The reason for the long menopause in these whales remains one of nature's great mysteries. Very few species have a prolonged period of their lifespan when they no longer reproduce, like humans do. However, female killer whales also stop reproducing in their 30s or 40s but can survive into their 90s. While different theories have been put forward for the evolution of menopause in humans, including the well-established 'grandmother' hypothesis, there has been no definitive answer to why females of a small number of other species, including killer whales, stop reproducing part-way through their lives.
Researchers analyzed records spanning 36 years of the members of two populations of killer whales (Orcinus orca) in the North Pacific ocean, off the coast of the USA and Canada.
They found the presence of a female who was no longer reproducing significantly increased her older offspring's survival. In the case of males over the age of 30, a mother's death meant a 14-fold increase in the likelihood of their death within a year. Females also stay within their mother's group but for daughters of the same age, the difference is just under three-fold. For females under the age of 30, the death of their mothers had no effect on their survival rates.
The research team included Dr. Dan Franks from York's Departments of Biology and Computer Science, and Dr Sonia Mazzi from the Department of Mathematics, as well as scientists from the University of Exeter, the Center for Whale Research in the USA and the Pacific Biological Station in Canada. Franks said, "Our analysis shows that male killer whales are pretty much mommy's boys and struggle to survive without their mother's help, even into adulthood. In humans, it is thought that menopause evolved so that women can focus on helping to nurture their grandchildren. In killer whales, however, females appear to be important lifelong carers for their adult sons. The need for mothers to care for their sons into adulthood explains why killer whales have evolved the longest post-reproductive lifespan of any non-human animal."
Killer whales live in unusual social groups, with sons and daughters staying with their mothers in a single group throughout their lives. With this close association, older mothers have the opportunity to increase the transmission of their genes by helping their adult offspring survive and reproduce.
When sons mate, their offspring are cared for by females in another group, whereas when daughters reproduce the offspring stay in the group, which increases local competition for resources within the group. Theory predicts that in order to have the best chance of spreading their genes, without carrying an additional burden, mothers should focus their efforts on their sons. This research backs up the theory and demonstrates the extent to which older sons are dependent on their mothers for survival.
Lead author on the paper, University of Exeter PhD student Emma Foster, said: "Killer whales are extraordinary animals and their social groups are really unusual in that mothers and their sons are lifelong companions. Our research suggests that they have developed the longest menopause of any non-human species so that they can offer this level of commitment to their older offspring."
Published in Science.