Old mothers know best: Killer whale study sheds light on the evolution of menopause

Menopause is one of nature's greatest mysteries. Most animals die around the same time they stop reproducing. Killer whales are one of just three species -- alongside humans and another type of whale -- where females continue to live for many years after giving birth to their last baby. Female killer whales generally breed between the ages of 12-40, but can survive into their 90s. Previous studies have shown that menopausal females greatly increase their childrens' and grandchildrens' chances of survival, but how old females help their relatives to survive remained a mystery.

One leading idea is that wisdom accumulates with age and that old females store vital information about the environment which they share with their relatives to help them during environmental hardships. The research team, from the Universities of Exeter and York (UK) and the Center for Whale Research (USA) tested this idea by studying leadership in the southern resident killer whale (Orcinus orca) in the North Pacific ocean, off the coasts of the USA and Canada.

The study, funded by NERC and published today in the journal Current Biology, found that post-reproductive female killer whales act as 'repositories of ecological knowledge', leading groups when they are moving together in salmon foraging grounds. Critically, the researchers discovered that leadership by menopausal females is especially prominent in difficult years when there are fewer salmon. Shortage of salmon is a major contributing factor to mortality in this population and so the benefits of older females knowing when and where to find salmon could be considerable.

Lauren Brent of the University of Exeter said: "Our results show for the first time that one way post reproductive females may boost the survival of their kin is through the transfer of ecological knowledge. The value gained from the wisdom of elders can help explain why female killer whales and humans continue to live long after they have stopped reproducing."

As humans did not develop writing for almost the entirety of our evolution, information was necessarily stored in the minds of individuals. The oldest and most experienced people were those most likely to know where and when to find food, particularly during dangerous conditions such as drought.

The theory that menopausal women store ecological knowledge is difficult to test in modern human populations, but as non-literate and highly social animals, killer whales can provide insights into how menopause evolved in humans.

"In humans, it has been suggested that menopause is simply an artefact of modern medicine and improved living conditions," said Professor Darren Croft of the University of Exeter. "However, mounting evidence suggests that menopause in humans is adaptive. In hunter-gatherers, one way that menopausal women help their relatives, and thus increase the transmission of their own genes, is by sharing food. Menopausal women may have also shared another key commodity -- information."

The researchers also found that females are more likely to lead their sons compared to their daughters. Dr Daniel Franks of the University of York explained: "Killer whale mothers direct more help toward sons than daughters because sons offer greater potential benefits for her to pass on her genes. Sons have higher reproductive potential and they mate outside the group, thus their offspring are born into another group and do not compete for resources within the mother's matriline. Consistent with this, we find that males follow their mothers more closely than daughters."

The team used information collected over the last 35 years and made observations on a total of 102 individual killer whales. The unique dataset includes birth and death dates as well as more complex data, like the genetic and social relationships between the different animals.