Image courtesy of NOAA.
Why menopause happens in some animals is a longstanding evolutionary mystery. Many biologists have argued that menopause forces grandmothers to support the offspring of their own children rather than continuing to have kids of their own but number-crunchers have dismissed this "grandmother hypothesis" on the grounds that the magnitude of the benefits granted by grandmotherly support are insufficient to account for the monumental genetic cost of giving up reproduction. In recent years an alternative theory has emerged suggesting that menopause reduces the cost of inter-generational reproductive conflict by preventing weakening older females from becoming pregnant and using up valuable resources that could go to their actively reproducing daughters. It is a nice idea but proving it has been difficult. Now a team is demonstrating that this theory is correct by cleverly making use of an animal that lives for decades after menopause sets in: the orca.
The researchers tapped into a long-term dataset on wild resident killer whales where females frequently live for 20 to 40 years after menopause begins. Using 43 years of orca data they were able to show that when mothers and daughters co-breed, mortality in calves from older generation females was 1.7 times that of calves from younger generation females. They point out that when the cost of this intergenerational effect is combined with the known benefits conveyed to grandchildren by grandmothers, the numbers to do work out and it becomes clear that menopause is a wise biological investment for a long lived species. You can read more in The Economist article that I wrote on this here.