A 40-year study of a population of killer whales off the US Pacific coast has helped British researchers to solve an evolutionary mystery - why killer whales and humans are two of only three species that go through what we call menopause - stopping reproduction part-way through their lives.

By examining a record of every birth and death in every orca family, scientists discovered that the menopause gave new calves a better chance at survival - preventing what they called "reproductive conflict" between mothers and daughters.

Prof Darren Croft from the University of Exeter, who collaborated with the Center for Whale Research on San Juan Island and Fisheries and Oceans Canada in the study, says the findings, published in the journal Current Biology, could reveal how and why the same phenomenon evolved in humans.

The study is part of an ongoing project to study the lives and family bonds of this unique population of orcas.

Video by Victoria Gill and David Cheeseman