I think that crocodiles
are quite fascinating animals and the more that people know about them,
the better they can understand them. Hopefully by understanding them, people may develop an appreciation of crocodiles which in turn may
lead to some rational decisions being made about their long-term conservation
and management here in Australia.
are the worlds largest and perhaps most exciting reptiles. They are
also great survivors and their prehistoric ancestors, the Archosaurs,
date back over 240 million years to the Triassic period. They have survived
major upheavals such as the break up of the continents and the ice ages.
They have witnessed the rise and fall of the dinosaurs and have seen
the evolution of mammals and birds. Since man colonised the world, no
species of crocodile has become extinct; however 17 out of the 23 species
of crocodilians around the world are endangered.
They have distinctive features such as long jaws, protective armour,
streamlined body and long tail. These, together with various anatomical
and physiological adaptations, make the crocodile perfectly suited to
an aquatic and predatory lifestyle. Their features have changed very
little from those of their prehistoric ancestors, proving that their
body form has been highly successful in nature. Some experts believe
that the crocodile, in its present form, has not changed for the last
100 million years. Maybe a perfect design?
Crocodiles are cold blooded and have a body temperature similar
to the surrounding air, land or water. Since they lack a reptilian thermostat,
they seek a habitat with warm water and air temperature all year round.
Though much maligned, crocodiles play an important role in wetland environments.
They help keep the balance in the complex web of life in freshwater
and estuarine ecosystems. They are key predators at the top of the food
chain and eat a wide range of prey. They are also prey, when smaller,
to other animals such as feral pigs, goannas, turtles, barramundi, sea
eagles and even other crocodiles. Being predator and prey, the crocodile
plays an important role in keeping a wetland ecosystem healthy and when
a wetland habitat is healthy, the fishery is considered to be healthy
The Freshwater crocodile (Crocodylus johnstoni) is endemic to Australia.
It is found nowhere else in the world. "Freshies" occur only
in inland waters of Northern Australia, and in Queensland; they are
found mainly in the rivers and swamps of Cape York Peninsula, areas
bordering the Gulf of Carpentaria and the northwest. They also live
in the tidal reaches of some rivers and therefore co-exist at times
with estuarine crocodiles. They have a narrow snout and reasonably straight
jaw line with even sized teeth. Males can grow to 3 metres but animals
larger than 2.5 metres are rare. Females rarely exceed 2.5 metres. They
are not "man-eaters" and feed mainly on insects, frogs, lizards,
turtles, with bats, birds and small mammals being taken occasionally
at the water's edge. Even larger "freshies" tend to eat very
small food items.
Estuarine (Saltwater) crocodile (Crocodylus porosus) is found in the
warm climate from Sri Lanka and India in the west to the Caroline Islands
in the east, to the north from Burma and South-East Asia, to Australia,
Vanuatu and the Solomon Islands in the south. In Australia, they are
restricted to northern parts of the continent, from about Rockhampton
on the east coast of Queensland to Broome on the west coast of Western
Australia. Estuarine crocodiles are the largest of living crocodilians
and quite probably the largest living reptiles on earth. They have broader
snouts than "freshies" with an uneven "up and down"
jaw line and irregular sized teeth. They vary in colour from grey, olive-brown
to almost black, with ragged dark mottling. Males can grow to 7 metres
(23 feet) but most are less than 5 metres. Females are usually less
than 4 metres in length and may begin nesting at about 12 years of age.
Maximum lifespan is unknown however it is estimated that they can live
for at least 70 to 100 years.
The skin of the saltwater crocodile, especially from the belly surfaces,
is the most prized of all crocodilian skins for fashion leather. Demand
for skins (for use in shoes, belts, suitcases, briefcases and handbags)
supported a significant crocodile harvesting industry in northern Australia
between the end of World War II and the 1960s. Two factors led to the
industry's decline and final cessation. Firstly, saltwater crocodiles
progressively became more difficult to find. Their populations declined
due to relentless and skilled hunting. Secondly, by the late 1960's
early 1970's, conservationists and governments were concerned that the
species might become extinct in Australia. At a time when the industry
had almost literally 'killed the goose that laid the golden egg', federal
and state governments protected crocodiles by legislation. "Freshies"
have oesteoderms (boney buttons) in their belly scales, are much smaller and
have slower growth rates. This greatly reduces the value of their hides
and makes them less attractive for leather.
"Salties" breed in the wet season (Nov-Mar) and build
a nest consisting of a large mound of vegetation and soil. The nest
is usually located in grass or fringing forest along the banks of a
watercourse or freshwater swamp. About 50 eggs are laid inside the nest
mound and incubation takes between 65 and 110 days. The female usually
guards the nest vigorously as she hides in a nearby wallow. The incubation
temperature determines the gender of the hatchlings with very high or
low temperatures producing females, and temperatures of 31º to
32ºC producing males. Pigs and goannas eat crocodile eggs and floods
destroy many nests. These factors contribute to the what sounds like
an alarming statistic that about 25% of eggs laid will hatch. From those
hatchlings that emerge, less than 1% survives to adulthood. This is
about average for most species in nature.
Young crocodiles are born with a horn or egg tooth on the tip of
their snout, which later drops off. This enables them to break out of
the egg. When hatching, young crocodiles squawk to attract the mother,
who may in turn dig them out and carry them gently to the water in her
mouth. Unhatched eggs are sometimes rolled around on the mother's tongue
to help the young emerge. Hatchlings are about 70 grams and about 25-30
centimetres long and may be protected as a crèche for several
months in the water by the mother. More than half the hatchlings die
in their first year of life because young crocodiles become prey for
other animals. A high proportion of juveniles are displaced from rivers
by larger crocodiles and many die during their exile. They are also
known to have a homing instinct with some specimens having travelled
more than 60km to return to their capture site.
