Incredible Shells (S04E05)

David Attenborough investigates two shells that have proved to be winners in evolution: the tortoise's shell and the shells of birds' eggs. The ostrich egg is so strong it is possible for a person to stand on it without it breaking, but how does the chick break out of this fortress? The evolution of the tortoise shell was for a long time a mystery and this bony box offers a lot more than just protection. (BBC)