The Freshwater Crocodile
is the more timid relation of the much larger and aggressive Estuarine
(Saltwater)
Crocodile (Crocodylus porosus). Unlike the Estuarine Crocodile,
there have been no reports
of serious unprovoked attacks by Freshwater Crocodiles.
The Freshwater Crocodile
is native to Australia, inhabiting rivers, creeks, permanent streams
and billabongs from the Kimberley to Cape
York Peninsula. Although it can tolerate the salty water of estuaries,
it is usually outcompeted by the Estuarine Crocodile.
The Freshwater Crocodile is an
ambush predator, lying in wait for prey to come within range, when,
in a flash, it catches its prey. The long, narrow
snout and
sharp teeth are adaptations for catching aquatic prey, including
fish, frogs, lizards and turtles but it also eats crayfish,
insects and spiders. Digestion is assisted by gastroliths, stones
swallowed
by the animal, which aid in grinding up food. Although it has
long been thought that these stones might also play a role in controlling
flotation of the animal, recent work suggests that this role might
be minimal.
Freshwater
Crocodiles can live for 50 or more years, most of their growth
occurring in the first 20 years. Males are up to 50kg and 3m long
(but typically less than 2.5m); females up to about 2m. Their growth
rate
(and therefore,
size) depends very much on food availability. Males become
sexually mature at about 16 years; females at about 12 years.
Like many other animals of the Top End, Freshwater
Crocodiles move late in the wet season to spend the dry season near
permanent water.
Mating occurs around July (in the season that the Bininj/Mungguy
people of Kakadu call Wurrgeng) in the water and about 12 eggs are
laid on one night in August or
September
(in the season of Gurrung).
Females use the same nesting sites repeatedly and
the nesting hole is dug in the sandy river bank, about 10m from the
water. Good maternal instinct is vital to the survival of the brood.
The location of the nest must be above flood waters before hatching
or an early wet season can flood nest, drowning the embryos. The
depth of the nest determines heat gain from the sun, which affects
development of the embryos.
Temperature plays another role, too. As in other
crocodiles, some lizards, snakes, fish and turtles, the sex of hatchlings
is determined by incubation temperature: 31-33°C produces mostly
males; above or below, mostly females. (Climate change may have some
implications here.)
The eggs are left unprotected. In the
season of Gunumeleng, after some 9-13 weeks of incubation (and, hopefully,
before the first floods),
the
adult
females return to the nest. When
a young crocodile hears the female patrolling the area, it calls
to her from within the egg and uses its small egg-tooth to start
breaking through the shell. The female digs out the eggs, helps the
hatchling out of the egg and carries the 25cm hatchlings to the water.
A large proportion of the nests are dug up by goannas
and feral pigs that eat the eggs. The eggs are also a favoured seasonal
food of the indigenous people. Only about 30% of the
eggs laid will hatch (in some areas, it might be as low as 5%)
and only about 1% result in mature adults.
Hunted for their skins until they became rare,
Freshwater Crocodiles have been protected from hunting since 1963
and numbers are
now estimated at around
100,000. Whilst habitat destruction is a threat, they occur mainly
in uninhabited or lightly habited areas and the biggest current threat
to Freshwater Crocodiles appears to be poisoning by the
Cane
Toad (Bufo marinus).
The other common name for this species
is the 'Johnston River Crocodile'. It's not named after the 'Johnston River'
but, rather, after a Queenslander named, ... well, you'll see in
a moment!
The species was first named 'C.
johnsoni'
by Krefft in 1873 but it is often referred to as 'C. johnstoni'
because Krefft (in a later letter) indicated that he had named
it after Robert Johnston of Queensland. (In fact, the man's name
was
Robert Johnstone, so the species should have been originally named
'C. johnstonei'!)
Under the rules that govern the naming of animals
(the International Code of Zoological Nomenclature, Article 32.5.1),
the name cannot be automatically corrected because the ICZN does
NOT regard this as a spelling error. One of the intentions of the
ICZN is to maintain stability of names, so that scientists around
the world can reliably refer to a particular species (which is not
possible by common names which can change with place and time). Accordingly,
(a very simplified interpretation of) one of the principal rules is that an animal's
correct name is the one it was first given.
When Krefft originally published
the name, he
dedicated it to 'Johnson' and the dedication to 'Johnston' occurred
only LATER, not in the ORIGINAL publication. Furthermore, the
validity of a proposed change via personal letter is also questionable.
The problem is that 'Johnson' is a real name. Had Krefft's original
publication spelled the
name 'ojhnstonei' instead of 'johnstonei',
the correction to 'johnstonei' would have been easy, i.e.
correction of what was clearly a spelling error.
So,
even though the dedication was wrong, the rules state that the
earlier name takes priority over the others, making the correct name ‘Crocodylus
johnsoni’.
Bad luck for Krefft and Johnstone!
Although this interpretation follows
strictly the rules for naming, there are people who would like to
see Krefft's original intentions fulfilled.
Traditionally, the crocodiles were
classified with lizards, snakes, turtles and dinosaurs into a group
called the ‘reptiles’. However, evidence from molecular
biology now suggests that the reptile grouping is unnatural, using
superficial features and not reflecting the evolution of these animals.
Surprisingly, the nearest living relations of crocodiles
are the turtles and birds; lizards and snakes are more distant
relations of crocodiles!
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