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
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
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.
Tell the differences: freshies and
The best way to tell the two Australian
crocodile species apart is to look at the skull and jaws. The skull
of an estuarine crocodile (below left) is broad and the jawline is
irregular, whereas the skull of a freshie (right) is narrower and
the jawline straighter.
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
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.
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
Don't make a
meal of yourself!
• Obey crocodile warning signs, even if
you can’t see crocodiles.
• Don’t go swimming or paddling in
areas where crocodiles live.
• Keep well away from the water’s
• Don’t gut fish, discard food
scraps or wash dishes at the water’s edge.
• Don’t camp within 50 metres of
• Be careful in boats; don’t hang a
foot over the edge.
• Don’t sit in branches above the
water—crocs can jump.
• Supervise children and be moderate with
alcohol intake in croc country—many victims of crocodile
attacks have become careless when intoxicated.
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.
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.