Facultative Embryonic Diapause and a Transitional Stage Between Oviparity and Viviparity in the South African Tortoise Chersina angulata

Abstract. Chersina angulata females ovulate sequentially and oviposit multiple, mostly single egg clutches from early autumn (March) throughout winter and spring until early summer (December). No matter when eggs are laid, they usually hatch in autumn (March/April). Evidence based on circumstantial observations also demonstrates that wild and captive females occasionally retain the last clutch of the season until completion of embryonic development in autumn, conforming to the accepted definition of viviparity in reptiles. However, developmental progress throughout the year and embryonic stages of freshly deposited last clutches has not yet been reported. We candled all eggs laid from April to December in a captive colony of C. angulata in an outdoor enclosure in Cape Town, South Africa, to estimate developmental progress and embryonic stages. In all eggs laid during autumn, winter, and early spring (March to mid-October), vitelline blood vessels, indicating development had progressed to somite formation, only started to become visible at approximately the same time in late November. Until mid-spring, embryonic development of those eggs had been arrested prior to somite formation; the embryos underwent diapause until, presumably, increasing temperatures reinitiated development. Development then progressed throughout summer until the hatching stage was reached in early autumn. However, eggs laid in late spring (mid-October to November) skipped diapause, and embryonic development progressed without interruption, demonstrating that, depending on the season of oviposition, diapause in C. angulata is facultative. Candling of an egg laid in late December showed that, prior to oviposition, embryonic development inside the female had already advanced to an embryonic stage with well-developed vitelline circulation corresponding to Yntema stage 12. Contrary to the generally accepted wisdom that intrauterine embryonic development in the order Testudines is always arrested at the gastrula stage, embryonic development inside the female had already reached the stage of somite formation. This phenotypic plasticity in the expression or abolishment of preovipositional developmental arrest represents a transitional stage in the evolution towards viviparity.