David Attenborough's Natural Curiosities

The giant panda gives birth to the smallest baby of any mammal and has to care for and protect it for many months. The kiwi lays one of the largest eggs in the bird world, which produces a very well developed chick. Why don’t pandas give birth to more developed, robust young and why do kiwis produce a single egg that is a quarter of its body mass and almost too big to lay?