CSL
Crocodylus johnstoni (KREFFT, 1873)


NAMES | DISTRIBUTION | HABITAT | STATUS | APPEARANCE | IMAGES | DIET | BREEDING | CONSERVATION

FAMILY:
ALLIGATORIDAE

A. mississippiensis
A. sinensis
C. crocodilus
C. c. apaporiensis
C. c. fuscus
C. latirostris
C. yacare
M. niger
P. palpebrosus
P. trigonatus

FAMILY:
CROCODYLIDAE

C. acutus
C. intermedius
C. johnstoni
C. mindorensis
C. moreletii
C. niloticus
C. novaeguineae
C. palustris
C. porosus
C. rhombifer
C. siamensis
M. cataphractus
O. tetraspis
T. schlegelii

FAMILY:
GAVIALIDAE

G. gangeticus

DICHOTOMOUS KEY
[German]

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STATUS OF INFORMATION:
This information was most-recently updated January 2009 and is considered up-to-date. Please contact me directly regarding updates or corrections.

COMMON NAMES:
Australian freshwater crocodile, Johnston's crocodile, "Freshie" [less common: fish crocodile, Johnston's river crocodile]

> Crocodylus is derived from the Greek krokodeilos which means literally "pebble worm" (kroko = pebble; deilos = worm, or man) referring to the appearance of a crocodile.
> johnstoni means "of Johnston", derived from the name of the first European to discover and report it to Krefft. Unfortunately Krefft mis-spelled the name "johnsoni" in his initial description and his subsequent correction was ignored until 1983 when the nomenclature was reviewed thoroughly by Hal Cogger (Cogger 1983). Although the majority of scientific literature, including all Australian Federal, State and Territory legislation has been using "johnstoni" correctly since then, the uncorrected version is still popular especially in the US on the basis of a later taxonomic review (King and Burke 1989) that ignored Cogger's revision.

SUBSPECIES:
There are currently no recognised subspecies. Escarpment populations show a significant reduced maximum size (1.7 m) and size at maturity (0.7 m for females) and are referred to as "pygmy", "dwarf", or "Stone Country" crocodiles, but there is presently no evidence that these differences are due to genetic traits. Their reduced size has been linked to limited food availability.

[CLICK ON MAP FOR DETAILED RANGE]
Distribution map Northern Australia (Northern Territory, Queensland and Western Australia)

HABITAT:
Various permanent freshwater areas such as lakes, billabongs and swamps, plus less saline upstream areas of river systems and creeks. Marginal populations are found in upstream sandstone escarpment country. Generally not found near the coast, where the high salinity and competition with the more dominant C. porosus makes the habitat far less favourable. However, when overhunting reduced populations of C. porosus, the range of C. johnstoni expanded somewhat further into more saline areas. Recovered C. porosus populations have reversed this trend.

STATUS:
     CITES: Appendix II
     IUCN Red List: LRlc (LOW RISK, LEAST CONCERN)
     Estimated wild population: 50,000 to 100,000
Summary: Healthy populations and remaining habitat, considered recovered from historical population declines. Recent impacts from cane toads in some areas have led to localised population declines.

APPEARANCE:
[click on image for enlargement]
Head drawing The Australian freshwater crocodile is a relatively small crocodilian which rarely exceeds 2.5 to 3 m in the wild and takes many years (at least 30) to reach this size. Females general reach a maximum size of 2.0 to 2.1 metres. The shape of the snout is unusually narrow and tapering, lined with numerous sharp teeth (unlike the blunter teeth found in larger C. porosus). The body colour is light brown with darker bands on the body and tail - these tend to be broken up near the neck. Some individuals possess distinct bands or speckling on the snout. Body scales are relatively large, with wide, closely-knit armoured plates on the back. Rounded, pebbly scales cover the flanks and outsides of the legs.

Dentition codeDENTITION:
5 pre-maxillary; 14-16 maxillary; 15 mandibular
Total no. of teeth = 68-72

IMAGES:
[click on image for enlargement]

Click Adult freshwater crocodile Click Head of sub-adult crocodile Click Adult female gaping

DIET:
The "longirostrine" (long and slender) shape of the snout, like that of the gharial, suggests a primary adaptation to a piscivorous (fish-based) diet, and fish are certainly handled with ease. However, a wide variety of other prey items are often taken - generally invertebrates and small vertebrates, especially when younger. Larger individuals may take terrestrial species. C. johnstoni favours a sit-and-wait hunting strategy, snatching prey up with a lightning-fast sideways movement of the head. It rarely feeds during the dry season because of a lack of prey availability, low temperatures at night and a reduction in available habitat due to drying of pools and rivers.

