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A male orca's best friend is his old mother.

That's according to a new scientific paper published Thursday in Current Biology, which found female killer whales that have undergone menopause prevent their sons — though not their daughters — from getting injured in fights.

Lead author Charli Grimes told AFP what drew her to the subject was just how rare menopause is in the natural world. Most animals with ovaries remain capable of reproducing until they die, with humans and five species of toothed whales the only known exceptions.

"A big question in our own evolutionary history is, how and why has menopause evolved?" she said.

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