Why do killer whales go through menopause?

Avoiding mother-daughter conflict is key.

Menopause is an exclusive trait, shared only among humans, killer whales, and short finned pilot whales. According to a new Current Biology study, killer whales go through menopause to avoid reproduction competition with their daughters, allowing them to instead share knowledge to help their family succeed. This explanation also sheds light on why humans share this unique trait with toothed whales.

Darren P. Croft, an animal behaviorist at the University of Exeter, and colleagues used 43 years of demographic data on killer whales to investigate menopause. The evolutionary benefit became clear when they found that in cases where mothers and daughters do have calves at the same time, the calves of the old-generation females are 1.7 times more likely to die than the calves of younger females.

We spoke with Croft to find out more.

ResearchGate: Could you explain the main findings of your study and their significance?

Darren P Croft: Killer whales are one of just three species known to science that go through menopause. Our previous work has shown why female killer whales live such a long time—they increase the survival of their adult offspring by sharing food and knowledge of where to find food. However, this does not explain why females stop reproducing. Our new work provides an explanation as to why females stop reproduction midway through life— because they lose out in competition over reproduction with their daughters.

Kenneth Balcomb, Center for Whale Research.
Kenneth Balcomb, Center for Whale Research.

RG: What other animals go through menopause?

Croft: Only humans, killer whales, and short finned pilot whales are known to go through menopause. It really is remarkable that as humans we share this unusual trait with the toothed whales.

RG: Why is menopause limited to such few species?

Croft: The key to unlocking the puzzle of menopause is in understanding the relatedness structures in social groups. In both the menopausal toothed whales and ancestral humans, older females are more related to their local group than younger females. The opposite pattern is observed in most mammals. This, combined with the opportunity for food sharing, explains why females from these three species are predisposed to evolve menopause.

RG: What do your findings mean for what we know about human menopause?

Croft: We know that grandmother benefits are key in explaining why women live for such a long time after menopause. A long-standing puzzle however is why the reproductive lifespan has not evolved to go along with the total lifespan. Many long-lived mammals continue to reproduce until the end of life. What then has held back the reproductive lifespan of humans?

The reproductive conflict hypothesis has been proposed to explain why human women go through menopause and don't reproduce in late life. This, in combination with the known benefits of grand mothering, can explain the evolution of menopause. However, this hypothesis has proven difficult to test in humans, because modern societies have very different patterns of mortality and fertility to our ancestral populations. The killer whales of the Pacific Northwest provide a unique opportunity to test the generality of the theory and to explain the evolution of menopause in humans. Our new work shows that indeed, as predicted by theory, generational conflict within family groups and the known benefits that old females can provide to their offspring and grand-offspring can explain the evolution of menopause.

This photograph shows a killer whale spyhopping. Emma Foster.
This photograph shows a killer whale spyhopping. Emma Foster.

RG: What are the next steps in this research?

Croft: We want to use drones to look at the behavioral interactions between individuals. How are old and young females competing? For example, who is sharing food with whom? Who is doing the babysitting. We are very excited about this new work because a bird’s eye view on the whales has the potential to transform our understanding of their social behavior.

Articles: Current Biology, Croft et al.: "Reproductive Conflict and the Evolution of Menopause in Killer Whales" http://www.cell.com/current-biology/fulltext/S0960-9822(16)31462-2

Featured image courtesy of David Ellifrit, Center for Whale Research.