2017. Reproductive conflict and the evolution of menopause in killer whales. Current Biology, 27(2), pp.298-304.
- Foster, E.A., Franks, D.W., Mazzi, S., Darden, S.K., Balcomb, K.C., Ford, J.K. and Croft, D.P., 2012. Adaptive prolonged postreproductive life span in killer whales. Science, 337(6100), pp.1313-1313.
- Foote, A.D., 2008. Mortality rate acceleration and post-reproductive lifespan in matrilineal whale species. Biology letters, 4(2), pp.189-191.
- Johnstone, R.A. and Cant, M.A., 2010. The evolution of menopause in cetaceans and humans: the role of demography. Proceedings of the Royal Society B: Biological Sciences, 277(1701), pp.3765-3771.
Make sure to read our other posts on Southern Resident killer whales here.
Eline van Aalderink is a recent MSc Marine Biology graduate from the University of Groningen (the Netherlands), where she specialised in marine mammal ecology and conservation biology. She is currently working as a marine mammal research assistant/supervisor at Archipelagos Institute of Marine Conservation in Greece.