Five animals, four of which live underwater, are now known to go through menopause, scientists have found in a new study.
The researchers added beluga whales and narwhals to the list of animals that evolved to stop reproducing once they reach a certain age.
Menopause In The Wild
For the majority of the animal kingdom, the reproductive system ages at the same rate as their somatic tissue, allowing them to bear offspring until they die. However, in three species, including humans, the reproductive system ages far more rapidly than the somatic tissue.
"For menopause to make sense in evolutionary terms, a species needs both a reason to stop reproducing and a reason to live on afterwards," stated Sam Ellis, author of the study.
Scientists already have evidence that killer whales and short-finned pilot whales go through menopause and have post-reproductive lifespans. Now, for the first time, scientists have added belugas and narwhals to the list.
The research from the University of Exeter, University of York, and the Center for Whale Research went through every available data on 16 species. They found dormant ovaries in older female beluga and narwhals.
However, the question of why these species have stopped reproducing after a certain age still stands. Based on the data, scientists predict that once they stop reproducing, female whales stop competing with their offsprings for mating opportunities, allowing them to stay with their families long after. In killer whales, this is important because male and female offsprings tend to stay with their mothers for life.
"This increasing relatedness means that, if she keeps having young, they compete with her own direct descendants for resources such as food," added Ellis. "The reason to continue living is that older females are of great benefit to their offspring and grand-offspring."
The findings were published in the journal Nature.
Menopause In Humans
Previous studies suggest that same is the case for female humans. The "Grandmother Hypothesis" predicts that if the older female improves the survival and reproduction of her offsprings, she can ensure the continuation of her own genes.
In a previous study, researchers found that children were more likely to survive long into adulthood if their grandmothers are still around. In fact, women with prolonged post-reproductive lifespan have more grandchildren.
In addition to "late-life helping," or benefiting the society long after a female stopped reproducing, might explain why humans evolve to experience menopause.
