How grandmother orcas help explain the evolution of menopause
Killer whales are one of just three species, including humans, that go through menopause
The hot flashes, the hair loss, the bone depletion … menopause isn't fun. But you aren't alone: Shamu felt it, too.
New research from Fisheries and Oceans Canada and the University of Exeter has shown that killer whales go through menopause for the same reasons humans do — and they might help explain the evolution of menopause for humans and orcas alike.
What does menopause in an orca look like?
No one can say if they have hot flashes, but biologists simply point to the fact that orca females stop having calves in their 30s or 40s yet can live to be 100 years old.
Needless to say, it's difficult to study orcas at all, let alone their hormone swings later in life. But we do know that orcas, short-finned pilot whales and humans are the only known animals to have a post-reproductive lifespan, which we call menopause.
Even chimpanzees, our closest evolutionary cousins, don't see a full stop of their child-bearing abilities. Chimps see a fertility decline but have never been known to enter into the hormonal state we know as menopause.
Why did menopause evolve in the first place?
No one really knows. It can be theoretically explained by some basic evolutionary biology — that having successful offspring is the goal of all creatures. Whatever strategy it takes to have those successful offspring will be maintained in your population.
For a turtle mom, that means laying hundreds of eggs and then taking off into the ocean hoping a few survive to lead long, happy reproductive lives.
For elephants, it means having few offspring but nurturing them and spending a great deal of your life parenting to make sure that they go on to reproduce as well.
So, where does a mom running out of eggs — essentially, going through menopause — fit in? When a female runs out of eggs, she's presumably gone through many reproductive cycles and could, in theory, be a grandmother.
The grandmother hypothesis is quite simple: one way to make sure your offspring are successful is helping raise the grandchildren. That way you are sure your genes survive not only to the next generation, but to the second generation as well.
What does this new research help explain about whale menopause?
This research provides evidence for something called reproductive conflict. The group studied 43 years of demographic data on orcas in B.C.'s Salish Sea, and they found something remarkable: when mothers and daughters of the same population breed at the same time, the mother's offspring are 1.7 times more likely to die than the daughter's offspring.
That means, in the continual cost-benefit analysis of nature, that it's more valuable to put resources into your grandchildren than your own kids since they have a better chance of survival.
So, instead of competing against your daughter for those precious resources, it's better to stick around to ensure the success of your grandchildren. Nature has taken care of your ability to reproduce lest you be tempted.
Why do grandma orcas stick around at all?
This is an important question to answer.
It is beneficial for grandmother orcas to stay alive because they contain decades of knowledge to help the entire pod survive. The researchers watched whale behaviour and noted a number of learned behaviours that aided the group. For example, matriarchs will take a greater lead role in the pod when salmon stocks are low. They also observed that momma's boys — full-grown adult males who swim right beside their mothers — have an increased risk of death if their mother dies.
So having grandma around is very good… As long as she doesn't decide to have kids, too.
Does this help us understand human menopause?
In fact it does. One of the fascinating things about the whale research is that it might help confirm long-held theories about the evolutionary role of human menopause.
"What we've been able to do with the resident killer whales," said Darren Croft from the University of Exeter, "is actually test the theory that's been developed to explain menopause in humans in a non-human species, and show that the same mechanisms can also explain menopause in humans."
The grandmother effect is definitely powerful in humans, as anyone who has tried to raise kids without their mom around can attest. But it's still unclear if it evolved from reproductive conflict or not.
Regardless, post-reproductive females in both humans and orcas are a cornerstone of survival for the entire population.