BREEDING:
Males reach sexual maturity around 1.5 m in length, females only slightly smaller. Courtship occurs in the early months of the dry season (around May) and nesting generally occurs between July and September depending on geographic region. Females dig hole nests in sand embankments, exposed after the wet season water levels fall. Research has found that all females within a particular population usually nest within the same three week period - behaviour described as 'pulse' nesting. They may also nest in groups, with many nests being laid in close proximity (some females even dig up the clutches of other females when nest density becomes very high). Eggs are normally laid at night 6 weeks after mating, at a depth of 12 to 20 cm below the surface - too close to the surface and they risk being overheated from the sun. Incubation temperatures of between 30 and 33 degrees are desirable. Clutch size averages 13 eggs, but can range from 4 to 20. The incubation period is usually between 75 to 85 days (maximum range 65 to 95 days). Studies on temperature effects have shown that eggs incubated at 32C produce male embryos, whereas those a couple of degrees above and below this produce female embryos - however, greater sexual differentiation seems to occur with fluctuating nest temperatures. Varanus lizards and feral pigs are major predators of eggs during this period, when neither parent guards the nest. Varanids can easily find nests laid within 24 to 48 hours. On average for all nests laid, under one third of the eggs survive to hatching. The female parents return at the end of the incubation period and wait for the neonates to begin calling. They may then carry the newly-hatched juveniles to the water in their mouths, although the nests will still hatch out without the female being present. Adult females stay close to and protect the small crocodiles for a variable period following this, but protection is not as long as the sympatric C. porosus. Disturbance is more likely to result in the female abandoning the nest and her offspring. Cannibalism has been reported, often due to a scarcity of food. Only 1% of these hatchlings will survive to reach maturity, and in some years predation pressures are so high that it is unlikely that any new animals are recruited into the adult population. In some years, early rains at the end of the dry season can destroy almost all the nests through flooding. Juveniles which survive to maturity have been found returning to the same breeding and nesting areas.

CONSERVATION:
Good population data exist for this species, as well as the sympatric C. porosus. Hunting has been less of a problem than with C. porosus, mainly due to the presence of ventral osteoderms in the belly scales of adults. Long-term aboriginal hunting did not significantly affect the population, but advances in tanning processes at the end of the 1950s meant that C. johnstoni skins could be utilised when C. porosus populations became depleted. This hunting caused a widespread reduction in the population until protection measures were implemented in the 1960s and 1970s. Although a small amount of illegal poaching and killing no doubt persists, the major threat at the moment is the omnipresent one of habitat destruction. Recently, invasive species such as cane toads (Bufo marinus) have led to mortality of adults and presumably juveniles in otherwise healthy populations. The threat of these toads has not yet been ascertained properly, although it is likely that only dwarf populations are under any real long-term threat. Populations have recently recovered to a significant extent. Small-scale farming and ranching programs exist for commercial purposes, and monitoring and management studies which were initiated in the 1970s are still ongoing. Valuable long-term research is being conducted upon the population dynamics of this species, together with other studies looking at biology, physiology and development. Population estimates vary, as the species can be difficult to survey effectively, but it is not unreasonable to assume that there are at least 100,000 individuals in the wild.

MORE INFORMATION:
For more information on distribution and conservation issues for this species,see the CSG
Action Plan resource.

SIGNIFICANT REFERENCES:

  • Cogger, HG, Cameron, EE, Sadlier, RA & Eggler, P (1993). The Action Plan for Australian Reptiles. Australian Nature Conservancy Agency.
  • Cooper-Preston, H & Jenkins, RWG (1992). Natural history of the Crocodylia. In: Fauna of Australia Vol.2 (Eds. Glasby, CJ, Ross, GJB & Beesley, PL). Australian Govt. Publishing Service, Canberra. pp.339-343
  • Tucker, AD, Limpus, CJ, McCallum, HI & McDonald, KR (1996). Ontogenetic dietary partitioning by Crocodylus johnstoni during the dry season. J. Herpetol. 1996(4): 978-988
  • Tucker, AD, McCallum, HI, Limpus, CJ & McDonald, KR (1993). Crocodylus johnstoni in the Lynd River, Queensland. continuation of a long term field study. In: Crocodiles. Proceedings of the 2nd Regional Meeting of the Crocodile Specialist Group. IUCN, Gland, Switzerland. pp. 469-473
  • Webb, GJW, Manolis, SC & Buckworth, R (1983). Crocodylus johnstoni in the McKinlay river area, NT. I. Variation in the diet, and a new method of assessing the relative importance of prey. Aust. J. Zool. 30: 877-899
  • Webb, GJW, Manolis, SC & Sack, GC (1983). Crocodylus johnstoni and C. porosus coexisting in a tidal river. Aust. Wildl. Res. 10: 639-650
  • Webb, GJW & Smith, AMA (1984). Sex ratio and survivorship in the Australian Freshwater Crocodile Crocodylus johnstoni. Symp. Zool. Soc. Lond. 52: 319-355
  • Whitehead, PJ, Webb, GJW & Seymour, RS (1990). Effect of incubation temperature on development of Crocodylus johnstoni embryos. Physiol. Zool. 63(5): 949-964
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