It's one of the golden rules of the natural world – birds live in trees, fish live in water. The trouble is, no one bothered to tell the mangrove killifish. Scientists have discovered that it spends several months of every year out of the water and living inside trees. Hidden away inside rotten branches and trunks, the remarkable creatures temporarily alter their biological makeup so they can breathe air. Biologists studying the killifish say they astonished it can cope for so long out of its natural habitat. The discovery, along with its ability to breed without a mate, must make...