Measuring extinction, species by species

The Yangtze River dolphin, the Christmas Island shrew and the Venezuelan skunk frog are all victims in an alarming flood of extinctions, but how do scientists decide when such "possibly extinct" creatures no longer exist?

The United Nations says the world faces the worst spate of extinctions since the dinosaurs vanished 65 million years ago, with man-made threats such as rising populations, felling of forests, hunting, pollution and climate change.

Yet proving that any individual species has gone the way of the dodo necessarily demands long, fruitless searching.