Fight or flee? Resist or surrender? Aggression over food, territory or mates is widespread in animals. But teeth, hooves, horns and spines can cause severe injury and sometimes death. So zebras in theSerengeti, moose in Alaska and even bizarre stalk-eyed flies in Malaysia assess their opponent's prowess and choose to fight depending on what they stand to lose or gain. And despite the proliferation of weapons in nature's trials of strength, many disputes are settled without a blow being struck. Risking injury is the last resort. Written and presented by Sir David Attenborough.