Introduction

In contrast to the open nests of birds, nests of reptiles are generally located in the soil or in decaying vegetation where, effectively separated from the atmospheric gaseous environment, eggs may be exposed to relatively low partial pressures of oxygen and correspondingly high partial pressures of carbon dioxide (Ackerman, 1977; Seymour & Ackerman, 1980; Ferguson, 1985; Packard & Packard, 1988). The deviation from atmospheric oxygen and carbon dioxide tensions may be greater during the later stages of incubation due to a high rate of respiration of the embryos (Booth & Thompson, Chapter 20). In addition, the humidity of these nests is high. Consequently, problems of desiccation are reduced thereby allowing eggs to have high conductances to respiratory gases which result in smaller gradients of respiratory gases across the eggshell. These presumably help to maintain the internal gaseous environment within physiological limits even in nests with low oxygen and high carbon dioxide tensions (Booth & Thompson, Chapter 20). In this chapter, we discuss conductance of different types of reptilian eggshell to water and to respiratory gases, making comparisons, where possible, with avian eggs. As eggshell structure is highly variable (Packard & DeMarco, Chapter 5) conductance and allometric relationships are described separately for each major type of shell.

The eggshell as a mediating barrier

Water vapour

In avian eggs, water vapour, oxygen and carbon dioxide all diffuse across the eggshell through the same pathway – discrete pores in the calcareous eggshell; the relative conductance of the shell to these gases is related to their different rates of diffusion (Paganelli, Ackerman & Rahn, 1978).