In 1932, Max Kleiber showed that basal metabolic rate scales as body mass to the 3/4 power, across nearly all animals.
P ∝ M^¾
An elephant masses ~75,000× more than a mouse but burns only ~14,000× the energy per day. Per kg, an elephant is far more efficient — which is why mice eat their body weight every few days while elephants eat ~5% of theirs.
Why ¾? Disputed for decades. The best modern theory: fractal vascular networks (West, Brown, Enquist 1997) constrain how energy is delivered to every cell of a 3D body through a hierarchical pipe system.