Is rate of ontogenetic growth constrained by resource supply or tissue growth potential?: A comment on West et al's model

1. West et al . (2001) describe a general model of ontogenetic growth in which growth represents the surplus of energy acquisition over maintenance energy requirement, which themselves are allometric (power) functions of body mass with exponents (3)/(4) and 1, respectively. Furthermore, the intercept of the maintenance requirement decreases with the -(1)/(4) power of adult mass. 2. These relationships determine the sigmoid shape of the postnatal growth curve and the (3)/(4) allometric slope of time to complete growth as a function of adult mass. 3. West et al.'s growth function is a particular form of the generalized Putter growth model dm/dt=am(y)-bm(z), which includes other familiar growth functions, such as the logistic and von Bertalanffy equations. 4. Empirical estimates of the allometry of resting metabolism in growing birds are close to z=1 in some precocial species, but are approximately 50% higher in altricial and semiprecocial birds, which challenges the generality of any single growth function for birds. 5. Rather than growth rate being limited by the difference between energy acquisition and maintenance expenditure, it may also be determined by the growth potential of tissues, which is traded off against functional capacity. Predicted growth curves based on increasing tissue function with age are indistinguishable from the West et al . and related equations. 6. Models based on growth rate-functional maturity trade-offs also can produce observed allometric relationships of growth rate to adult mass by incorporating the relative size of the chick at hatching. 7. In contrast to West et al.'s model, declining variance in size among individuals as chicks approach adult size implies targeted growth and control of growth rate by feedbacks of body size itself on growth rate. 8. The existence of plausible alternative hypotheses for the control of postnatal growth should stimulate additional experimental studies to elucidate mechanisms of growth rate regulation and to understand the evolutionary diversification of postnatal growth.