The Australian estuarine crocodile, Crocodylus porosus is the world’s largest living reptile. Best known from the ‘Crocodile Dundee’ movies, and the late Australian wildlife expert and crocodile hunter Steve Irwin’s documentaries, the reptile inhabits freshwater and estuarine habitats in Northern Queensland and the Northern Territory.
However, the widespread distribution of this one species of reptile throughout the islands of the whole South-east Pacific led researchers to believe that individual crocodiles don’t stay in their estuaries but regularly go on quite long ocean voyages.
In 2006, Steve Irwin and other Australian researchers were able to track salt-water crocodiles by satellite, showing they could travel more than 400 kilometres, swimming about 30 kilmetres a day.
Catching a Wave Saves Crocs’ Energy
Now the same team from the University of Queensland’s School of Biological Sciences with colleagues from Australia Zoo and Queensland Parks and Wildlife Service, has used underwater acoustic tags together with satellite tracking to discover how the crocodiles can travel so far and so fast without tiring.
Speaking at the media briefing on the study, UQ’s Dr Hamish Campbell said there had been many anecdotal accounts of large crocodiles being seen far out to sea, but that the study was the first to show that the crocodiles used the ocean currents just like a surfer riding a wave, to travel between islands.
Crocs Wait Until Current is Going their Way
Dr Campbell and his team tagged 27 adult crocodiles from the remote Kennedy River area of far North Queensland, attaching sonar transmitters to the reptiles’ bellies, then tracked their movements over 12 months with underwater receivers.
They found that every individual crocodile, both male and female, regularly travelled more than 50 km, going from their home area to the river mouth and beyond into open sea.
Over the same period, the surface currents between the Kennedy River mouth and the open sea were measured. These measurements were then overlaid onto the data from the crocodiles’ acoustic tracking and satellite imaging, when it could be seen the crocs were using the currents to save their energy.
“The data showed that crocodiles always began long-distance travel within an hour of the tide changing, allowing them to go with the flow, and that they halted their journeys by hauling out on to the river bank when the tide was against them,” Dr Campbell said.
“By only travelling when surface currents were favourable, individuals would be able to move long distance by sea,” he said.
“This not only helps to explains how estuarine crocodiles move between oceanic islands, but also contributes to the theory that crocodilians have crossed major marine barriers during their evolutionary past.”
Crocs Travel Hundreds of Kilometres Over the Ocean
The estuarine crocodile’s geographical range is huge. It covers over 10,000 square kilometres of the South-East Pacific, from East India and Sri Lanka throughout Southern China to Thailand, the Philippine and Sunda islands including Sumatra, Java, Borneo, Celebes, and Timor to North Australia, Vanuatu, Fiji and the Solomon islands.
The UQ study showed some large individuals were capable of travelling almost 600 kilometres in less than four weeks, an average of 23 kilometres a day. Dr Campbell described a 3.84 metre-long male which left the Kennedy River and travelled 590 km over 25 days down the west coast of Cape York Peninsula. The reptile timed its journey to take advantage of a seasonal current system that develops in the Gulf of Carpentaria and flows down the coast.
Another large male, 4.84 metres long, travelled more than 411 km in only 20 days from the east coast of Cape York Peninsula through the Torres Straits to the Wenlock river on the west coast of Cape York. Dr Campbell said the Torres Straits currents are very strong, and by the time the animal had reached the Straits the current was flowing against him. The reptile slipped into a sheltered cove and waited for four days until the current was flowing in the right direction for it to continue its journey.
The study, ‘Estuarine Crocodiles Ride Surface Currents to Facilitate Long-distance Travel’ was published in the June 2010 edition of the British Ecological Society's Journal of Animal Ecology.
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Sue Cartledge - I'm a science, health, nutrition and lifestyle journalist, fascinated by the way the physical world operates in all its forms, and how ...
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