Metabolic rate, allometry কী?

Example 1.16 (Mouse and elephant)

A $20\,\mathrm{g}$ mouse: $B = 3.4\times 0.02^{0.75} = 0.18\,\mathrm{W}$, or $9\,\mathrm{W}/\mathrm{kg}$. A $4\,\mathrm{t}$ elephant: $B = 3.4\times 4000^{0.75} = 1700\,\mathrm{W}$, or $0.43\,\mathrm{W}/\mathrm{kg}$. The elephant burns ten thousand times more than the mouse, but each of its grams burns twenty-one times less: $(4000/0.02)^{-1/4} = 1/21$. The mouse must eat a tenth of its mass every day; the elephant, a fiftieth. The mouse’s heart beats $600\,$ times a minute, the elephant’s $30\,$: heart rate, like $B/M$, scales as $M^{-1/4}$, and both animals’ hearts beat about a billion times in a lifetime.