Alligator osteoderms as a source of labile calcium for eggshell formation
The calcium (Ca) demand on alligators in active reproduction is significant, yet the source of this additional Ca is unclear. Three possible sources for Ca mobilization are endolymphatic deposits, as in anurans and some lizards; short lived skeletal medullary bone or analogous deposits, as in birds; or some other source such as the osteoderm layer of the integument or simply mobilization of structural bone. Here, we investigate possible extra skeletal sources for labile Ca in the reproducing alligator, including endolymphatic Ca deposits, by analogy with anuran amphibian and some reptiles and integumentary osteodermal (scale) Ca deposits. We conducted X ray image analyses of skulls for the presence of significant endolymphatic Ca deposits. We also examined dermal bone of scutes (osteoderm, scales) from the dorsal integument using both X ray and histological analyses. Tissues from reproducing females containing mature but unovulated follicles were compared with those from specimens that had nested (laid eggs) or contained eggs within the oviduct at advanced stages of calcification. A small number of immature specimens and an adult male were also compared. No clear differences were observed in endolymphatic deposits between pre and post ovulatory specimens. Scute (osteoderm) X ray density was significantly greater in females with ripe ovarian follicles compared with those that had recently laid (nested) or contained heavily calcified eggs within their oviducts. The latter groups also showed histological evidence of scute resorption compared with the former, suggesting that the scutes play a role in Ca storage during egglay.