Smaller crocodiles appear to feed throughout the year, reducing
their intake during cooler periods. Larger crocodiles are affected more
by cool weather and their food intake is greatly reduced or can stop
altogether. They can live for months at a time without feeding as they
carry extensive energy supplies in the form of fat. The wet season seems
to be the period when growth and feeding are maximised in crocodiles
of all sizes. Young crocodiles eat small animals such as crabs, prawns,
fish, frogs and insects. Larger crocodiles take bigger prey including
pigs, birds, reptiles, turtles, wallabies and even other crocodiles.
This cannibalistic behaviour is believed to be an important population
Crocodiles have a tendency to retain hard, indigestible objects
in their stomach and most crocodiles over two metres long have been
found to have stones in their stomach. It is believed that the stones
are used as gastroliths (to aid in grinding up food) and that they also
may function as ballast (just as in a ship's hull). This tendency for
retention of hard objects can also confound coroners' inquests where
people have been killed by crocodiles. Along with the human remains,
bullets are often recovered from the stomachs of large crocodiles. The
bullets usually come from other animals that have been shot and have died,
and are later eaten by crocodiles. The animals are digested but the
A unique feature of crocodilian physiology is their ability to maintain
strenuous activity for only short periods of time, after which they
become totally exhausted. This can occur during the capture of prey,
being captured or even fighting other crocodiles. This 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.
The result of anaerobic activity is a build up of lactic acid in the
blood, making it acidic. Although crocodiles can withstand higher levels
of blood acidity than other animals, sometimes it can prove fatal. This
is why larger crocodiles, over 5m, often die during capture operations,
if they are allowed to struggle excessively.
Crocodiles use a combination of active hunting and the more passive
"sit and wait" strategy. Juveniles tend to position themselves
in shallow water with all four feet on the bottom and wait until potential
prey comes within striking distance of the jaws. The movement of prey
is detected by the enlarged sensory pits along the sides of the jaw.
The most common strategy of larger crocodiles involves an underwater
approach to potential prey on the bank, in the water at the bank or
in overhanging vegetation. Once a crocodile is attracted by the movement,
sound and perhaps smell of potential prey, it will orient its head towards
the prey, submerge (usually without a ripple), and swim underwater until
it reaches the immediate vicinity of the prey. Then as the head silently
emerges, if the prey is within striking distance, it will lunge with
the jaws opening then slamming shut. A crocodile can lunge more than
half its body length into the air or out on the bank. Once caught, small
prey is usually crushed and swallowed. Larger prey is squeezed tightly
until all movement stops. The largest prey evokes the full attack sequence.
Once the crocodile has a grip, it will roll to throw the prey off balance
so it can be dragged into deeper water and drowned. Because the stomach
of the crocodile is small relative to the size of some prey taken, head
shaking, thrashing and rolling is used to dismember large prey into
smaller pieces for eating.
Estuarine crocodiles are unique in the reptile world and use their
blood system to remove salt from the body. Lingual glands at the back
part of their fleshy tongue excrete excess salt when the animal is living
in a highly saline environment. They also are one of the few reptiles
to have a four chambered heart (just like us) and have the ability to
slow their heart rate down to one beat every thirty seconds or so. They have been observed in captivity holding their breath for up to four to six hours underwater. In the wild in Queensland, one crocodile was observed making a voluntary dive and holding its breath for three hours and ten minutes. Salties have
a protective translucent third eyelid called the nictitating membrane,
which closes sideways across the eye. This allows them to see and swim
at the same time. It is just like swimmers' goggles, "crocodile
style". At night, crocodiles are easily detected by their "eye
shine" which is the red reflector look that they assume when illuminated
in the dark. The tail of the estuarine crocodile is 49.5% of its total
body length, the longest of any crocodile. The tail is used to propel
the animal through the water and scutes (spikes) along the top of the
tail are an important part of the tail. Not only do they increase the
surface area and therefore thrust for the tail, they are made of cartilage,
have a good blood supply and are an important device used for temperature
Although crocodilians have a rich repertoire of behaviours, there
is little information available on those of saltwater crocodiles. In
the "daily life" category of behaviours, regulating body temperature
is perhaps the most obvious. With a preferred body temperature of 30°
to 33° C, crocodiles use the water, sun and shade to regulate their
body temperature and move between these warm and cool parts of their
environment to adjust it. They are regularly seen basking in the sun
with a mouth gaping posture. One theory is that this is done to cool
the brain through evaporation from the palate while the body is heated
by the air and sun. Other theories include strengthening jaw muscles
or maybe a display to other crocodiles. One of the most common displays
seen during interactions between crocodiles is "snout lifting".
This signals, "I give in". It is very common when large crocodiles
approach smaller ones. Salties adopt an inflated posture that seems
to be a threat display. This is often accompanied by "tail arching",
a display in its own right but also a means of making the body rigid
to enable the head to be swung with power. The same posture can be adopted
before "head slapping", which signals a crocodile's presence
to all around. Saltwater crocodiles are rated out of all crocodilians
as the most intolerant of other members of the same species. One behavioural
trait that is well documented is that saltwater crocodiles are highly
territorial. Little is known about the territory sizes or the frequency
with which successful challenges to territories are made.
The Environmental Protection Agency, Queensland's nature conservation
authority, are attempting to meet the challenge of protecting crocodiles
and preventing their extinction, while trying to ensure that people
can safely co-exist with these animals. However, despite wide community
awareness of and response to warning signs, and educational information
provided by fauna authorities, occasionally crocodiles continue to pose
a threat to people.
"Australian Crocodiles - A Natural History" - Graham Webb
Attack in Australia" - Hugh Edwards 1998
Mark Read - Senior Conservation Officer - Queensland Environmental Protection
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