Thermal preferences of hatchling saltwater crocodiles (Crocodylus porosus) in response to time of day, social aggregation and feeding

Three month old hatchling Crocodylus porosus with data loggers in their stomachs were placed in thermal gradients, in isolation (N=16) and in groups of 4 (N=8 groups; 32 individuals). Mean Tb and variation in Tb (SD) was not different whether individual crocodiles in isolation were fasted or fed, or if individuals were housed in isolation (I) or in groups (G). However, individuals in isolation (N=16) maintained slightly lower Tbs than those in groups (N=32) during the early morning (06:00–11:00 h). The overall mean Tb recorded for fasted individuals in the isolated and group treatments (N=48) was 30.9±2.3 °C SD, with 50% of Tbs (Tset) between 29.4 °C and 32.6 °C, and a voluntary maximum and minimum of 37.6 °C and 23.2 °C respectively. During the day (11:00–17:00 h), individuals in isolation and in groups selected the warmer parts of the gradient on land, where they moved little. Outside of this quiescent period (QP), activity levels were much higher and they used the water more. There was a strong diurnal cycle for fasted individuals in isolation and in groups, with Tb during the QP (31.9±2.09 °C; N=48) significantly higher than during the non-quiescent period (NQP: 30.6±2.31 °C). Thermal variation (SD) in Tb was relatively stable throughout the day, with the highest variation at around dusk and early evening (18:00–20:00 h), which coincided with a period of highest activity. The diurnal activity cycle appears innate, and may reflect the need to engage in feeding activity at the water's edge in the early evening, despite ambient temperatures being cooler, with reduced activity and basking during the day. If so, preferred Tb may be more accurately defined as the mean Tb during the QP rather than the NQP. Implications for the thermal environment best suited for captive C. porosus hatchlings are discussed.

► Hatchling C. porosus in a thermal gradient had a mean Tb of 30.9±2.3 °C SD and Tset of 29.4–32.6 °C.
► There was a strong diurnal cycle, with Tb during the day (11:00–17:00 h) higher and less variable.
► Tb was similar regardless of whether crocodiles were fasted or fed, in a group or isolated.