The effects of estrogen on skeletal calcium metabolism and on plasma parameters of vitellogenesis in the male, three-toed box turtle (Terrapene carolina triunguis).

The box turtle (Terrapene carolina triunguis) does not appear to have evolved a mechanism for secondary bone formation similar to Aves inasmuch as several parameters of bone calcium metabolism were unaffected by estrogen treatment. This suggests that reproduction does not constitute a substantial stress to the calcium stores of the female, even though the box turtle is a terrestrial species for which a supply of calcium may be limited. However, the dermal bone of the shell of the box turtle appears to take up calcium at a rate equivalent to that of endoskeletal bones, suggesting that it participates in mineral homeostasis at least to the extent of other osseous elements. Calcium metabolism in the turtle may be unique among vertebrates in this respect. Estrogen treatment of male box turtles results in the appearance of a specific plasma protein in parallel with pronounced changes in plasma calcium and magnesium, suggesting the induction of vitellogenin. The magnitude of the vitellogenic response may have a seasonal component.