Green sea turtle

Imagine leaving home as a baby and finding your way back years later with no one to guide you. This sea turtle may do it with Earth's magnetic field. Scientists think youngsters memorise the magnetic pattern of their birth area. Some make journeys of thousands of kilometres across the ocean and stay away for more than ten years. Females may then use that memory to return to the same area to lay eggs. In tanks, babies of its close relative, the loggerhead turtle, swam in directions that would keep them in warm currents in the sea when scientists changed the field around them.

Surprisingly, its closest living relatives are crocodiles and birds, not lizards or snakes. DNA from more than a thousand parts of the genome shows this link. It looks very different, with a smooth, heart-shaped upper shell in mixes of black, grey, olive, brown and yellow, and a plain pale yellow underside. Its small head cannot pull back inside its shell. Long, paddle-shaped front flippers make it a strong swimmer.

Typical adults have a shell about a metre long and weigh about 150 kilograms, about as heavy as two grown-ups. The biggest have a shell about one and a half metres long and weigh about 320 kilograms. They live in warm tropical and subtropical seas around the world. Youngsters drift for years in open-ocean currents and hide in floating seaweed. Adults feed in shallow coastal waters, and females crawl onto sandy beaches to nest.