Body Size, Physiological Time, and Longevity of Homeothermic Animals

The Concept of physiological time is extended from contributions of Adolph, Hill, and Stahl to included a wider range of events in the life histories of mammals and birds. The durations of physiological, developmental, and ecological cycles show nearly parallel exponential relationships to body size (masss1/4). Maximum life span approximates a fixed multiple of shorter events, such as muscle-twitch time, circulation and filtration of blood plasma, respiratory and metabolic cycles, embryonic development, growth, sexual maturation, and even minimum population-doubling time. Endothermy evolved independently in birds and mammals, which share the approximate M1/4 allometry of physiological time, although with different absolute values of cycle times. Birds develop faster than similar-sized mammals, live longer, and their slower hearts actually beat more before expiring. A time-scale proportional to M1/4 may be difficult to synchronize with environmental cycles. Thus, the mouse experiences more events in a day's fast or a winter' cold than a moose (a long cold winter being relatively longer for mouse than moose). This asynchrony of physiological time and astronomical time has resulted in the need for biological clocks. Life span, like the other scales of physiological time, may be regarded, not as a direct product of natural selection, but rather as an allometric consequence of other characteristics subjected to natural selection.