Joseph Robert Burger, Chen Hou, James H. Brown

Significance
Data and theory reveal how organisms allocate metabolic energy to components of the life history that determine fitness. In each generation animals take up biomass energy from the environment and expended it on survival, growth, and reproduction. Life histories of animals exhibit enormous diversity – from large fish and invertebrates that produce literally millions of tiny eggs and suffer enormous mortality, to mammals and birds that produce a few large offspring with much lower mortality. Yet, underlying this enormous diversity, are general life history rules and tradeoffs due to universal biophysical constraints on the channels of selection. These rules are characterized by general equations that underscore the unity of life.

Abstract
The life histories of animals reflect the allocation of metabolic energy to traits that determine fitness and the pace of living. Here we extend metabolic theories to address how demography and mass-energy balance constrain allocation of biomass to survival, growth, and reproduction over a life cycle of one generation. We first present data for diverse kinds of animals showing empirical patterns of variation in life history traits. These patterns are predicted by new theory that highlights the effects of two fundamental biophysical constraints: demography on number and mortality of offspring; and mass-energy balance on allocation of energy to growth and reproduction. These constraints impose two fundamental tradeoffs on allocation of assimilated biomass energy to production: between number and size of offspring, and between parental investment and offspring growth. Evolution has generated enormous diversity of body sizes, morphologies, physiologies, ecologies, and life histories across the millions of animal, plant and microbe species, yet simple rules specified by general equations highlight the underlying unity of life.