Lifespan differs radically among organisms ever lived on earth, even among those roughly similar in size, shape, form, and physiology; Yet, in general, there exists a strong positive relationship between lifespan and body size. Although lifespans of humans and human-related (domestic) animals are becoming increasingly longer than that of other animals of similar sizes, the slope of the regression (lifespan-body size) line and the intercepts have been surprisingly stable over the course of the dramatic human population growth, indicating substantial depression in lifespans of many other animals probably due to shrunk and fragmented natural habitats. This article addresses two questions related to the lifespan-size relationship: (1) what caused the exceptions (e.g., a few remote human-related animals are also located above the regression line with great residuals) and why (e.g., could brain size or intelligence be a covariate in addition to body size in predicting lifespan?), and (2) whether continued human activities can eventually alter the ' natural' regression line in the future, and if so, how much. We also suggest similar research efforts to be extended to the plant world as well.
