Female orcas go through menopause, living up to 90 years—a longstanding mystery. Now, a new study suggests there’s a reason why.
The orca is one of only a handful of mammals known to go through menopause. The reason has remained murky, but now, new research suggests why: Grandmothers boost the survival of their grandcalves.
Scientists who analysed decades of orca populations in the Pacific Northwest found that young orcas with grandmothers were more likely to stay alive than those without. What’s more, a calf’s risk of death rose dramatically for two years following the death of its grandmother. Because orca (also known as killer whale) societies are matriarchal, it’s likely that these older females carry with them crucial knowledge about food resources that can mean life or death for their kin.
“[A killer whale grandmother’s] greater knowledge and their leadership, especially when times are hard, are helping calves,” says senior author Dan Franks, an evolutionary ecologist at the University of York in the U.K. (Read about a mysterious new orca species.)
Ranging from polar regions to the Equator, orcas live in close-knit family groups of up 40 individuals. The predators work together to hunt a variety of prey, from fish to whales, depending on where they live. Generally, both male and female orcas stay in their natal pod throughout their entire lives, although both sexes search for mates from other pods to prevent inbreeding. Orca females stop reproducing around 40 and can live to 90, whereas males tend to live around 50 years.
Although orcas are listed as data deficient by the International Union for Conservation of Nature, populations are in decline—including those in the Pacific Northwest, which are the most studied orcas in the world. A triple whammy of exposure to toxic chemicals known as PCBs; a drop in the populations of their main prey, chinook salmon; and noise pollution from ocean vessels are all contributing factors to their demise.
That’s why this study, published December 9 in the , is a powerful piece of evidence for conservationists to protect orcas from such threats, Franks says.
“The death of a post-menopausal grandmother has an outsized impact on her family group,” he says, “which makes this an important conservation tool.”
“It’s really important work,” adds Janet Mann, an animal behaviourist at Georgetown University in Washington, D.C., who was not involved with the study. “We’re just scratching the surface of what these grandmothers are doing.”
Explaining menopause
Scientists have long been intrigued by menopause in women. A woman’s life expectancy at birth is generally longer than that of a man, yet women live for decades after they stop having children, whereas men can become fathers right up until death.
“Males don’t have menopause. They still have a few swimmers, even at the end,” says Mann.
From an evolutionary standpoint, that makes sense—a woman living for 40 years longer than she’s able to reproduce, not so much, Mann says. (Read how grandmothers also experience changes in their brain chemicals.)
Natural selection would seemingly prioritise a female’s ability to have as many surviving offspring as possible, and ceasing to reproduce long before the end of life would interfere with that.
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An orca helps herd a school of herring in the deep waters of the Andfjorden in Norway.
PHOTOGRAPH BY PAUL NICKLEN, NAT GEO IMAGE COLLECTION
Evolutionary biologists have developed several hypotheses that can explain this dilemma. For one, menopause can help prevent grandmothers and mothers from competing for scarce resources to feed their own young. Giving birth at older ages may also be risky, putting not just mother and baby in peril, but also the mother’s existing children.
Then there’s the grandmother hypothesis, popularised by American anthropologist Kristen Hawkes and her work with the Hadza, a modern group of hunter-gatherers in Tanzania, argues that grandmothers boost the survival of their grandchildren by supplementing food and childcare.
The idea is supported by a variety of studies, including a 2004 analysis of pre-industrial Finns and Canadians, which showed that children with grandmothers were much more likely to make it to adulthood.
Grandmas matter
Intrigued by such human research, Franks and colleagues wanted to see if such an effect occurs in orcas. The scientists pored over 40 years of data sets detailing births, deaths, and various events in the lives of two orca populations that live off the coasts of Washington State and British Columbia.
In 2017, a team of scientists descended on a beach in Western Cape of South Africa to investigate the carcasses of three great whites. Their autopsy concluded that orcas were responsible for killing the great whites and removing their livers. The incident came twenty years after killer whales were filmed killing a great white shark off San Francisco. The recent events shed new light on the orcas’ taste for shark meat.
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Biologists in South Africa investigate the remains of great white sharks killed by orcas, a behaviour rarely witnessed firsthand.
NATIONAL GEOGRAPHIC CHANNEL
In all, the team analysed the survival rate of 378 “grandchildren,” and discovered that the risk of death for a calf was greater when their grandmother had stopped reproducing herself, and when the calf was male. Franks says that post-menopausal orcas are likely able to devote more resources to their grandcalves, which made the grandmothers’ eventual deaths especially devastating; the male factor is more of a mystery.
The risk was also highest when salmon populations were low or moderate, suggesting grandmothers are most useful in times of scarcity. (Watch a rare video of young orcas hunting sea turtles.)
“We know that they are leading their family group around foraging grounds, especially in times of need, and we know they share their salmon catches with their young relatives,” Franks says. “But we suspect there is more” to discover about how grandmothers keep their families going.
And as the Pacific Northwest orca populations dwindle, we may never find out, Mann notes.
“We’re losing the opportunity to understand how menopause is evolving,” she says, “because of our own actions.”
Sourse: www.nationalgeographic.com.au
