Thermal profiles associated with nest site selection of Nile crocodiles (Crocodylus niloticus) on a commercial crocodile farm

Abstract

Understanding crocodile nest site selection is important in the context of climate change and related habitat alterations. This study assessed a current nesting environment on a crocodile farm in South Africa, examining associations between various nest site selection parameters, with a particular emphasis on the role of temperature. It was hypothesized that thermal profiles of nests and factors affecting nest temperatures (orientation, shading, grassy cover) would directly impact nest site selections, nests closer to waterbodies would be preferred, dominant females would dictate nesting area use, and human presence would not impact nesting behaviours as farmed crocodiles are accustomed to this. Nile crocodiles in this study produced nests of similar depth to wild Nile crocodiles, and subsurface temperatures varied with nesting layouts (section, orientation, shading), climate factors, and grass growth. Although a complex interaction of factors affected nest site selections, mean subsurface nest temperatures tended to fit into the narrow range of 25–26 °C, highlighting a measure of stability within the nesting environment. Daily temperatures and temperature ranges did however vary significantly between crocodile-selected nesting depths. Behaviour played an important role in the nest site selections, highlighting how nesting sites must be more than just thermally viable in farmed settings. Grassy growth over nesting sites reduced the surface and subsurface temperatures of those nests. Although this did not affect nesting site occupancy, crocodiles selected against depositing eggs in these sites. Nests closer to waterbodies and tourist walkway were occupied more frequently; however, successful nesting occurred further from the walkway. The size (snout-hindlimb length) of crocodiles within nesting sites did not correlate to preferred nesting sections within the pen. Further research is needed to determine if thermally optimal nesting conditions might be complicated by climate change related nesting environment alterations on commercial farms. HIGHLIGHTS • Nest site selections were assessed using drones and iButtons on a crocodile farm. • Temperatures varied significantly between crocodile-selected nesting depths. • Mean subsurface nest temperatures tended to fit into the range of 25–26 °C. • Crocodiles selected against depositing eggs in cooler, grass-covered nests. • Crocodiles deposited eggs in nest sites further away from a tourist walkway.