Orcas are among the few animals known to stop reproducing once they reach a certain age, but why do killer whales go through menopause like humans do?
A report from The Atlantic looked at the story of the killer whale named Granny, who hasn’t been sighted for the past three months, and is believed to be dead. Granny is one of the 24 whales in the Pacific Northwest group scientists have codenamed J-pod and have studied for more than four decades, and prior to her last sighting in October 2016, she was frequently spotted, and easily identified due to her unique physical features. And while her exact age in years is unknown, researchers believe that she may have been as young as 74, or maybe as old as 105 at the time she disappeared.
Yet, it’s not her old age that has piqued researchers, but rather the fact that she is, or was proof that killer whales do have menopause. In the 40-plus years scientists have observed J-pod, Granny has never given birth, meaning she has yet to reproduce since the first time she caught the attention of human observers. Typically, animals are capable of giving birth even in old age and close to death, and that too applies to elephants, blue whales, and other creatures with exceptionally long lifespans.
There are only three known species of mammals – humans, short-finned pilot whales, and killer whales – that go through menopause. But a new multinational study sought to answer the question of why female orcas stop giving birth once they reach the age of 30 to 40, much like women typically experience menopause between the age of 45 and 55.
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“Why females of some species cease ovulation prior to the end of their natural lifespan is a long-standing evolutionary puzzle,” wrote the authors, who published their study this week in Current Biology. “The fitness benefits of post-reproductive helping could in principle select for menopause, but the magnitude of these benefits appears insufficient to explain the timing of menopause.”
The study, which was led by a team of British scientists, looked into 43 years of data from two Pacific Northwest killer whale populations, including the aforementioned J-pod. Based on the data, the reason why killer whales go through menstruation has something to do with female orcas’ relationships with their daughters.
According to the researchers, older mothers who give birth at a similar time as their daughters are 1.67 times more likely to lose their children before they turn 15-years-old, meaning right around the time orcas reach the age of sexual maturity.
A report from the Los Angeles Times explained that this may be because male and female killer whales usually stay with their maternal pod, with males venturing out to mate in other pods before returning to their mothers. That dynamic results in female orcas “being more related” to other whales in their pod over time than her daughters are.
“At the start of her reproductive life, a female’s relatedness to males in her local group is relatively low, because her father is from a different social group. As a female reproduces, her sons will remain in her group, increasing her overall age-specific local relatedness.”
— Chuck Dalldorf (@ChuckDalldorf) January 15, 2017
In other words, this means older female killer whales may rely on their maternal instincts to make sure more members of her pod survive, regardless of how the whales are related to her. But since her daughter(s) aren’t as related to the pod, that leads to mother vs. daughter competition as both try to feed their own respective offspring, with the latter usually outdoing the former. As such, the scientists believe that the strain of competition on older female killer whales may be behind their menopause.
“Models incorporating both the inclusive fitness costs of reproductive conflict and the inclusive fitness benefits of late-life helping (grandmother and mother benefits) may explain why, of all long-lived social mammals, prolonged post-reproductive life appears to have evolved only in humans and toothed whales,” wrote the authors in conclusion.
[Featured Image by Handout/Getty Images]