Introduction

The eggs of birds and oviparous reptiles exchange water and respiratory gases with the environment in which they are incubated, primarily via diffusion across the eggshell. Depending on the incubation environment, such eggs may experience either net gains or net losses of water during incubation (Packard, Chapter 13; Ar, Chapter 14). Successful hatching requires that the developing embryo be capable of dealing with the resulting variation in egg water content.

Most bird eggs are incubated in environments where potential gradients favour the continual loss of water throughout incubation. Surfaces on which they lie are usually relatively dry, and parental brooding, plus embryonic metabolism, keep their temperature, and hence water activity, well above that of their environment. Some of the loss is replaced by metabolic water derived from the oxidation of organic molecules within the egg, but water content of all avian eggs declines during incubation, usually by 10-20% of the initial mass of the egg (Ar, Chapter 14). Avian embryos must therefore contend with a gradual dehydration of the egg in which they are developing. Within oviparous reptiles, water budgets are much more variable. The shells of reptile eggs constitute much less of a barrier to water exchange, and reptile eggs are usually laid in close contact with soil of varying moisture content. Depending on soil moisture, reptile eggs may experience either net gain or net loss of water during incubation (Packard, Chapter 13).