What Do Humans and Killer Whales Have in Common? Among Other Things, Menopause

​Humans, orcas, and short-finned pilot whales seem like an odd trio, but they actually have something very interesting in common: menopause, the transition from fecundity to age-based infertility in females. Though other an​imals can reach menopause, only these three species produce elder females that live for decades beyond their reproductive years. But what could be driving such a unique adaptation, and why is it limited to humans and two seemingly random whale species?

These questions are tackled in a comprehensiv​e study on killer whales, published today in Current Biology. The authors, led by biologist La​uren Brent of the University of Exeter, found that there is some truth in the "wise old woman" archetype so common across human cultures. According to the study, it's a staple of killer whale culture as well.

"Our results provide the first evidence that a benefit of prolonged life after reproduction is that postreproductive individuals act as repositories of ecological knowledge," the authors wrote.

In short: What elder females lose in fecundity, they gain in experience. The team reached this conclusion by observing 102 killer whales in the Pacific Northwest, as well as studying 35 years worth of data from the Center for Whale Research. They repeatedly observed influential postmenopausal females leading pods to salmon feeding grounds, and even observed an uptick in this behavior when salmon harvests were particularly meagre. These females can live t​o up to 90 years old, about a half-century after menopause.

The advantage of seasoned leaders is self-evident. Indeed, what's bizarre is that postreproductive guidance hasn't caught on in other species. Brent and her colleagues suggest this is due to the unconventional family structures of humans and whales.

"Humans, short-finned pilot whales, and resident killer whales have unusual social systems that may predispose them to menopause and prolonged life after reproduction," Brent told me over email.

"The families of these three species are structured in a very kin-focused way," she continued. "Mothers have very high levels of relatedness to their social group, which increases as they age and new family members are born. This is different from many large mammals, where the dispersal of family members away from the parent group causes relatedness levels to be stable throughout a mother's lifetime."

In other words, elder female humans and killer whales have a lot more genetic investment in their social communities.

"Since the benefits of helping their relatives are amplified in elder humans, short-finned pilot whales, and resident killer whales, it makes sense that they stop reproducing and focus on caring for their families," Brent said. "This is the main underlying commonality–these are highly social species who live with their kin in societies structured in such a way that helping in later life is highly beneficial."

The reason that only females tend to inhabit these leadership roles is because males in all three species tend to have higher mortality earlier in life. "Males probably have the potential to live as long as females," Brent said. "It is not that they have evolved a different maximum lifespan. Instead, what is almost certainly happening is that males face higher mortality risks early in life and thus are much more likely to die sooner than females."

But though males die much earlier, rarely living past 50, elder females place greater investment into leading their sons. Sons, in turn, are more likely to follow their mothers than daughters, and more likely to die if they lose their mothers.

The explanation again has to do with evolutionary payoff. Males are more likely to mate outside the family pod, thus expanding the mother's genetic reach to surrounding communities. Because the extended families don't compete for the same resources as the mother's pod, all of her descendents gain a survival edge. Evolution, once again, proves to be intricately economical.

The team's research is the first to reveal the finer genetic mechanics behind menopause in mammals. But due to the host of new questions it raises, it will not be the last. "The current leadership study is part of a much larger project that looks at the forces driving the evolution of menopause and prolonged life after reproduction," Brent said.

"In addition to the benefits mothers gain from living a long time, there must also be benefits to no longer producing offspring," she added. "So next we are going to focus on exactly how mothers benefit from stopping reproduction."