Ferocious Fighters (S04E06)

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, with the most impressive male kangaroos being solid blocks of muscle with a kick that can kill. Why do they fight and what skills must a winner have? (BBC)