From Tropical Topics newsletter, No. 78 June 2003, produced by Stella Martin at the Queensland Environmental Protection Agency. Download the PDF to read the whole issue.

Estuarine crocodile  |Freshwater crocodiles | Big hearts |Croc origins | Tell the difference: freshies and salties | Be wise in croc country | 

There are about 23 species of crocodilians in the world, two of them in Australia. Estuarine crocodile (Crocodylus porosus) and the freshwater crocodile (Crocodylus johnstoni). Pictured is the estuarine (saltwater crocodile). Photo: Deb Bisa

Estuarine crocodile (Crocodylus porosus)

This crocodile is found not just in Australia but also in South-east Asia, India and around western Pacific islands. It is sometimes called the saltwater crocodile, but this is a deceptive name because these animals are found not just on the coast (and occasionally in the open sea) but also far upstream and in many freshwater swamps and billabongs.

Almost any body of water in tropical Australia must be suspected of harbouring crocodiles of at least one species. Young estuarine crocodiles feed on insects, crabs, prawns and shrimps, but as they grow in size the amount of vertebrate material in the diet increases. Larger animals attack sea turtles, goannas, wallabies, pigs and even cattle; estuarine crocodiles over 3 metres in length are a danger to people. Very strong muscles are employed for closing the jaws, which can easily crush a pig’s skull—but a rubber band around the snout of a 2 metre crocodile is enough to keep it from using its weak ‘opening’ muscles.

Prey, grabbed in the jaws, is usually drowned. Contrary to popular myth, crocodiles do not store uneaten food and most certainly do not prefer rotten food. Adult male estuarine crocodiles are usually between 3.3 metres and 4.5 metres, but larger individuals over 6 metres have been recorded. Females grow to about 3 or 3.5 metres long.

Freshwater crocodiles (Crocodylus johnstoni)

This species is found only in Australia. It inhabits freshwater rivers, swamps and billabongs throughout northern tropical Australia from Broome in Western Australia to Princess Charlotte Bay in Queensland—but no further south on the east coast except where it has been introduced artificially. It does occur in tidal estuaries, but tends to avoid areas inhabited by estuarine crocodiles, which sometimes eat freshies.

This species is much smaller than the estuarine crocodile, males growing to about 2 metres in length—although a 3 metre male has been caught—and females to about 1.8 metres. Freshwater crocodiles feed on fish, insects, crustaceans, small birds, reptiles and frogs, often grasping them with the tip of the jaw and manoeuvring them further into the mouth. They do not prey on people but will bite in self-defence. Since some large freshies take wallabies, children should always be supervised near freshwater crocodiles.

Big-hearted crocs

Crocodiles have four-chambered hearts, like mammals and birds, whereas all other reptile hearts have three chambers. This difference in structure ensures that the blood flow system in crocodiles is more efficient. In addition, crocodiles can control the flow of blood throughout their bodies by increasing or decreasing the heart rate or by ‘shunting’ the blood flow to areas of importance such as the heart, brain and muscles and restricting the flow to nonessential areas like the intestines. However, some reptilian features allow oxygenated and deoxygenated blood to mix, an advantage for a crocodile when diving.

Croc origins

All vertebrates evolved from reptiles, which in turn developed from amphibians. However, the branch on the evolutionary tree which was to become lizards and snakes diverged from the branch which would give rise to birds, dinosaurs and crocodiles more than 300 million years ago. Interestingly, birds, dinosaurs and crocodiles all developed the same arrangement for the joining leg and foot at the ankle. This connection in lizard legs is quite different. Nonetheless, crocodiles share many characteristics with lizards. For example, they bask in the sun to raise their body temperature, lying sideways to give maximum exposure to the blood-rich bumpy scales which act as solar panels on their backs.

As with lizards, crocodiles can go without food for long periods. If a crocodile’s body temperature drops too much, digestive enzymes cannot work and food rots in the stomach instead. Both species decrease their food intake during the cooler months, and in more southern locations crocodiles may cease feeding altogether during winter. The growth of crocodiles during the cooler months decreases.