- Image courtesy of Daniel W. Franks.
- Humans and orca whales have something in common: in both species, females survive long after they stop bearing children.
- In humans, anthropologists use the term “grandmother effect” to refer to the added benefit of having grandmothers available to feed and care for grandchildren.
- Now, researchers have discovered that the grandmother effect also holds true for orcas: The presence of a grandmother orca increases the survival rate of that whale’s grandchildren.
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In most animals, the end of a female’s reproductive years aligns with the end of her life. In humans, however, women live long past menopause – and there’s an evolutionary reason for that.
Anthropologists refer to the high survival rate of post-menopausal women as “the grandmother effect,” since the presence of grandmothers boosts the chances that their kin will survive (and pass on their genetic information to future generations). That’s because these older women help their children care for and feed their grandchildren.
A new study, published today in the journal Proceedings of the National Academy of Sciences, reveals that another species benefits from the grandmother effect, too: orcas.
Female killer whales also survive long after they stop bearing children, and the new research found that the presence of maternal orca grandmothers increases baby whales’ chances of survival.
“Post-reproductive grandmothers use their superior ecological knowledge to lead their group around foraging grounds,” Daniel Franks, the lead author of the study, told Business Insider.
Grandmother whales are also known to share food with their daughter’s calves.
- Chase Dekker/Shutterstock
A grandmother orca boosts babies’ chances of survival
Franks and his team examined more than 40 years of census data on two orca groups that live off the coast of Washington state and British Columbia. The researchers analyzed the survival rate of 378 whale calves known to have living grandmothers, and found that in the two years after a grandmother whale’s death, her grandchildren’s chances of survival decreased.
“Grand-offspring whose maternal grandmother died within the last two years have a mortality rate 4.5 times higher than an individual with a living grandmother,” the study authors wrote.
The study authors also found that the loss of a grandmother in years when there were fewer Chinook salmon – which these Pacific northwest whale groups like to eat – had an even greater impact on the chances of a calf’s death.
“We found the impact of a grandmother dying was worse in times of need, when the food supply was low in abundance,” Franks said. (Research has already shown that the more abundant these salmon are, the less likely the orcas are to die.)
- Tobias Brehm/Shutterstock
Whales have close-knit families
According to Franks, the new research may reveal why orcas are one of only five known mammal species (including humans) that go through menopause and have a long post-reproductive lifespan.
Beluga whales, narwhals, and short-finned pilot whales also go through menopause. But orcas live the longest after they’ve stopped reproducing: Female killer whales stop bearing offspring around age 45, then can live for at least 16 more years.
Some female orcas are estimated to be in their 80s or 90s, according to Franks.
One common thread among the few mammal species that go through menopause is that they all have social structures in which females do not disperse. That means offspring stay with their mothers for their entire lives.
“This kind of dispersal means that the older a female gets, the more related she becomes to her local group,” Franks said. “This means that around the age of menopause, a female is well placed, in evolutionary terms, to pass on her genetic legacy through helping family members rather than through reproducing herself.”
Post-reproductive orcas whales act as ” repositories for ecological knowledge” and play an important leadership role for the group when foraging in salmon grounds, according to the researchers.
- NOAA Fisheries
That’s why, Franks said, the cost of losing a grandmother is higher in years when salmon are more scarce.
Grandmother orcas have also been known to babysit their grand-calves while their daughters hunt, he added.