Influence of Hydration of the Environment on Eggs and Embryos of the Terrestrial Turtle Terrapene ornata

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Abstract
Flexible-shelled eggs of the terrestrial turtle Terrapene ornata were incubated on wet (-150 kPa) and dry (-800 kPa) substrates at 29 C. Eggs on the wet medium absorbed water from the environment and increased in mass by 6% over the course of incubation, whereas eggs on the dry substrate lost water throughout development and weighed 17% less late in incubation than they did at oviposition. Availability of water to embryos had no apparent influence on hatching success, but embryos in the wet environment incubated longer, mobilized more of the nutrient reserve in their yolk, and grew larger before hatching than did those in the dry setting. Most of the ammonia released in catabolism of protein by embryonic Terrapene was detoxified by converting it to urea, which accumulated in eggs during incubation. In later stages of development urea attained concentrations inside eggs that may have been sufficient to inhibit metabolism of embryos, with the inhibition potentially being greater in dry settings than in wet environments. Differential inhibition of metabolism could have led to differences in rates of growth that contributed to the differences in size of turtles at hatching, The relatively large eggs of Terrapene ornata may represent the one adaptation of this species for development in terrestrial conditions, because large eggs of other turtles are more likely to hatch following incubation in a stressful hydric environment than are smaller eggs laid by conspecifics.