A 40-year study of a population of killer whales off the US Pacific coast has helped British researchers to solve an evolutionary mystery.

Orcas and humans are two of only three species that go through what we call menopause, stopping reproduction part-way through their lives.

And scientists say the killer whales' lives have shown them why any species, including ours, may have evolved to stop having babies so early in life.

Produced by Vic Gill