Growth models based on first principles or phenomenology?

1. Most models of growth are simply mathematical descriptions of growth trajectories. They are evaluated empirically based on goodness of statistical fit. 2. Recently we (West et al. 2001) presented a theory of growth based on first principles of energy conservation and allocation. The critical parameters are defined precisely and operationally in terms of measurable fundamental parameters not directly connected to growth, so the mechanistic basis of the model can be evaluated by testing both the assumptions and the predictions. To the extent that available data permit such evaluation, they generally support the model for many classes of organisms including mammals, birds, fish and crustacea. 3. Our model provides a basis for understanding the general and fundamental features governing ontogenetic growth. It is intended to explain the major patterns in terms of first principles of energy allocation to production (of new cells) and maintenance (of existing cells) that are applicable to all organisms. While the model is not intended to account for all of the observed variation in growth rates and life histories it does provide a baseline for developing more detailed treatments of ontogenetic growth. 4. Ricklefs (2003) has criticized the conceptual foundation of our theory as it applies to birds and asserts that it cannot account for many of their growth and life-history attributes. 5. He correctly points to similarities in mathematical form and statistical fit between our growth model and that of von Bertalanffy 1938, but fails to point out the fundamental conceptual differences between the two models. 6. With respect to birds, our model suggests a new hypothesis based on water balance, for the difference in growth allometry between altricial and precocial species. In particular we show that ours is the only model discussed by Ricklefs that correctly predicts the absolute value and scaling characteristics of the total energy metabolized by altricial birds from hatching to fledging. 7. In contrast, Ricklefs's models are of limited utility since they are primarily designed for birds, and are based on qualitative concepts such as 'growth potential' and 'functionality' of tissues and therefore do not readily lead to quantitative predictions. 8. Our model does not imply that mature or asymptotic size is determined simply by resource supply. It allows for a mature body size that is set by natural selection and that results in a balance of energy allocation between production of new biomass and maintenance of existing biomass.