Early killer whale menopause

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Picture of killer whales.
Picture of killer whales.
Killer whales near the San Juan Islands, USA and British Columbia. Both male and female killer whales remain with their mothers throughout their lives. Adult male killer whales are easily distinguishable from females by their considerably larger dorsal fins. Images: David Ellifrit Centre for Whale Research

Sept. 27, 2012 — Scientists believe they have found out why female killer whales have the longest menopause of any non-human mammal — to care for their adult sons. Led by scientists at the universities of Exeter and York and published in the journal Science (Sept. 14, 2012) the research shows that, for a male over 30, the death of his mother means an almost 14-fold-increase in the likelihood of his death within the following year.

The reason for menopause has been one of nature's mysteries. Most animals do not have a prolonged period during their lifespans when they no longer reproduce, as do humans. However, female killer whales 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 there has been no definitive answer as to why in certain other mammals, including killer whales, females stop reproducing part way through their lives.

The research team, from the Universities of Exeter and York (UK), the Center for Whale Research (USA) and Pacific Biological Station (Canada) analysed 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 that 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.

Killer whales live in unusual social groups, with sons and daughters staying with their mothers in a single group throughout their lives. The researcher say that with this close association, older mothers have the opportunity to increase the transmission of their genes by helping their adult offspring survive and reproduce. They also claim that 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. The findings in their report tend to back up this idea.

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."

Dan Franks, of the Department of Biology at the University of York, said: "Our analysis shows that male killer whales are pretty much mommy's boys and struggle to survive without their mother's help. 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."

Darren Croft of the University of Exeter added: "Both humans and killer whales are unusual in having a long menopause. Although they share this trait, the way older females benefit from ceasing reproduction differs, reflecting the different structure of human and killer whale societies. While it is believed that the menopause evolved in humans partly to allow women to focus on providing support for their grandchildren, it seems that female killer whales act as lifelong carers for their own offspring, particularly their adult sons. It is just incredible that these sons stick by their mothers' sides their entire lives."

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Adapted from materials obtained from the AAAS

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