The "fire of life cycles": age-specific metabolic scaling within and among species.

The tempo of the “fire of life” (metabolic rate) relates strongly to adult body size. Most metabolic scaling theory has been based on these adult relationships. However, organisms are not just static adults but have dynamic life histories encompassing multiple developmental stages. Therefore, I advocate that empirical and theoretical studies of metabolic scaling be expanded to include allometric relationships between the metabolic rates and body sizes of various life-history stages. In support, I review observed ontogenetic scaling relationships within species and interspecific relationships based on the body sizes of specific immature developmental stages, including animal eggs/embryos/larvae/juveniles, and plant seeds/seedlings. A diversity of linear and nonlinear scaling relationships, which often do not follow simple 2/3- or 3/4-power laws, is revealed that compels one to consider multiple mechanistic models other than just resource-transport network models that have been used to explain adult-based metabolic scaling relationships. Allometric analyses focused on the “fire of life cycles” show much promise for stimulating the development and testing of a comprehensive, multi-faceted theory that can explain the diversity of observed intra- and inter-specific relationships. These studies may also help bridge the disciplines of developmental biology, gerontology, and biological scaling in mutually beneficial ways. Additional new research opportunities are discussed, including analyses of whether and how metabolic scaling parameters vary with developmental age from conception to death, and in response to various intrinsic and extrinsic factors.