Sizing up life and death

Determining the “rules” that govern the lifespans and birth and death rates of organisms is a central goal of population biology and lies at the heart of understanding a broad range of ecological and evolutionary phenomena (1, 2). Yet, until recently, quantifying these rules and, more importantly, providing a mechanistic explanation for how they operate have eluded biologists. This gap in our knowledge is understandable. Organisms manifest lifespans that range on the order of minutes or hours in the case of bacteria to many hundreds or thousands of years in the case of some tree species. In addition, for organisms like some unicellular algae and plants, which can reproduce asexually, the application of concepts like “birth” and “death” can be problematic or ambiguous. Nevertheless, the work of Marbà et al. (3) in this issue of PNAS provides reason to hope that the life and death dynamics of otherwise very dissimilar organisms abide by predictable and explicable rules. Equally important, their work demonstrates that lifespan and birth/death rates scale with respect to body size across aquatic and terrestrial and unicellular and multicellular plant species in ways that accord remarkably well with those predicted by a generalized theory for the metabolic optimization of life history traits (4, 5). To understand this theory and fully appreciate the significance of the analyses of Marbà et al. (3), we must first consider the observation that many biological traits vary with body size in a manner conveniently described by the allometric equation Y = β0 M α, where Y is the variable of interest, β0 is the normalization “allometric constant” (that can nevertheless vary numerically with the nature of Y, the kind of organism studied, or the environmental conditions attending growth and development), M is total individual body mass, …