That coddling appears to help the male orcas to mate and reproduce.
The study released Thursday by the University of Exeter, in the United Kingdom, looked at more than three decades of research by Fisheries and Oceans Canada in B.C. and the Center for Whale Research in Washington state, and found that orca females live many years after they are no longer capable of reproducing.
"This long period of post-reproduction is not very common in mammals, outside of humans," said Dr. John Ford, a research scientist at federal fisheries' Pacific Biological Station.
A female killer whale typically stops bearing offspring in her 40s and lives, on average, another decade. Ford said whale researchers believe they can live as much as another 50 years.
Using the census data collected off the Pacific coast since 1974, researchers at the University of Exeter were able to model the lifespans of the 300-plus members of the northern and southern pods of resident killer whales and found that these whale grannies have little effect on their adult daughters but a profound effect on the survival of sons.
"We have discovered that female killer whales have evolved the longest menopause of any non-human species so they can care for their adult sons," Emma Foster, the lead author of the paper, said in an email interview.
By looking at the Canadian and American whale census data, Foster and her colleagues found that for a male orca over 30, the death of his mother results in an almost 14-fold increase in his own death within the next year. The effect on female offspring's reproductive success was negligible.
Why that is remains something of a mystery, Foster said, because whales are difficult to observe under water where they spend the majority of their lives.
"However, we have a few ideas about what could be going on here we can speculate that they may provide help with foraging, or support during encounters with other whales," she said. "This is something we hope to explore in the future."
It's an intriguing addition to the growing book of knowledge about the stunning black-and-white ocean mammals, Ford said.
"In the case of killer whales, it's the first time it's been shown that there is a positive effect of the grannies on, ultimately, their son's survival and the number of offspring that the son will sire," he said.
The northern and southern resident pods ply the waters off the B.C. and Washington state coast for much of the year, feeding on salmon.
Their routine offers a unique opportunity, and Canadian and American researchers have documented the lives of these two unique pods of killer whales for decades, identifying them using unique fin and body markings that act like fingerprints.
"It's the longest running study on any species of whales in the world now, and I think it's really starting to pay off with some interesting insights into the animals' lifestyle, their societies, their biology and so on," Ford said.
The whales are matrilineal, meaning that offspring remain with their mothers their entire lives, although they mate outside the group.
Females typically reproduce from about 15 to 40, averaging five or six surviving offspring. Males don't begin to reproduce until they're in their 20s, and live on average into their 30s.
The number-crunching at the University of Exeter found that the mortality rate for males under 30 that lose a mother that's still bearing offspring is three times that of males that have their mothers, and more than eight times more for males over 30. The mortality rate increases to almost 14 times for males over 30 that lose a post-reproductive mother, or granny.
"They (grannies) seem to play a key role in the social fabric of the group," Ford said.
Foster said menopause remains one of nature's great mysteries, and the orca research is an exciting breakthrough in the understanding of the evolution of the menopause.
This year's orca census is currently underway.
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