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"Homing" Crocs Voyage Hundreds of Miles

Brian Handwerk
for National Geographic News
September 26, 2007
 
Captured and relocated saltwater crocodiles can swim hundreds of miles to return to their home rivers, according to a new study.

A team of crocodile researchers, including the late Steve Irwin, seized three crocs near bays in Queensland, Australia, and flew them by helicopter to coastal spots between 32 and 81 miles (52 and 130 kilometers) away.

In the first satellite tracking study of wild crocodiles ever conducted, researchers kept tabs on the reptiles with a transmitter attached to the back of their heads. The device collected the data and relayed it via satellite.

The data revealed that once in ocean waters, the animals covered surprising distances—between 6 and 19 miles (10 and 30 kilometers) each day.

The reptiles headed homeward following navigational cues that scientists do not yet understand.

Because of the crocodiles' speedy return home, the study also concluded that relocating "problem" animals—which favor living near people—is not an effective strategy, and that other methods of controlling them should be developed.

Super Croc

Craig Franklin, of the University of Queensland's School of Integrative Biology, is a co-author of the study, which appears in today's issue of the journal Public Library of Science.

Franklin and colleagues were particularly shocked by the endurance of a croc that was transported from one side of the northern tip of Australia to the other.

The animal simply circumnavigated the cape on a long, speedy swim home.

"It blew us away," Franklin said.

The animal spent several months roaming the coastline in various rivers, Franklin said.

"Then all of a sudden it seemed to decide to return home.

"We don't know whether or not it used those several months to [orient itself]—to get clues and later use them to return home. But on the fourth of December it just made a beeline [for home] and moved 249 miles [400 kilometers] in 20 days.

"We say that it headed home for Christmas."

Reptiles on the Move

The study was not the first to document crocodiles' ability to travel great distances on their way back to their original habitat. (Related news: "Costa Rica's "Problem Crocs" Return After Removal" [September 26, 2003].)

However, most other studies have relied on radiotelemetry, a difficult technique for studying animals that live in remote locations, have huge geographic ranges, and are easily disturbed by human presence, the study authors wrote.

Satellite tracking allows data to be accessed continuously from the crocodiles without human interference, giving scientists a clearer picture of their habits.

Crocodiles are thought to live mostly within their home rivers, but scientists say not enough data exist to be sure.

"More studies may show that large animals go out of river systems naturally," Franklin said. "It's clear that they can undertake rather large voyages."

To Franklin's knowledge, the new study represented the first time satellite tracking was used to study the movements of wild crocs.

The technology allows researchers to continually gather movement data without disturbing the unpredictable animals or influencing their behavior.

Bird Cues?

No one knows how crocodiles are able to navigate.

"We can probably look to some of the well-known mechanisms of other organisms as a place to start," said Perran Ross, a crocodile biologist at the University of Florida (UF) who was not involved in the study.

"Whether crocodiles are different would be interesting to see."

Notable navigators in the animal world use visual cues such as the sun's position, familiar landmarks, a keen sense of smell, and the Earth's magnetic fields.

Pigeons, probably the best studied animal navigators, appear to rely on several methods, the first of which is recognizing landmarks.

After many years of flying around, pigeons know their way—much like humans can drive around their hometowns without using street signs.

"Crocodiles are long-lived and active," UF's Ross explained. "It's quite conceivable that over a 30-year lifetime, a crocodile would know a very large area around it. We know that they have the ability to learn."

Birds, lobsters, sharks, and other animals may also find their way by following the Earth's magnetic field. (Related news: "Lobsters Navigate by Magnetism, Study Says" [January 6, 2003].)

Whatever methods crocodiles use, their homing ability may highlight their distant relation to birds.

"Crocs are actually more closely related to birds than they are to all other reptiles," study lead author Franklin said.

"So ... they might be using similar cues to navigate."

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