Water and Heat Exchange between Parchment-Shelled Reptile Eggs and Their Surroundings

Abstract

The heat and water exchange of incubating reptile [Chrysemys picta] eggs with their surroundings can be described by a series of thermodynamic equations relating heat and water vapor fluxes to temperature, and to water vapor pressure differences. Heat and mass transport coefficients must be defined and estimated both for the fraction of the egg exposed to substrate and for the fraction exposed to atmospheric gases. Model solutions closely resemble empirical data reported for the changes in mass occurring during incubation of various reptile eggs. Exchange of water vapor rather than liquid water appears to account for the observed changes in egg mass. The thermal conductivity of the substrate has an important effect on the exchange of heat and water by the eggs. Differences in the patterns of water exchange reported for reptile eggs can be attributed to quantitative differences in the thermal regimes in which the eggs are incubated. The greater the thermal conductivity of the substrate in which eggs are incubating, the more water the eggs will take up under wet conditions and the less water they will lose under dry conditions. Large eggs will be more strongly buffered against water exchange than small eggs and, in similar conditions, they will exchange relatively less water than small eggs. The effect is due to the relationships between egg mass, surface area and water vapor conductance. An egg clutch may be viewed as a very large egg which will be much less sensitive to its hydric environment than a single, smaller egg. Temperature and water vapor pressure differences existing inside egg clutches will determine the exchange of heat and water within the clutch, and influence the exchange between the clutch and the substrate surrounding it.