A new model of avian embryonic metabolism.

A new model of avian embryonic metabolism is proposed that more accurately describes the available data. The model assumes that embryonic metabolism can be described by the equation VO2 = A X massB + C X growth rate. Empirically determined values of A, B, and C were used to predict the values of these parameters for the species whose preinternal pipping rate of O2 consumption (PIP) and total energy used during incubation (TOT) have been measured. Different patterns of growth were assumed for altricial and precocial species. Predicted values of PIP and TOT are highly correlated with observed values (r2 = 0.966 and 0.979, respectively). The model predicts the pattern of metabolism in precocial species will vary with hatchling mass and incubation period; TOT will be greater in eggs with longer incubation periods and greater in precocial species than in altricial species because 70-80% of TOT is devoted to maintenance; PIP will be relatively insensitive to interspecific differences in incubation period, because the 80% of PIP devoted to maintenance is insensitive to incubation period; the energy cost of biosynthesis will average only one fifth of the energy content of the tissues produced; and the maintenance metabolic rate of an avian embryo is more like that of an adult reptile than that of an adult bird. Shell conductance may not be exactly inversely proportional to incubation period, because it represents a compromise between the requirements for limiting water loss and permitting adequate exchange of respiratory gases. The model overestimates PIP and TOT in species with very long incubation periods; these embryos may have unusually low metabolic intensities.