The giant crocodiles that ply Australia's rivers and bays are also savvy ocean voyagers: The 5-meter-long reptiles make good use of ocean and river currents to travel hundreds of kilometers, according to one of the most extensive monitoring studies of the animals ever attempted. And like experienced sailors, they prefer to navigate when the tides are in their favor.
Estuarine crocodiles (Crocodylus porosus) can be found throughout the southeast Pacific, from East India to southern China and from the Philippines and Java to northern Australia. They typically live in rivers but have been sighted in the open ocean. Two years ago researchers in Australia tracked crocs with satellites and discovered that they can swim 400 kilometers in the ocean in just 20 days—much farther than previously suspected.
Now, Hamish Campbell, a zoologist at the University of Queensland, Brisbane, in Australia and his colleagues have shown that to travel such astounding distances, crocs ride surface currents. To study crocodile movements in greater detail, they set up an acoustic tracking system in a remote river in North Queensland. They placed 20 listening devices at regular intervals along the 63 kilometers of the river that were tidal. Outfitting the crocodiles with ultrasonic transmitters was the hard part. "This takes a large team of experienced individuals," says Hamish. The team included experts from the Australia Zoo who had been trained by the famous crocodile hunter and TV celebrity Steve Irwin, who died in 2006. The researchers implanted transmitters, as well as temperature and depth sensors, in 27 adult crocs and collected data for a year.
Most of the crocodiles stayed local, cruising less than 3 kilometers in any given day. But eight ventured to the river mouth at least once and headed out to sea for up to 64 days before returning to their original place in the river, Hamish and his colleagues report today in the Journal of Animal Ecology. These travelers took to the water only when the tides were moving in their direction of travel. Otherwise, they climbed on land, as indicated by the body temperature and the loss of an acoustic signal, or sank to the bottom, as indicated by the depth recorders.
As part of the study, the team reanalyzed the data from one of the satellite-tagged crocodiles with respect to the ocean currents. They found the crocs moved slowly when the currents decreased and sped up when the currents were stronger over the course of the journey. Hamish doesn't know how they sense when the currents or tides are favorable. "But this study shows they possess a detailed understanding of their local environment and current systems," he points out. Crocodiles might have a natural magnetic compass within their heads, like turtles and pigeons do, Hamish speculates. He hopes to test that idea in the near future.
"I was astounded by the shear distances involved in the travels of these animals," herpetologist Hannes Botha of the Mpumalanga Tourism and Parks Agency in South Africa writes in an e-mail. Botha also studied migration in South African crocodiles. This homing ability over long distances has important management implications. Crocodiles are the only animals to deliberately hunt humans, and one common solution is to transport dangerous animals far from settled areas. That means that problem crocs will have to be released where they can't ride the currents, Botha explains. Otherwise, good luck telling a 1-ton croc it can't sail home again.