They may be among the largest predators on the planet, but in some ways orca males are mommy's boys too.
A massive 36-year study of nearly 600 killer whales off the coast of Washington state and British Columbia shows that adult males are far more likely to die soon after their mothers than are adult females.
The study throws new light on why female killer whales are the only non-human animals - apart from pilot whales - known to go through an extended menopause.
Female orcas live to be 90 but stop reproducing when they are relatively young, at between 30 and 40. In an effort to understand why, Darren Croft of the University of Exeter, UK, analysed death patterns in 589 individually identifiable orca observed by his US and Canadian colleagues between 1974 and 2010.
For orcas between about 5 and 50 years old, Croft and his colleagues looked at the probability that an animal would die in any given year. They compared the prospects of animals whose mothers had died with others of the same age whose mothers were still alive.
This revealed that having a living mother considerably increased an orca's chances of survival. This appeared to be particularly important for males over 30. "There's a 14-fold increase in the risk that sons over 30 years old die in the year after their mothers die," says Croft.
The result suggests that older mothers play a significant role in ensuring the survival of their sons, suggesting orca females go through an extended menopause so they can invest their energies in doing this. Intriguingly, they appear to be less crucial to the survival of their daughters.
Cause for menopause
There are two principal theories for why humans evolved a menopause. One is that, by preventing further childbirth, the menopause means women survive long enough to raise their youngest child. Another is the grandmother hypothesis. This states that by being around to help raise her grandchildren a woman ensures the continuance of her genes down the generations.
Croft says it not clear exactly how menopausal orca help their sons, but he has some ideas. "There's some suggestion from whale watchers that they may guide their sons' foraging for food such as salmon, or provide support for their sons in aggressive encounters with other orcas," he says.
The researchers suggest the menopause may have evolved because protecting sons instead of having further offspring may give orcas the best chance of ensuring they pass on their genes.
Male orcas breed with females in other pods, then return to the pod they were born into. "That means the matriarchs are not paying costs of rearing their own sons' children, although there are extra costs of raising the offspring of their daughters," says Croft.
"There are differences between the human and orca case," says Ruth Mace, who studies human evolution at University College London. "The new data suggest the mother is crucial to the males' ability to stay with his group, which is not common in human societies."
Croft's collaborators at the US Center for Whale Research in Friday Harbor, Washington, and Canada's Pacific Biological Station in Nanaimo, British Columbia, collected data for the study. Researchers at the University of York in the UK also participated.
Journal reference: Science, DOI: 10.1126/science.1224198
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