Nesting environment may drive variation in eggshell structure and egg characteristics in the Testudinata.

Testudines exhibit considerable variation in the degree of eggshell calcification, which affects eggshell conductance, water physiology of the embryos, and calcium metabolism of embryos. However, the underlying reason for different shell types has not been explored. Phylogenetically controlled analyses examined relationships between egg size, shell mass, and clutch size in ∼200 turtle species from a range of body sizes and assigned by family as laying either rigid- or pliable-shelled eggs. Shell type affected egg breadth relative to pelvic dimensions, egg mass, and relative shell mass but did not affect size, mass, or total shell mass of the clutch. These results suggest that calcium availability may be a function of body size and the type of shell may reflect in part the interplay between clutch size and egg size. It was further concluded that the eggshell probably evolved as a means of physical protection. Differences in shell calcification may not primarily reflect reproductive parameters but rather correlate with the acidity of a species' nesting environment. Low pH environments may have thicker calcareous layer to counteract the erosion caused by the soil and maintain the integrity of the physical barrier. Limited calcium availability may constrain clutch size. More neutral nesting substrates expose eggshells to less erosion so calcification per egg can be reduced and this allows larger clutch sizes. This pattern is also reflected in thick, calcified crocodilian eggs. Further research is needed to test whether eggshell calcification in the testudines correlates with nest pH in order to verify this relationship.