Embryonic Energetics

Abstract

Avian eggs are an attractive system for the study of energy use. In eggs the sources and quantity of energy are well defined and the partitioning of that enery for different uses is simplified compared to adult birds. Essentially no energy is used for reproduction, locomotion, behavior, or temperature regulation. Rather, part of the original energy in the egg is transferred to stored chemical potential energy in tissues of the hatchling, in residual yolk, or in the extraembryonic tissues and other waste products. The rest of it is metabolized during the developmental process primarily to support (1) the biosynthetic work of producing the new tissue and (2) for maintenance of existing tissue. The muscular work of hatching requires additional energy expenditure late in development. Muscular activity occurs within the egg before hatching but has not been quantified. Our goal is to describe the energetics of avian development to the point of hatching and to identify patterns that are apparent in the available data. Developmental strategies vary widely, and the evolution of those strategies can be elucidated only by using a comparative approach in a wide variety of taxa. Embryo-genesis constitutes a complex package of co-evolving traits involving the tempo and mode of development, the resources packaged in the egg, and the eggshell. Its success depends on the physical environment around the egg and the parent who behaviorally modifies that environment. The size, maturity, and energy reserves of hatchlings must depend in a proximate (or mechanistic) sense on the energy provided in fresh egg and on how that energy is apportioned during development. Considering energy as the currency of interest in these complex interactions provides a tool with which to examine some of these interactions.