Skin colour varies depending on where they live and their size. Colours range from almost black through to grey and olive-brown, with ragged, dark mottling. From the side, the jaw line is uneven with irregular-sized rounded teeth. The head is generally broad and knobbly. An average male may be 3-4m long and weigh 200-300kg. Females rarely reach over 3.5m and weigh up to 150kg.
Habitat and Range
Estuarine crocodiles are found from India, throughout south-east Asia and New Guinea, across to northern Australia, Vanuatu and the Solomon Islands. In Queensland, they are known to occur between Gladstone and Cape York Peninsula, and throughout the Gulf of Carpentaria. Although most commonly seen in tidal reaches of rivers, they also occur in freshwater lagoons, rivers, and swamps hundreds of kilometres inland from the coast. They can even be found along beaches and around offshore islands in the Great Barrier Reef and Torres Strait.
Crocodiles use the water, sun and shade to maintain their preferred body temperature of 30–33 °C. When basking, they orientate their bodies to ensure the maximum surface area is exposed to the sun. Crocodiles are unable to sweat. To avoid over-heating they may return to the water or lie with their jaws agape, allowing cool air to circulate over the skin in their mouths. This process of heating and cooling their bodies is called thermoregulation and is crucial for many bodily functions including digestion and locomotion, and ultimately for their survival.
Often observed basking on the banks of watercourses where they are generally inactive, crocodiles are less likely to be seen when they are in the water. Although livelier in the water, crocodiles are able to swim just below the surface, with only their eyes and nostrils visible.
Crocodiles are one of the few reptiles to have a four-chambered heart (like mammals). They can also stay underwater for extended periods of time because they have the ability to slow their heart rate, allowing them to hold their breath.
A unique feature of crocodiles is their inability to maintain strenuous activity for extended periods of time. They can easily become exhausted while capturing prey or fighting other crocodiles. Extreme exertion is carried out anaerobically (without oxygen) and must be followed by a period of rest so that the “oxygen debt” can be repaid to their muscles. The result of anaerobic activity is a build-up of lactic acid in the blood. Although crocodiles can withstand higher levels of blood acidity than other animals, sometimes it can be fatal.
Until 1974, estuarine crocodiles in Queensland were hunted to the brink of extinction for their prized skins. Both species are now protected throughout Australia, but other pressures continue to threaten these animals.
It is estimated that less than one percent of eggs laid by estuarine crocodiles hatch and survive to adulthood. Overheating, flooding and predation by goannas and feral pigs claim a high proportion of unhatched embryos (an estimated 70–80 percent). From the small numbers that do hatch, more than half die in their first year of life, mainly from predation by birds of prey, fish, snake-necked turtles and other crocodiles. Once they have reached maturity their only enemies are each other and humans.
Habitat destruction is now considered a major threat to crocodile survival in Queensland. Increasingly, humans are crowding in on crocodile territory – developments in swamps, mangroves and rivers are displacing crocodiles from their homes.
Did you know?
Estuarine crocodiles are unique in the reptile world in that they use their blood system to remove salt from their body. Salt glands embedded in their tongue tissue excrete excess salt when the animal is living in a highly saline environment.
Text © Queensland Goverment (Dept of Environment and Heritage Protection)
Image © Jona Photography