The Australian freshwater crocodile is endemic to Australia and found in freshwater creeks and rivers across northern regions. It is a small species, often less than three metres in length, and recognisable by its long slender snout. ‘Freshies’ have received less attention than their larger estuarine cousins (‘salties’), and many aspects of their ecology remains unknown. In order to address this issue, Australia Zoo recently studied the movements of 28 individuals over a number of months in Lakefield National Park, North Queensland. The study revealed some fascinating insights into why crocodiles dive, what they do underwater, and what factors affect their diving behaviour.
The team captured the crocodiles in billabongs during the dry season, attached miniature electronic devices (tags), and released them back into the wild. The crocodiles would go about their usual daily routine, and the tags would record information about their physiology and behaviour. This information was transmitted by radio waves to remote receiving stations, and the research team were able to observe their secretive underwater lives.
All of the tagged crocodiles increased the duration and depth of dives in the early hours of the morning, and undertook fewer, shorter and shallower dives throughout the afternoon and during the night. It was previously thought that these high periods of diving in the early morning were foraging behaviours; this study revealed that the crocodiles were in fact resting within deep holes, and may have even been undergoing a crocodile version of sleeping.
The crocodiles exhibited some of the longest dive durations ever recorded for an air-breathing vertebrate. These were even more remarkable considering the warm water temperature (> 21.5oC) and the small size of the crocodiles – a 5kg crocodile dived for 5.7 hours and a 42kg crocodile dived for 6.7 hours. The ability of these relatively small diving animals to remain submerged for such long periods of time reveals a unique biology, one which enables crocodiles to stay underwater longer than elephant seals and even whales.
The research team also observed that the crocodiles would adapt diving behaviour to modify body temperature. The deep water within the water hole was cooler than the air temperature in summer, and the crocodiles would stay cool during the middle of the day by diving down into the cooler water. In winter, the deep water was warmer than the air temperature throughout the night, and the crocodiles would maintain a warmer body temperature by remaining for long periods in deep water during the night.