The nesting site selected by a female turtle influences the temperature and humidity of the incubating eggs, which affects the development, survival, and sex of the embryo and the subsequent developmental phases. The present study evaluated the effects of environmental variables on the duration of incubation, hatching success, and sex ratio of hatchlings of Podocnemis unifilis in an area of várzea swamp on the lower Amazon River in Brazil, during the 2007 and 2009 nesting seasons. Nests were located, marked, and monitored, and physical and environmental characteristics were measured. The temperature of the nests was measured, and hatchlings were collected to determine the sex ratio. The mean temperature, vegetation cover, and nesting date influenced the duration of the incubation period, and the grain size of the substrate influenced hatchling survival. The sex ratio differed between the years and was female-biased in 2007 and male-biased in 2009, emphasizing the need for further long-term studies of the reproductive cycle in chelonians.
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