What do whales and females have in common?
WHAT DO PILOT WHALES, orcas and women have in common? They represent the only species in which the females go through a menopause and live long after they’ve finished breeding. Now two researchers in the UK think they’ve finally discovered why.
There is already good evidence that having grandmothers around boosts the survival of their ‘grand-offspring’, which is one evolutionary explanation for the role of post-menopausal women. But biologists have puzzled over why – for people, pilot whales and killer whales – females lose the ability to reproduce decades before they die.
Other long-lived animals keep producing offspring throughout their lives – female elephants do it into their 60s, for example, and baleen whales can give birth into their 90s.
Michael Cant at the University of Exeter and Rufus Johnstone at the University of Cambridge studied the social structures of great apes, whales and other long-lived species for clues.
They found that even though the social structures of great apes and pilot and killer whales are very different, the relationships within families are the same. “Females get more closely related to other group members as they get older,” Michael says. This just isn’t the case for other long-lived, socially minded animals, like elephants or other whales.
Female bonding
In societies of great apes such as chimps, bonobos and gorillas, females leave their family groups to mate – and the researchers think this probably also happened in ancestral human societies.
In this case, females start their breeding lives in families comprising completely unrelated members. “But as they get older and their sons start to breed, their relatedness to local offspring will increase,” says Michael.
Contrary to apes, female whales start their breeding lives in groups that include their sons but not their fathers. But the overall effect is the same: “Average relatedness to other group members increases over their lifespan,” Michael says.
Human nature
The social structure of great apes suggests menopause would be evident, but humans seem to be the only ones to have evolved this trait. This is because other factors are also involved, the scientists say.
One crucial factor is that humans are different to other great apes in that they help to raise young that are not their own. In all other animals that do this, females compete to be the sole breeder in the family.
By stopping breeding in middle age, older women don’t compete with their daughters-in-law. “We believe that together, the costs of reproductive competition and the benefits of helping can explain why women stop breeding so early and live so long afterwards,” says Michael.
More work is now needed, he says, to find out whether grandmother killer and pilot whales also help raise their grand-offspring.
“This is an extremely important model,” says Andy Russell at the University of Sheffield, UK, who wasn’t involved in the work. “There will always be debate on the evolution of the menopause but Cant and Johnstone provide a new hypothesis with testable predictions, and this should be commended.”
The findings are reported in the journal Proceedings of the Royal Society B.
