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Temperature is crucial for reptiles, also during embryonic development, particularly for species with temperature-dependent sex determination. Under natural conditions, Broad-snouted caiman (Caiman latirostris) eggs are influenced by thermal changes in the interior of the nest related to the external environmental temperature. As nests are subject to variations in temperature and most lab studies on crocodilian incubation have been carried out at constant temperatures, we were interested in determining how temperature fluctuations may affect the development of caiman embryos. We investigated the effects of incubation at constant temperatures (31 degrees C, 32 degrees C, and 33 degrees C) and fluctuating temperatures (31 +/- 2, 32 +/- 1, and 32 +/- 2 degrees C) on the following aspects: incubation period duration, hatching success, sex ratio, total length, and body mass of C. latirostris hatchlings. Eggs incubated at 31 degrees C produced 100% females, those at 32 degrees C produced 71.6% females (however, the sex ratio was nest related), and at 33 degrees C produced 100% males. We found a masculinizing effect when incubation was at 31 +/- 2 degrees C compared with a constant 31 degrees C; and temperature fluctuations at 32 degrees C (32 +/- 1 and 32 +/- 2 degrees C) had a negative effect on hatchlings size and mass, and hatching success compared with constant incubation temperatures of 32 degrees C and 33 degrees C. Finally, the effect of temperature variation during the incubation period on sex ratio, hatching success, and phenotype depends on the mean temperature, as the fluctuation around 31 degrees C affected the sex ratios and incubation period, and the fluctuation around 32 degrees C affected hatchling success and size.