[S]ize seems to predict lifespan. The formula seems to be nature’s way to preserve larger creatures who need time to grow and prosper, and it not only operates in all living things, but even in the cells of living things. It tells animals for example, that there’s a universal limit to life, that though they come in different sizes, they have roughly a billion and a half heart beats; elephant hearts beat slowly, hummingbird hearts beat fast, but when your count is up, you are over. Plants pulse as well, moving nourishment through their veins. They obey the same commands of scale, and when the formula says “you’re done,” amazingly, the buttercup and the redwood tree obey. Why a specific mathematical formula should govern all of us, I don’t completely understand, but when the math says, “it’s time,” off we go … —NPR