Energy and Water Vapor Exchange by Parchment-Shelled Reptile Eggs

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Abstract
The mass and heat transfer coefficients were determined for the parchment-shelled eggs of 11 reptile species. The eggs experience high rates of water loss in atmospheres that are less than saturated with water vapor. The concomitant transfer of heat that occurs as water evaporates is sufficient to lower the temperature of the egg well below ambient temperature. The mass transfer coefficients vary from 71,102 mg·day⁻¹·m⁻²·kpa⁻¹ for the smallest eggs (10 g). The water vapor conductance of the parchment-shelled eggs ranged from 483 mg·day⁻¹ · kpa⁻¹ for the .32-g eggs of the lizard Eremias to 5,155 mg·day⁻¹·kpa⁻¹ for the 23-g eggs of the snake Spalerosophis. The egg conductances are two orders of magnitude greater than reported for bird eggs of similar mass and are composed of an eggshell conductance and an external film (boundary layer) conductance. The eggshell conductance and the external film conductance appear to be of similar magnitude; thus, for parchment-shelled eggs, the external film of gas surrounding the egg presents a quantitatively important resistance to water vapor exchange. Buried or partially buried eggs will have a water vapor conductance for the fraction of the shell that is buried which will not include the film conductance. The water vapor conductance of the fraction of the egg which is buried will thus be substantially greater than that of the unburied fraction. The convective heat transfer coefficients which we measured averaged 21.4 (±SD 4.6) W·m⁻¹·C⁻¹. This value is appropriate for spherically shaped objects experiencing free convection. Heat loss caused by metabolic energy conversion inside the egg will have an important effect on the exchange of water vapor and heat by the egg.