Patterns in the mass-independent energetics of avian development

Metabolic data from embryos and hatchlings of several species, when analyzed using allometric relationships, are more typical of precocial species than of altricial species even though the same species are classified as altricial or semialtricial by the criteria of Nice (Trans. Linn. Soc. N.Y., 13:212, 1962). An analysis of existing embryonic and nestling growth and metabolic data utilizing Heusner's model for generating mass-independent measures of metabolism (MIM values) reveals various patterns: Anseriformes and Galliformes have eggs with high energy density, and these species have relatively high and variable embryo/adult MIM ratios. Species whose eggs are less energy dense than the eggs of precocial species have relatively low and invariant embryo/adult MIM ratios throughout most of incubation. This suggests that there are energetic constraints on rates of growth and maturation in the less energy-dense eggs. I propose that changes in the length of incubation periods may have been a result of natural selection upon adult birds that resulted in the production of eggs with a lower energy content and of natural selection favoring changes in adult basal metabolic rate (BMR). In this scheme altriciality and rapid growth are not the traits upon which natural selection was acting directly. Once pipping and then hatching occur, more divergent patterns of energy metabolism between species are evident. This hypothesis does not exclude others that suggest why different growth patterns occur, but it reinforces the general caution that when one analyzes patterns in any one part of the life cycle, one must consider the possible effects of change in all other parts of the cycle.