Animal Species:Freshwater Crocodile
This shy and secretive crocodile was described by Australian Museum curator, Gerard Krefft, after receiving a specimen from Mr R. A. Johnstone who collected the first specimen from Queensland. Unfortunately, over one hundred years passed before proper study of this species was undertaken.
Freshie, Johnstone’s River Crocodile, Johnstone’s Crocodile, Fish Crocodile
The Freshwater Crocodile is slender-snouted and considerably smaller in build and overall size compared to its cousin, the Saltwater Crocodile Crocodylus porosus. Its colour ranges from grey to tan-brown, with dark patches along the sides and top of the body. The nostrils and eyes sit at the top of the head and the fine sharp teeth are clearly visible even when the mouth is closed. The powerful tail features large triangular scales known as ‘scutes’ along its length, which is almost half the total length of the animal. The hind limbs are considerably larger and longer than the forelimbs; this is most noticeable when the legs are stretched in the water. When resting, the limbs are held tight to the body.
to 3 metres
The species occurs along all but the near coastal reaches of the rivers, streams and creeks that flow into the waters off northern Australia between King Sound in the south-western Kimberley, Western Australia and the northern part of Cape York Peninsula, Queensland. They occasionally occur in near coastal areas where Saltwater Crocodiles are absent, but are much more common in inland areas.
Freshwater Crocodiles inhabit various freshwater environments, including rivers, creeks, pools, billabongs, lagoons, and swamps. During the wet season these habitats become inundated with flood waters which allow the crocodiles to move throughout the flood plains. As the water levels drop the crocodiles tend to congregate in the larger and deeper water bodies, where they prefer to inhabit the shallower waters at the pool edges. Despite the periodic flooding and drying of their habitat, Freshwater Crocodiles show a strong fidelity to their dry season water body, e.g. along the McKinlay River in the Northern Territory, 72.8 percent of marked crocodiles were found to return to the same water body in two successive dry seasons.
Freshwater Crocodiles may shelter in burrows among the roots of trees fringing the water bodies they inhabit.
Despite the common name, Freshwater Crocodiles may also occur in brackish waters up to 24% salinity (seawater is 35%).
Freshwater Habitat: freshwater
In areas where there is permanent water, Freshwater Crocodiles can be active year-round. However they may become dormant in areas where the water dries up during the dry winter season. These crocodiles over-winter in shelters dug into the creek bank, and a number of animals will often share the same shelter. A well-studied aestivation site in the Northern Territory consisted of a cavern in an undercut creek bank, 2m below the top of the bank, where the crocodiles would lay dormant between late winter and late spring (last week of August – mid-December).
Feeding and Diet
In the wild, Freshwater Crocodiles will eat a variety of invertebrate and vertebrate prey, including crustaceans, insects, spiders, fishes, frogs, turtles, lizards, snakes, birds and mammals. Insects (both aquatic and terrestrial) appear to be the most common food item, followed by fish. Larger crocodiles tend to eat larger prey items, however the average size of prey for all Freshwater Crocodiles is generally small (mostly less than 2cm²). Small prey is usually obtained by a ‘sit-and-wait’ method, whereby the crocodile lies motionless in shallow water and waits for fish or insects to come within close range, before they are snapped up in a sideways action. However, larger prey like wallabies and waterbirds may be stalked and ambushed in a manner similar to that of the Saltwater Crocodile.
Freshwater Crocodiles are known to be cannibalistic, with larger individuals sometimes preying on hatchlings.
In captivity, hatchlings will feed on crickets and grasshoppers, while larger young eat dead baby mice and chopped adult mice.
Other behaviours and adaptations
Freshwater Crocodiles can perform a ‘high walk’ to move overland, whereby the body is held high so that the belly and most of the tail is not touching the ground. Track marks indicate these crocodiles may walk considerable distances at the end of the wet season in search of a dry season refuge. When startled on land a Freshwater Crocodile may leap into a fast ‘gallop’ and rapidly sprint to the water.
