Understanding reproductive biology and embryonic development is vital to guide conservation programs for endangered species. The Red-footed Tortoise, Chelonoidis carbonarius, is a vulnerable species for which the sex determination mechanism and the transitional temperatures range are unknown. In an attempt to obtain this information, we incubated 145 eggs (38 nests) at 3 constant temperatures: 29°C, 31°C, and 33°C. All embryos incubated at 33°C (n = 49) died during the first third of the incubation period. Of the eggs incubated at 31°C (n = 47), only 10.6% hatched, and 2.12% of the embryos showed malformations of the carapace, head, or legs. Of the eggs incubated at 29°C (n = 49), 52% hatched successfully with 10.2% exhibiting body malformations. From the 30 neonates obtained, we were able to sex 20 individuals by laparoscopies, 2 by gonads histology, 1 was impossible to identify because of the small sizes of the gonads at the time of death, and the remaining were released back to their tortoise origin of collection without being sexed. These results suggest that C. carbonarius exhibits a temperature-dependent sex determination mechanism with females produced at, or above, 29°C. Our incidental findings of embryo malformations, asynchronous hatching, and incomplete yolk absorption suggest that this species may be able to arrest embryonic development via embryonic diapause, aestivation, or both. Future studies should focus on the developmental biology of this species including confirmation of the production of males at temperatures below 29°C.

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