Thermal ecology and embryonic development in black caiman's (Melanosuchus niger, Spix 1758)

The black caiman (Melanosuchus niger, Spix, 1825) is a crocodilian species endemic to the Amazon biome, that nests within a defined nesting season bridging the local dry and wet seasons. Eggs are deposited in a mound of fresh and old vegetation, but little is known about the thermal relations of M. niger nests and their influence on embryo development rates and total incubation time. We present data for 7 wild nests from the southwestern Brazilian Amazon, gathered in 2019 and 2024. The mean incubation period from laying to hatching was 77.1 ± 8.3 days (SD; range 71.3-93.8 days). Nest temperatures were subject to daily cycling and baseline shifts over time, and had an overall mean of 31.6 ± 1.65 °C (range 27.2 °C-35.9 °C). The nests insulate and buffer the eggs against ambient temperature extremes. Mean nest temperature increases with nest size, and precipitation causes baseline declines in temperature. High nest temperatures (33.6 ± 1.39 °C) in one nest disrupted normal embryo development, and may have affected another. Embryo development rates within 6 nests, and their interactions with nest temperatures, survival to hatching and development time between laying and hatching are investigated. Based on nest temperatures we consider it likely that the M. niger nests we studied would have produced hatchlings with a female bias but could not confirm it.