The Relevance of Time in Biological Scaling

SIMPLE SUMMARY: Large organisms are not simply proportionately magnified versions of smaller related organisms. The magnitude of various features of organisms often changes disproportionately (allometrically) with increasing body size, thus causing fundamental shifts in body shape and function. These “biological scaling” patterns, especially for the rates and durations of various living activities, have traditionally been explained as being driven by body-size-related changes in the rate of metabolism or energy use. Here, I explore whether a “time perspective” may help explain biological scaling patterns as much as or even more than that of an “energy perspective”. After identifying problems with existing time perspectives based on simple universal “four-dimensional space-time” and “biological clock” concepts, I advocate further investigation of three other new or unappreciated time perspectives. They include (1) biological scaling based on time (allochrony) rather than size (allometry) and viewing the body-size scaling of the “pace of life” in relation to (2) fundamental time limits set by the “pace of death” and (3) evolutionary patterns of the origination/extinction of large-bodied species during geological “deep” time. These new or neglected time perspectives have the potential to revolutionize our understanding of biological scaling and its applications. ABSTRACT: Various phenotypic traits relate to the size of a living system in regular but often disproportionate (allometric) ways. These “biological scaling” relationships have been studied by biologists for over a century, but their causes remain hotly debated. Here, I focus on the patterns and possible causes of the body-mass scaling of the rates/durations of various biological processes and life-history events, i.e., the “pace of life”. Many biologists have regarded the rate of metabolism or energy use as the master driver of the “pace of life” and its scaling with body size. Although this “energy perspective” has provided valuable insight, here I argue that a “time perspective” may be equally or even more important. I evaluate various major ways that time may be relevant in biological scaling, including as (1) an independent “fourth dimension” in biological dimensional analyses, (2) a universal “biological clock” that synchronizes various biological rates/durations, (3) a scaling method that uses various biological time periods (allochrony) as scaling metrics, rather than various measures of physical size (allometry), as traditionally performed, (4) an ultimate body-size-related constraint on the rates/timing of biological processes/events that is set by the inevitability of death, and (5) a geological “deep time” approach for viewing the evolution of biological scaling patterns. Although previously proposed universal four-dimensional space-time and “biological clock” views of biological scaling are problematic, novel approaches using allochronic analyses and time perspectives based on size-related rates of individual mortality and species origination/extinction may provide new valuable insights.