Effect of Embryonic Calcium Constraint on PostHatching Growth and Bone Microstructure in the American Alligator (Alligator mississippiensis)

RM 207

Among oviparous reptiles, archosaurs lay eggs with the thickest and most rigid eggshells. During embryonic development, archosaurs mobilize eggshell calcium to the yolk sac, and upon hatching rely on this calcium reservoir to supplement their dietary calcium intake. This additional source of calcium may have allowed archosaurs to achieve high post-hatching growth rates. We tested this hypothesis by incubating eggs of the American alligator, and following posthatching growth for over two months. The calcareous eggshells were either experimentally peeled or shamhandled with the fibrous shell membrane left intact in both treatment groups. At hatching, experimental animals were significantly smaller than controls. There was considerable variation in growth rates within both groups, but overall control animals grew significantly faster than experimental ones. Standardized to bone length, femora and lower jaws of three month-old experimental animals had smaller cross-sectional area, second moment of area, and polar moment of inertia. Cortical thickness was decreased, as was lacunar density. Incomplete osteone formation resulted in prominent vascular spaces in the lower jaws of experimental alligators. Considering a lower bone mineral content in the experimental group, these results suggest that insufficient calcium supply exerts negative feedback on bone tissue growth, and archosaurs cannot compensate for decreased material stiffness by augmenting the geometric properties of skeletal elements, even those critical to locomotion or feeding. We propose that selective forces on post-hatching survival drove the evolution of ever-thicker and mineralized eggshell of archosaur eggs. Eggshell and bone microstructure of extinct archosaurs may contain clues to their calciumhandling strategies.