When one morning a file titled ‘Killer Menopause.pdf’ landed in my inbox, I couldn’t NOT open it. The (not even that disappointing) title of the actual paper is “Adaptive Prolonged Postreproductive Life Span in Killer Whales” and the paper is all about post-menopausal killer whales and their sons.
As you may or may not know, it is incredibly rare for animals to live beyond their reproductive age. Once a male or female can no longer reproduce they usually die. Either instantly, or shortly after. There are two exceptions to this rule: killer whales and humans.
It is rare, because -evolutionary speaking- reproduction is the sole purpose of life (ugh how depressing). But humans and killer whales will happily live another 30-40 years after the menopause and no one is entirely sure why.
The killer menopause paper by scientists from York, Exeter and Nanaimo, Canada covered only the killer whale menopause. Mainly because it is very hard to study killer whales *and* humans in the same way as there are just too many differences in lifestyle. For a comparative study we’d need mermaids.
The aforementioned scientists used some long-term observational studies to record the lifespan of the killer whale population in the coastal waters of the state of Washington and British Columbia. The data revealed something about the whales that might be a possible reason as to why the females live so long after their reproductive age.
It turns out that male killer whales are 13.9 times more likely to die within the year following the death of their mothers. Now let me put that in context for you: The mortality rate for female killer whales only increases 5.4 fold after their mum has died.
This shows us one thing: Killer whale boys literally can’t live without their mummies.
The social dynamic of a killer whale family is as follows: The female is the matriarch, she lives with her sons and daughters whilst they roam the oceans. When it is time to mate, the males leave the herd to mate with females in other herds. Once the mating is done, the males return to their own family. Daughters are left to raise their offspring, so all the mated daughters have their kids hanging around with them. The males however, don’t participate in the rearing of their own young (the b*stards) (LINK TO GUARDIAN ARTICLE ON SEXISM IN BIOLOGY PAPERS (oh they don’t have one, but Science Now does)).
This is brilliant for the matriarch. She gets to spread her genes through her children. But the boys spread the genes,without any cost to the herd. The daughters on the other hand spread her genes, but they also burden the family with their offspring. The matriarch is therefore a lot keener on her sons. Because as much as she loves her grandchildren, she prefers the ones who don’t ever come to visit.
So grandma keeps her sons safe, they are her golden boys. The hypothesis is that she uses her life experience to point out where the boys can find food and when a fight breaks out, she’s the one to knock their heads together and break the fight up.
When she dies, the sons suddenly find themselves having to look for their own food and they start fights they can’t win without their mum breaking them up. And so with these extra strains, they end up 13.9 times more likely to die within that year, and the mortality rate increases dramatically for every year after that if they manage to survive.
The daughters, in the meantime, are fine without mum. They rear their kids and find food and start up their own herds with sons to pamper. Because female killer whales can be adults about these things, unlike their spoilt brothers.
Emma A. Foster, Daniel W. Franks, Sonia Mazzi, Safi K. Darden, Ken C. Balcomb, John K. B. Ford, Darren P. Croft –
“Adaptive Prolonged Postreproductive Life Span in Killer Whales” Science 14 September 2012: Vol. 337 no. 6100 p. 1313