Most animals breed throughout their lives. Only humans and four whale species are known to experience menopause, and scientists have long been puzzled about why this occurs. Killer whales are a diverse species made up of multiple separate ecotypes (different types within a species) across the world's oceans that differ in their prey specialisation and patterns of social behaviour.

 

Previous studies have found menopause in an ecotype called "resident" killer whales whose social structure appears to favour "grandmothering" (females using their energy and knowledge to help their offspring and grand-offspring, rather than competing to breed themselves). The new study looked at an ecotype of killer whales with a different social structure, where offspring are more likely to leave their mother -- and the evolutionary benefits of grandmothering are therefore reduced.