Patterns of Metabolism and Growth in Avian Embryos

Metabolic rates of embryos of precocial birds increase rapidlyuntil about 80% through incubation, then increase slowly remainconstant or even decline. In altricial species, embryo metabolicrates increase continuously and at an accelerating rate throughout incubation. Total energy cost of development is higher inprecocial than in altricial species. Growth patterns of altricialand precocial embryos differ in the same way as does metabolicrate. Embryo growth rates decline late in incubation in precocialspecies, but increase continuously in altricial species. Embryometabolic rate in cal/hr (P) is related to embryo mass in grams(M) and growth rate in grams/day (GR) by the equation P = 12.17GR+ 1.66M + 1.81. The energy cost of growth in avian embryos is292 cal/g. The energy cost of maintenance is 1.66 cal/g hr andappeals to be independent of embryo mass. Differences in growthpatterns account for the observed differences in metabolic ratesand total energy costs of development. High energy costs ofmaintenance account for high total developmental costs in piecocialspecies and in species that have unusually long incubation periods.