LONDON, Sept 13 (Reuters) - Scientists have found the answer
to why female killer whales have the longest menopause of any
non-human species - so that they stick around long enough to
care for their grown-up sons.
In a study published in the journal Science, researchers
found that for a male whale over 30, the death of his mother
means an almost 14-fold increase in the risk he will also die
within the following year.
Yet males whose mothers live well beyond their reproductive
years are more likely also to live to older age, they found.
"Male killer whales are pretty much mummy's boys and
struggle to survive without their mother's help," said Dan
Franks of the University of York, who worked on the study.
The reason for the menopause remains one of nature's great
mysteries. Very few species have a prolonged period of their
lifespan when they no longer reproduce, as humans do.
But women can look to female killer whales as kindred
spirits. They stop reproducing in their 30s and 40s, but can
survive into their 90s.
"Killer whales are extraordinary animals and their social
groups are really unusual in that mothers and their sons are
lifelong companions," said Emma Foster, a PhD student at
Britain's University of Exeter who led the study.
"Our research suggests they have developed the longest
menopause of any non-human species so they can offer this level
of commitment to their older offspring."
Foster's team, which included scientists at the United
States Center for Whale Research and Pacific Biological Station
in Canada, analysed 36 years of records on the members of two
populations of killer whales in the North Pacific ocean, off the
U.S. and Canadian coasts.
They found that the presence of a mother killer whale who
was not reproducing significantly increased her offspring's
survival.
For males over the age of 30, a mother's death meant a
14-fold increase in the likelihood of their death within a year.
But for daughters of the same age, the difference was just under
three-fold. For female killer whales under the age of 30, their
mothers death had no effect on survival rates.
Foster said the study suggests that female killer whales who
stop having offspring but increase the survival of their sons
can maximize the transmission of their genes without increasing
competition within their own group, as would be the case when a
daughter reproduces.
The researchers said they had little data on the specific
ways mothers may help their adult sons survive longer, but
aiding them in the hunt for food and helping them out in
dangerous situations may be two possible explanations.
(Editing by Paul Casciato)