Abstract

The evaluation of offspring sex ratios is important in any large-scale threatened species breeding programs if temperature-dependent sex determination is a possibility. We assessed the sex ratio in captive-bred juvenile Aldabra tortoises (Aldabrachelys gigantea or Dipsochelys dussumieri) at the La Vanille Crocodile and Tortoise Park in Mauritius. The gonads of small juvenile Aldabra tortoises are thin and elongate and fixed to the dorsal part of the body cavity, with ovaries appearing as transparent sheaths with some visible oocytes and testes appearing as small, transparent, thin, sausage-like structures with a net of fine blood vessels on the surface. With growth, ovaries expand and masses of previtellogenic follicles appear on the surface; testes first turn pinkish-white and then yellowish, with tubular structures visible through a thin, transparent theca containing a network of fine blood vessels but no melanocytes. Three tortoises had both testes and ovaries and were classified as intersexes. Aldabra tortoises bred in La Vanille in the late 1990s and 2000s show a male∶female ratio of 1∶4 (n  =  106), suggesting that Aldabra tortoises have temperature-dependent sex determination. However, the incubation conditions were not monitored rigorously enough to allow the determination of pivotal temperatures or the transitional range of male- and female-producing temperatures. Because a female-biased sex ratio seems to be preferable to a male-biased or even to a balanced sex ratio in recovery programs of threatened species, there does not seem to be an immediate need to change egg incubation methods to adjust sex ratios.

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