Light cues override temperature as a cue for basking in a critically endangered crocodile, the gharial

Ectothermic species depend on ambient temperature to regulate their body temperature. As such, they may be particularly vulnerable to changes in climate. However, the magnitude of this vulnerability varies by species and their behavioural and physiological thermoregulation capacity. Crocodilians bask to benefit from warmer temperatures on land than in water during the daytime. However, whether they use temperature or light conditions as a cue for basking is unknown. We investigated if basking of gharial (Gavialis gangeticus) correlates with light or temperature conditions, by observing gharials in enclosures at the Deori EcoCentre in India. We found that the proportion of individuals basking in the morning was more strongly associated with illuminance than with air temperature or with the difference between water and air temperature. This suggests that light is the primary cue for basking. Younger individuals responded to lower light levels than older ones and varied more in body temperature, while having the lowest body temperatures of all age groups in the morning. Therefore, it may be useful for managers of captive populations to manage light and temperature conditions for hatchlings during mornings. Moreover, a mismatch between light conditions and environmental temperature because of climate change and/or light pollution can disrupt basking timing and affect especially young individuals. Considering the threatened status of the species, their natural habitats should be protected from artificial light at night. Our results highlight the importance of assessing the environmental cues driving behavioural decisions in changing habitats when estimating effects of environmental change on populations.