They are all “power laws” and are typically governed by an exponent (the slope of the graph), which is a simple multiple of 1⁄4, the classic example being the 3⁄4 for metabolic rate. So, for example, if the size of a mammal is doubled, its heart rate decreases by about 25 percent. The number 4 therefore plays a fundamental and almost magically universal role in all of life.
