Australian freshwater crocodile, Johnstone’s river crocodile, ‘Freshie’
Freshwater crocodiles are significantly smaller than their cousins the Saltwater crocodile. They grow to 3m long and are much more slender in the snout and the body. Their colour is grey to tan-brown and they have dark patches on the sides and the top of the body and large smooth scales covering their body. They have a laterally compressed tail to help them swim fast and propel them vertically out of the water. Their tail also has scutes or spines running down the length that help regulate temperature. Their 66 teeth poke out the sides of their mouth on the top and bottom jaw when it is closed, differentiating crocodiles from alligators. Crocodiles are excellent in the water, they have webbed feet, and their eyes, ears and nostrils are on the top of their head to help them hunt undetected by ambushing their prey. Their eyes help them to find their prey underwater, as each eye has a third transparent eyelid called a nictitating membrane that acts as a pair of goggles.
Freshwater crocodiles will feed on a variety of animals including crustaceans, insects, spiders, frogs, turtles, lizards, snakes, birds and mammals. Their most common food item is fish and they use the ‘sit and wait’ method to catch their prey. Smaller crocodiles will eat insects and baby dead mice.
Freshwater crocodiles live in various freshwater habitats including rivers, creeks, pools, billabongs, lagoons and swamps, however despite their name they can also live in brackish waters.
Freshwater crocodiles are commonly found in all rivers, streams and creeks that flow into the waters of Northern Australia. Their range extends from the south west kimberley region in WA to Queensland.
Freshwater crocodiles are active all year round. During the wet season their habitat floods and they are able to move freely between bodies of water. During the dry season they can travel long distances in search of a large pool that they can inhabit, which they do by walking up on all fours with their belly off the ground. Despite mainly living in freshwater habitats, they can also survive in saltwater due to their internal salt glands. Like all other crocodiles these glands secrete sodium and potassium in concentrations higher than that of the blood and balances the levels in the body. In the dry season the crocodiles sometimes lie dormant on land, so these glands also help them to excrete excess salt.
Freshwater crocodiles, like turtles, are at risk of becoming endangered due to climate change because of their reproductive strategy. Courtship begins in the dry season, egg laying occurs during August and September, where the female digs burrows and lays the eggs in them for protection. The time of incubation and the gender of the hatchlings in these nests is dependent on the temperature. Eggs incubated at temperatures 31-32 degrees will produce males, 29-29 degrees will produce females, and eggs incubated at 30-31 degrees will produce embryos of both sexes. Therefore with the increase in overall climate, more males will be produced and will eventually result in a decline in population due to there being no females to mate with.