Like most crocodiles, Freshwater Crocodiles have salt glands in the tongue. These glands, numbering around 20-26, secrete sodium and potassium in concentrations higher than that of the blood. It is not clear why this primarily freshwater species has salt glands, however one explanation may be that the salt glands exist as an important means of excreting excess salt and maintaining internal water balance during the dry season when the crocodiles lie dormant on land. A second possible explanation is that, given that the species may occasionally inhabit saline waters, the excess salt can then be excreted though the salt glands.
In captivity, Freshwater Crocodiles can be very aggressive to one another. Juveniles less than three months old will bite each other on the head, body and limbs, and juveniles up to six months old continue to bite each other, sometimes with fatal consequences. In the wild, a large male will usually dominate a congregation, and attack and bite the tails of subordinates as a means of asserting dominance.
Life history mode
Mating and reproduction
In the Northern Territory courtship and mating begins at the start of the dry season (June), with egg-laying taking place around 6 weeks later. Courtship in captive Freshwater Crocodiles involved the male placing his head on top of the female’s and slowly rubbing the glands underneath his throat against her before copulation took place.
The egg-laying period typically lasts for four weeks through August and September. About three weeks before egg-laying starts the gravid female will begin excavating a number of ‘test’ holes at night, usually in a sandbank within 10m of the water’s edge. In areas where there are limited suitable nesting sites, many females may choose the same area, resulting in a number of nests being accidentally dug up. The egg chamber is hollowed out primarily with the hind foot, and its depth is largely determined by the length of the hind leg and the type of substrate.
Clutch size ranges 4-20, with on average a dozen eggs being laid. Larger females tend to have more eggs in a clutch than do smaller females. The hard-shelled eggs take between two to three months to hatch, depending on the nest temperature. Unlike Saltwater Crocodiles the females do not guard the nest; however they will return and excavate the nest when the eggs have hatched, honing in on the calls of the young inside. Once the young have been uncovered the female helps carry them down to the water and will aggressively protect them for a period of time.
Predators, Parasites and Diseases
Goannas are the major predator of eggs in nests - in one population in the Northern Territory, 55 percent of 93 nests were disturbed by goannas. Once they emerge the hatchlings face many predators including larger crocodiles, freshwater turtles, sea eagles and other predatory birds, large fish and pythons. Most will not survive to one year of age, however those that do have a much improved chance of survival.
Mature animals have few enemies besides other crocodiles and the toxic Cane Toad Bufo marinus, which is thought to have seriously impacted on some populations of Freshwater Crocodiles after the discovery of many dead crocodiles with toads in their stomachs.
The species' recorded parasites include nematodes (round worms) and trematodes (fluke worms).
Conservation Status (IUCN):
LEAST CONCERN (LC)
Both species of crocodiles are protected in Australia; wild specimens cannot be destroyed or collected without a permit from wildlife authorities. A permit is required to keep this species in captivity, with both species seen as unsuitable pets in some States and Territories. Please see the Resources for keeping live animals page on the Live Exhibits section and check with your local wildlife licensing agency.
Danger to humans and first aid
Unlike the highly dangerous Saltwater Crocodile, this species is generally shy and quick to flee from human disturbance. Swimmers however may risk being bitten if they accidentally come in contact with a submerged crocodile. When threatened in the water a defensive crocodile will inflate and shudder its body, causing the surrounding water to ripple violently, whilst gaping and emitting a low-pitched warning growl. If approached too closely, the crocodile will make a quick snapping bite, causing lacerations and puncture wounds. A bite from a large Freshwater Crocodile can cause serious damage and infection from the deep punctures which may take many months to heal.
- Webb, G. and Manolis, C. 1988. Australian Freshwater Crocodiles. Conservation Commission of the Northern Territory.
- Cogger, H. G. 2000. Reptiles and Amphibians of Australia. Reed New Holland. Sydney.
- Ehmann, H. 1992. Encyclopaedia of Australian Animals: Reptiles. Angus & Robertson. Pymble.
- Greer, A. E. 2006. Encyclopedia of Australian Reptiles : Crocodylidae. Australian Museum
- Webb, G. and Manolis, C. 1989. Crocodiles of Australia. Reed. Chatswood.
Chris Hosking , Interpretive Officer