What the hen can tell about her eggs: egg development on the basis of energy budgets.

By a simple model involving the state variables size and storage, it is possible to describe a wide variety of observations on the feeding, growth, energy storage and reproduction of animals. The model is based on the assumption that reproduction, growth as well as maintenance depend on the stored energy only and not directly on feeding. If an egg is thought of as a non-feeding animal, the model predicts the respiration ontogeny and growth of the embryo inside the egg. These predictions seem to hold well for published data on the development of eggs of fish and ratite, precocial and altricial birds. The latter two are known to follow different respiration ontogenies, but both are described well, differing only in one (compound) parameter value. The model explains why the incubation times of eggs of different species tend to increase linearly with egg size to the power 1/4, and why kiwis and petrels, which lay relatively large eggs, have to brood them much longer than larger birds with eggs of the same size. Conversely, it explains why the small eggs of the (parasitic) European cuckoo, hatch earlier than the still smaller eggs of their tiny hosts. Furthermore, it has been shown how the maintenance rate constant, which frequently appears in the microbial literature, can be obtained from measurements on the respiration and weight ontogeny in embryos, so linking independent lines of research. Application of the model shows an increase of the maintenance rate constant from bacteria, crustaceans, up to fish and birds, and a decrease from bacteria to green algae, suggesting lines of evolutionary development.