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April Flowers for redOrbit.com – Your Universe Online
Killer whale females have the longest period of menopause of any non-human species. A new study, led by the University of Exeter and the University of York, claims this is so the female Orcas can care for their adult sons. 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 own death within a year.
Very few species have a prolonged period of their lifespan when they no longer reproduce, and the biological reason for menopause remains one of nature’s great mysteries. The female killer whale, however, is unusual in this respect. She stops reproducing when she is 30 to 40 years old but can survive well into her 90’s.
Several theories explaining the evolution of menopause in humans have been proposed, but so far there are no definitive answers as to why females of a small number of other species, including killer whales, also stop reproducing partway through their lives.
The research team, which also included scientists from the Center for Whale Research and Pacific Biological Station 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.
What they found was that the presence of a female who was no longer reproducing significantly increased the survival rate of her older offspring. The male offspring were not the only ones to benefit, however. Females also commonly stay within their mother´s group, but their survival ratio only increases by three-fold. For females under 30, the death of their mother had no measurable 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. With this close association, older mothers have the opportunity to increase the chance that their genes will be passed on by helping their adult offspring survive and reproduce. When sons mate, their offspring are cared for by females in another group, whereas the offspring of daughters stay in the group, thus increasing competition for resources within the group.
“Killer whales are extraordinary animals and their social groups are really unusual in that mothers and their sons are lifelong companions,” explained University of Exeter PhD student Emma Foster. “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.”
Theories predict that in order to have the best chance of spreading their genes, without carrying any additional burden, mothers should focus their efforts on their sons, and the new research that appears in the journal Science seems to back up this theory by demonstrating the extent to which older sons are dependent on their mothers for survival.
Dr Dan Franks from 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.”
Dr 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.”