Killer whale moms live long past menopause, apparently to support and protect their adult sons.
Orca momma’s boys. I’m Bob Hirshon and this is Science Update.
Adult male killer whales, or orcas, are surprisingly dependent on their moms. This according to University of Exeter animal behaviorist Emma Foster. She and her colleagues studied survival rates in orcas.
If a mother died, a male over thirty had almost a fourteen-fold increase that he was going to die in the subsequent year. Whereas for a female of the same age, she only had a fivefold increase.
Foster says this may reflect the mother whales’ evolutionary strategy. She notes that orcas, along with humans, are among the few species where females live well past menopause. After they can’t reproduce themselves anymore, mothers can continue to pass on their genes by making sure their offspring produce lots of grandchildren. And since sons can sire more progeny, which end up being raised by other orca families, it’s in Grandma’s genetic interest to put more energy into protecting her boys. I’m Bob Hirshon for AAAS, the Science Society.
Making Sense of the Research
In human society, healthy thirty-year-old men who are extremely dependent on their mothers certainly exist, but they don't get a lot of respect. Not so among orcas, where depending on Mom is a completely normal part of male adulthood.
However, as you heard, orcas share an unusual trait with humans: females live well past menopause, or the end of childbearing age. In other mammals, females keep reproducing up to or near their life expectancy, whether that's a few years or several decades. One possible explanation for this in humans is called the “grandmother hypothesis:” that women, after a certain age, can best ensure the long-term survival of their genes by helping to care for their existing children and grandchildren, rather than bearing new ones. (This may be related to the long childhood and adolescence of humans, as compared with animals that become self-sufficient within a few months or years of birth.)
There's evidence that grandmothers matter among orcas: both adult male and female survival rates go down in the year after their mother dies. However, the risk posed by a lost mother is far higher for adult males—almost triple that of adult females. Why would that be?
The answer, according to Foster, may be that males have become more dependent on Mom because Mom benefits more from protecting them. Orcas live and travel in close-knit family groups called pods. When babies are born, they stay with their mother's pod, which takes responsibility for their care and feeding. Therefore, having grandchildren through a daughter means more work for an older female and her pod; on the other hand, grandchildren through a son are essentially “free of charge,” since another pod will take care of them. Additionally, as with all mammals, males can sire an unlimited number of offspring, while females' reproductive potential is limited by the time it takes to gestate and bear their young.
As a result, an orca grandmother that puts her energy into protecting her adult sons can boost her chances of having unlimited grandchildren that she won't have to lift a fin to care for. Protecting her daughters, on the other hand, comes with limitations on the number of grandchildren she can gain, and the tradeoff that it takes energy and resources to raise them. With that kind of evolutionary pressure, it makes sense that grandmother orcas would focus on protecting their sons. And over time, those sons might become so dependent on their protection that it's difficult to go on when Mom passes away.
Now try and answer these questions:
- What's similar about orcas and humans?
- How do orca pods assign responsibility for the offspring of adult males? Adult females?
- Why is it in an orca grandmother's genetic interest to focus on protecting her adult sons?
For more on animals and genetics, see the Science Updates Promiscuous Finches, Androgynous Snails, and Lizard Mates. As for human genetics, listen to Grandfathers and Telomeres and Testosterone Tradeoff.