Saltwater Crocodiles

Fast Fact Takeaways – Salties The focus of this edition will be another much maligned predator, one that doesn’t animate our thoughts too often, as it prefers places of low population density. In tropical tidal rivers, and out at sea, it is the top predator in a way that Great Whites could only dream about. The 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 Onslow on the west coast of Western Australia. 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. “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. Pigs and goannas eat crocodile eggs and only about 25% of eggs will hatch. 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 can starve. From those hatchlings that emerge, less than 1% survives to adulthood. 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. 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. Behaviour Crocodiles spend a fair bit of their time diving under the water, foraging, resting and – when they’re small – escaping predators. Since crocodiles are cold blooded animals, their body temperature and metabolic rate is affected by the temperature of the environment. Page 12 They 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. Crocs can maintain strenuous activity for only short periods of time, after which they become totally exhausted. This can occur during the capture of prey, or while fighting other crocodiles. Exercise must be followed by a period of rest as they build up an oxygen debt. Larger crocodiles, over 5m, often die during capture operations if they struggle excessively. The warmer it is, the higher their metabolism and oxygen use. In summer time they need more time to recover from longer dives, or fights. They are often seen resting on the surface or basking in the sun during the day. Crocodiles use a combination of active hunting and the more passive “sit and wait” strategy. Juveniles tend to wait until potential prey comes within striking distance. Once a large 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. Usually on the bank, in the water at the bank, or in overhanging vegetation They also like hanging around boat ramps where people are carelessly cleaning fish. After the strike, larger prey is squeezed tightly until all movement stops. The largest prey evokes the full attack sequence with rolling to throw the prey off balance so it can be dragged into deeper water and drowned. Because the stomach of the crocodile is small, head shaking, thrashing and rolling is used to dismember large prey into smaller pieces. They have a great memory for food and will identify and set up ambushes in spots where prey often visits. They can lie in wait for days. Never revisit the same spot in a river to collect water, and don’t linger. Movements Salt-water Crocodile hatchlings remain near the nest for up to two months although they can disperse up to 9 km in their first month, with movements over 20 km recorded within six months. Within one year, over 90% of surviving crocodiles are within 5 km of their nest site. Crocodiles between 2 and 6 years of age may travel up to 80 km from their nest site. A study in the Wenlock River showed that adult crocs tend to roam up and down one home river system most of the time. The majority of males stay in a routine home range and defend it, but some ‘nomadic batchelors’ move long distances around a river system, especially at night. These batchelors average 380 metres of travel each night and 230 metres during the day. Over a six month period they can disperse over 100 kilometres from their original stretch of river. Some of these nomadic males will occasionally try and move to places where there is less competition. A high majority (>75%) of crocs captured in relocation traps are males between 2 and 3 m total length. The movements of relocated animals demonstrate their ability to make long distance movements (up to 280 km) to and from home and their sites of capture. Females move a lot less and they were also less active than the males at night. While nesting from November through until March, the females will travel long distances looking for suitable nesting sites, especially favouring sand banks around the river mouth.