David Attenborough's Natural Curiosities Season 4

Hybrids can be bizarre and they can be deadly. We look at two hybrid animals that owe their existence to human interference – the pizzly bear (a cross between a polar bear and grizzly), which has come into being because of global warming, and the killer bee brought into existence because of the transfer of African bees to South America.

Some animals have an extraordinary ability to find their way. The dung beetle, an insect revered by ancient Egyptians, uses the sun, the moon and even the Milky Way to move its prized ball of dung in the right direction. Pigeons are often considered feeble birdbrains, but they have incredible memories that can recall several complex travel routes with amazing accuracy and they even use manmade roads and hedgerows to find the quickest way home.

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?

Can animals count? This is a question that has intrigued and fooled investigators for a long time. Just over a hundred years ago, a German horse called Hans was declared a mathematical genius but all was not as it seemed. And strangely, some bamboos around the world flower exactly at the same no matter where they are – are they counting down the years?

David Attenborough investigates two shells that have proved to be winners in evolution: the bird’s shell and the hard shell of the tortoise. The ostrich egg is so strong it’s possible for a person to stand on it without it breaking – 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.

The Siamese Fighting Fish is so aggressive it will fight its own reflection until it is exhausted. Recent research shows that the fighting behaviour varies and depends on the personality of the fish! Male kangaroos were once pitted against humans in the boxing ring – the most impressive male kangaroos are solid blocks of muscle with a kick that can kill. Why do they fight and what skills must a winner have?