(Image credit: C. Franklin/ PLoS One)
In the first study of its kind, a team of Australian researchers have used satellite telemetry to show that crocodiles can navigate hundreds of kilometres to return to their home rivers after being moved.
Mark Read, of the Queensland Parks and Wildlife Service, and his colleagues captured three large male estuarine corocodiles (Crocodylus porosus) from the Nesbit and Wenlock Rivers in northern Queensland. The crocodiles were transported 56, 99 and 411 km by helicopter, and fitted with specially designed satellite transmitters (above) before being released.
The crocodiles behaved in a similar way upon their release. All three made apparently random movements for up to 108 days at their release sites - during which they were perhaps orienting themselves - before taking the most direct routes to the locations from which they were captured. All three showed fidelity to their "homes" - once they had returned, they remained in the vicinity for the remainder of the tracking period.
It was previously thought that crocodiles do not travel long distances because they tire easily. But the crocodiles tracked in the study travelled up to 19 miles per day when returning to their capture sites. One of them swam 411 km along the coast in a period of 20 days, circumnavigating the northern tip of Australia (the Cape York Peninsula) on its return to the capture site.
The new study, which is published in the open access journal PLoS One, raises doubts about the method of moving crocodiles who pose a threat to humans. The researchers say that the study would not have been possible without the assistance of naturalist, conservationist and crocodile hunter Steve Irwin, who was killed last year by a stingray. Irwin and his colleagues from the Australia Zoo in Beerwah trained the researchers in how to capture the crocodiles.
The mechanisms by which these large reptiles navigate such long distances in unknown. Evolutionarily, crocodiles are more closely related to birds than to other reptiles, so it is possible that they use the same cues as birds for long distance navigation: the position of the sun, the Earth's magnetic field, sight, smell, or a combination of these.
Read, M. A., et al. (2007). Satellite tracking reveals long distance coastal travel and homing by translocated estuarine crocodiles, Crocodylus porosus. PLoS One 2 (9): e949. doi:10.1371/journal.pone.0000949. [Full text]
That's really fascinating. I wonder why they hung around the release site so long. Checking it out to see if they "prefer" it over their original? If it's a case of taking that long to get their bearings that would seem to rule out magnetic field as the primary means (wouldn't it?) and indicate they are relying on some kind of seasonal change - angle of the sun?
I caught a bit of TV documentary where they were placing these tags. Great to see the actual research that comes out of it for once.