Viciously, with full intention, he hurled his spear at Ralph. The point tore the skin and flesh over Ralph's ribs, then sheared off and fell in the water. Ralph stumbled, feeling not pain but panic, and the tribe, screaming now like the chief, began to advance. Another spear, a bent one that would not fly straight, went past his face and one fell from on high where Roger was. The twins lay hidden behind the tribe and the anonymous devils' faces swarmed across the neck. Ralph turned and ran. A great noise as of sea gulls rose behind him. He obeyed an instinct that he did not know he possessed and swerved over the open space so that the spears went wide. He saw the headless body of the sow and jumped in time. Then he was crashing through foliage and small boughs and was hidden by the forest.
What is the "instinct" referred to in the passage above?