Question

Why the requirement of oxygen is more in higher animals?

Answer 1

If all animals need the same concentration of oxygen throughout their tissues, larger animals need more O2 because they more tissue, but metabolism is probably more important than size alone. One thing to consider isn't so much the amount of oxygen but the diffusion of oxygen. Large animals have trouble getting oxygen to all of their tissues, so they have lungs, hemoglobin, and efficient circulatory systems to move the oxygen in and CO2 out. Small animals have less tissue to diffuse through, so they can rely more on simple diffusion of atmospheric oxygen through their bodies