A riverine thermostat
The Australian freshwater crocodile, Crocodylus johnstoni, is a master of behavioral thermoregulation. As an ectotherm, it cannot generate its own body heat and must rely on the environment to maintain its preferred body temperature, which fluctuates seasonally between 29°C and 33°C. The Ord River near Kununurra provides a perfect mosaic of microclimates for this purpose. The crocodiles constantly shuttle between the sun-baked mud of the riverbanks and the cooler, deeper water of the main channel.
This shuttling behavior is a precise calculation. A crocodile will bask on the bank to absorb solar radiation, raising its body temperature. To avoid overheating, it then slides into the river. But it doesn't just dive to the bottom. Studies have shown that these crocodiles use a range of postures, from being fully submerged (diving) to exposing just a small part of their back (20% exposure). These intermediate postures significantly slow heat loss, allowing them to remain in the cooler water for longer while keeping their body temperature within the optimal range. The greatest rates of heating happen while fully basking, and the greatest rates of cooling occur when fully submerged.
An energetic calculation
Maintaining this specific temperature range is not for comfort; it is about optimizing core physiological processes. Digestion, metabolic rate, and growth in reptiles are all highly dependent on temperature. By actively managing their body heat, the crocodiles ensure they can process food efficiently and devote energy to growth.
This thermoregulatory strategy is a delicate balance of posture and location. The crocodiles demonstrate a suite of behaviors besides basking or diving. They may lie with only their head and back exposed, or just the top of their head, fine-tuning the rate of heat exchange. The difference between the operative environmental temperature and the crocodile's internal temperature can be substantial. While diving, the water is well below their body temperature, but when basking, the temperature at the skin's surface can exceed their core temperature by more than 20°C.
The size of the crocodile also plays a role. Smaller crocodiles, with less thermal inertia, must move between heating and cooling spots more frequently. Larger animals can maintain their temperature for longer periods. This constant environmental negotiation allows Crocodylus johnstoni to thrive in the dynamic thermal environment of the Ord River.