Orca females also go through menopause | Polarjournal
Orcas live in clans led by a female. Depending on whether they are Residents or Transients, the members are more or less closely related to each other. Theoretically, therefore, menopause should occur less or not at all in the leader. But the researchers were wrong. Archive image: Michael Wenger

Menopause, the cessation of reproductive ability in females, rarely happens in the animal world. In fact, this biological mechanism is known only in humans and some species of whales. Researchers believe it could be a way to ensure the survival of offspring because the older females no longer have to compete with the younger ones, but can focus on improving the rearing of their grandchildren. With whales, such a behavior became known among other things with an orca group. An international research team has now made a surprising discovery of the same behavior in another, ecologically distinct group on the west coast of Canada.

The female orcas in the group studied, which lives between Washington State and British Columbia, show a similar menopause pattern to the only previously known orca group in the same region. But the latter belongs to the so-called “residents”, whose members hardly change groups. This leads to a higher degree of relatedness the older the females become and thus to a greater advantage of giving up one’s own reproductive ability in favor of the closely related offspring. The newly studied group, called the “Biggs orcas” do not show such pronounced social behavior. Despite this, first author Mia Lybkaer Kronborg Nielsen and her professor Darren Croft from the University of Exeter found a similar age for the onset of menopause and a similar lifespan for females as in the resident population. “These different whale populations both show increased female relatedness with age, but – as this is stronger in resident than Bigg’s killer whales – it’s not immediately clear why the age at menopause and the length of the post-reproductive lifespan seems to be the same in both,” Professor Croft explains. The study appeared in the July issue of the journal Ecology and Evolution.

Orcas show a wide range of prey and fish is the main food especially for the residents. Schools of fish are targeted and driven into shallow water or encircled. Young animals must learn such strategies from older females. Picture: Michael Wenger

According to the researchers’ theory, the Biggs orcas studied should actually show a weaker tendency to menopause. This means later onset and a less long age span of reproductive stop. “Previous research on the evolution of menopause has focussed on resident killer whales, where both males and females usually stay in the social group into which they were born,” explains Mia Lybkaer Kronborg Nielsen. Biggs orcas often leave the group when they reach adulthood. As a result, females are less strongly related to juveniles and their mothers from around 35-40, when they reach menopause, than in other orcas. “We expected this to be important in terms of menopause because weaker relatedness would appear to give females a weaker evolutionary reason to cease reproduction,” Darren Croft further explains. But the data, spanning more than 40 years and more than 500 individuals, showed that the researchers’ assumption was wrong. Females of the Biggs orcas entered menopause during the same period and lived virtually as long without reproducing as the other orca groups to which they are not related. It is true that the life span of Biggs’Orcas is longer (44 years on average for males, 59 for females). But for this the menopause also occurs later in the latter.

In addition to the orca groups studied, menopause is also known to occur in belugas, narwhals and pilot whales. All of these animals, like us, live in complex social structures where a lot of time and energy is invested in raising offspring. Image: Alexey Paramonov

According to the team’s conclusions, a long menopause is likely to be an ancient trait in orcas and also occurs in other populations and ecotypes. It is known that menopause still occurs in other toothed whales such as belugas, narwhals and pilot whales. Researchers can now use the results to further investigate the advantages of such a strategy, also with regard to interaction with humans. For one thing, orcas and humans aren’t necessarily always friends. Especially where residents fight with fishermen over the valuable food fish such as salmon or tuna, conflicts often arise. According to co-authors Thomas Doniol-Valcroze and Jared Towers of the Canadian Fisheries and Oceans Agency, “”Not only do the results contribute to a better understanding of animal evolution, they have significant implications for conservation by shedding light on the importance of social structure for the recovery of these populations.”

Dr Michael Wenger, PolarJournal

Link to the study: Lybkær Kronborg Nielsen et al. (2021) Ecol Evol 11 (13) A long postreproductive life span is a shared trait among genetically distinct killer whale populations; https://doi.org/10.1002/ece3.7756

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