Killer whales are one of only three species on the planet to experience menopause, and scientists believe the evolutionary trait is a result of whales' "unusual social structure," in which their adult children never really leave them. (On a side note: what the fuck is day two of a whale period like?)

Humans, pilot whales, and killer whales are the only species to continue living long enough after they stop reproducing. Female killer whales finish up their childbearing years in their 30s and 40s, but they can live until they're 90. Researchers from Exeter University and York University are currently studying why they have evolved to be so similar to humans. The theory is that it has to do with their social relationships.

Killer whale offspring live with their mothers for the rest of the mothers' lives. The matriarchs lead the family and, long after they stop ovulating, they are no longer in competition with their daughters to mate, so their time is freed up to care for their grandchildren, teaching them how to hunt for food.

Interestingly, the male offspring leave briefly to mate but then return to their moms. Research has shown that the sons "have a 14-fold increase in mortality risk the year after their mother dies," which suggests that the females have evolved to experience menopause so that they can look after their adult sons, and increase their chance of survival.

Image via Jan Daly/Shutterstock

Killer whales experience menopause – just like humans [The Guardian]