© 1999 by The Society for Integrative and Comparative Biology
Water Relations of Chelonian Eggs and Embryos: Is Wetter Better?1
Department of Biology, Colorado State University Fort Collins, Colorado 80523-1878
Correspondence: 2 E-mail: packard{at}lamar.colostate.edu
The exchange of water between a chelonian egg and its subterranean environment is influenced by numerous factors, the most important of which are (1) structure of the calcareous layer of the eggshell, (2) water potential and temperature in the nest, and (3) fraction of the eggshell that actually contacts soil in the nest cavity. Eggs with relatively porous shells tend to absorb large quantities of water from cool, moist environments and to lose large amounts of water to warm, dry ones. Net water-exchange by such eggs also tends to be more favorable (= positive) when soil contacts the entire eggshell than when a large fraction of the shell is exposed to air trapped inside the nest cavity. In contrast, eggs with relatively impermeable shells usually exchange only small amounts of water with their environment, regardless of the physical conditions that prevail inside the nest. The pattern of net water-exchange, together with size (and water content) of the freshly laid egg, determines the amount of water that is available to sustain the embryo. An embryo having access to a relatively large reserve of water will consume more of its yolk and grow to larger size before hatching than will an embryo having access to a smaller reserve of water. Large, well-hydrated hatchlings may survive better than small, dehydrated animals during the trek overland from nest to water. If so, a cooler, wetter nest will also be a better nest.