Killer whales live on after menopause to protect sons
Killer whale mothers live longer lives in order to protect their sons, a study has found.
Females give birth in their thirties but can live for a further 50 years after having their offspring.
Scientists from the University of Exeter used long-term records to identify possible reasons for this long non-reproductive phase of life.
They found that the presence of mothers ensured greater survival of adult sons to breeding age.
The findings are published in the journal Science.
"Prolonged life after menopause remains one of nature's great mysteries," said Dr Darren Croft from the University of Exeter who led the study.
Killer whales, also known as orca, are of particular interest because, after humans, they have one of the longest post-reproductive life spans in the natural world.
Most animals must survive on their own as an adult but in a small number of species, including chimpanzees and elephants, females continue to care for their adult sons.
In killer whale society, the young never leave their mothers, remaining in a single group.
"With this close association, older mothers have the opportunity to increase the transmission of their genes by helping their adult offspring survive and reproduce," said Dr Croft.
Researchers theorised that living longer in order to protect their sons could represent the wisest investment for orca mothers.
When males mate their offspring live in a different group whereas daughters' offspring stay with the group, limiting the transmission of genes and increasing competition for resources.
According to this theory, Dr Croft said that mother's should "focus their efforts on sons" for the best chance of spreading their genes with little additional burden.
To test their prediction, the team, including colleagues from the University of York, analysed 36 years of killer whale records to identify patterns of births and deaths among over 500 animals in US and Canadian waters.
Using this data, they were able to calculate the probability of survival of any individual orca at any age.
They then compared the survival probability of adult killer whales with a mother to those without one.
"Our 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," explained Dr Croft.
But for females, the chances were only three times greater for the over 30s, and remained unchanged for those that were younger.
The researchers described their findings as an "exciting breakthrough" in understanding why some species live for so long after their reproductive years.
But the question of exactly how mothers care for their adult sons remains to be answered.
"We simply don't know just how mothers are increasing the survival of their adult male offspring," said Dr Croft.
"Anecdotal observations suggest that mothers may help adult sons with foraging or providing support during aggressive interactions.
"This is one of the things we hope to work on in the future."