Thermal relations of large crocodiles, Crocodylus porosus, free—ranging in a naturalistic situation

Abstract

We monitored behaviour and environmental and body temperatures (T_b) in summer and winter in 11 salt–water crocodiles (Crocodylus porosus), of body mass 32–1010kg, free–ranging in naturalistic captivity in northern Australia. We found pronounced daily cycles in air and water temperatures in both winter (16–33 °C and 20–31 °C, respectively) and summer (21–45 °C and 24–36 °C, respectively). In winter, crocodiles exposed themselves to the sun during the day and stayed in the water at night. In summer, they remained in the water during the day and emerged onto land at night. Body temperature showed a daily cycle the amplitude of which decreased with increasing mass, from 3.5 °C (mass 32kg) to 1.0 °C (660kg) in summer, and from 3.5 °C (42kg) to 1.4 °C (1010kg) in winter. Underlying the daily cycles in T_b were intermediate (10–13 day, tidal?) and seasonal cycles. Overall, values of modal T_b ranged from 25.1–28.7 °C in winter and from 28.4 to 33.6 °C in summer, trending upwards with body size. This pattern of continuous oscillations in T_b, with no daily plateau, is conspicuously different from that seen in crocodilians of small sizes and from the pattern usually regarded as typical of reptiles in general.