Only three mammals in the world are known to go through menopause. Alongside humans, there are short-finned pilot whales and killer whales in something of a motley crew. The uniqueness of this stage of life has puzzled scientists for quite some time. Now, however, the mystery of menopause, at least for the orca, may have been solved.
There are plenty of mammals in which the older females become the leader of the group, such as with elephants, and impart their knowledge to the younger members, often consisting of their offspring. But while matriarchal elephants continue breeding throughout their entire lives, matriarchal orcas stop.
A team of researchers have found that the older female whales may stop reproducing in a bid to reduce competition between her calfs and those of her adult daughters. By looking at over 43 years’ worth of data collected from two populations of killer whales living off the North West coast of Canada, they found that the older mothers experienced a much higher cost than their daughters – to the extent that the mortality of the older mothers' offspring was 1.7 times that of the younger mothers.
“Our previous work shows how old females help but not why they stop reproducing,” says Professor Darren Croft, who led the study published in Current Biology, in a statement. “Females of many species act as leaders in late life but continue to reproduce, but this new research shows that old females go through the menopause because they lose out in reproductive competition with their own daughters.”
So rather than investing in more offspring that are more likely to die, the older females instead cease reproducing and focus on providing for their grandchildren and the pod as a whole. This also links into another curious behavior of the whales, where mothers will still partake in food sharing, giving up salmon they catch to other members of the pod.
The older females are vital for the survival of the pod not only because they feed other members, but because they have vital information that younger whales will not yet know. If there has been a bad year in salmon, for example, the older killer whales may know where to go for a more reliable source.
“Our new work shows that if an old female killer whale reproduces her late-life offspring suffer from being out-competed by her grandchildren,” explains co-author Dr Daniel Franks. “This, together with her investment in helping her grandchildren, can explain the evolution of menopause.”
