Saltwater crocodiles enjoy catching a wave and can travel hundreds of kilometres by "surfing" on ocean currents, a study suggests.

Australian researchers used sonar sensors and satellite transmitters to monitor 20 reptiles' movements.

They found the crocodiles undertook numerous trips of over 10km (6.2 miles), but only when a current flowed in their direction of travel.

The results of the research appear in the Journal of Animal Ecology.

The TV personality Steve Irwin, who was nicknamed The Crocodile Hunter, but died in 2006, took part in the study.

Estuarine or saltwater crocodiles are the world's largest reptiles and can grow up to 5.5m in length.

They are poor swimmers and despite their name, they inhabit fresh water as well as estuaries. Their "home" range, however, spans over thousands of kilometres of South-East Asia.

Researchers have long been puzzled by how crocodiles managed to spread themselves so widely.

"Of all the amazing things animals can do, the ability of certain species to migrate significant distances across formidable geographical barriers is one of the most remarkable," write the authors of the recent study.

Although the crocodiles spend most of their life in salt water, they are not considered marine animals as they rely on land for food and water.

The open sea

During the research, a team led by Dr Hamish Campbell, from the University of Queensland, captured 20 crocodiles living in the North Kennedy tidal river in Queensland, northern Australia, and tagged them with satellite transmitters.

They found that during the period of study, eight of them ventured out into the open ocean. One travelled from the river mouth all the way to the west coast of the Cape York Peninsula, in Queensland's far north. That amounts to a total of 590km covered over 25 days.

To do that, the ocean-trotter hitched a ride on a current within the Gulf of Carpentaria (that separates Cape York from Arnhem Land, to the west). This current occurs seasonally, during the summer monsoon.

Image caption 20 crocodiles were tagged with satellite transmitters

"[These crocodiles] can survive for long periods in saltwater without eating or drinking, so by only travelling when surface currents are favourable, they would be able to move long distances by sea," commented Dr Campbell.

It took another adventurer - a 4.84m-long male - just 20 days to go more than 411km from from the east coast of Australia's Cape York Peninsula through the Torres Strait (which divides Australia from New Guinea) to the Wenlock River on the west coast of Cape York.

When the crocodile arrived in the Torres Strait, strong currents were flowing in the opposite direction to where it was headed.

So the animal waited in a sheltered bay for four days and continued its trip when the currents changed direction.

Important clues

The scientists also tagged 27 crocodiles with sonar transmitters and spent a year tracking their every move inside the North Kennedy River with underwater receivers.

They found that both male and female crocodiles regularly travelled more than 50km from home, swimming to the river mouth and back.

But the team discovered that crocodiles would only set out on a long journey within an hour of the tide changing. This allowed them to "catch a wave".

They put their trips on hold when the tides reversed, moving out of the river and on to the banks.

Dr Campbell said that the results of the study gave important clues to understanding the evolution of the world's largest reptiles.

"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," he said.