"With our device, the cylinder containing the GPS and cell phone is affixed to the scales behind the head because it is normally the part of the body that is just above or below the surface. The cell phone is set to transmit readings at regular intervals. It might be that you miss a few, especially during the night when crocodiles hunt and they often go deep underwater. During the day they laze on sandbanks, making it easier to get regular readings. It is also possible to get readings between set times by sending the crocodile's cell phone a 'Where Is' message."
Catching the crocodile to fit the cylinder is a risky operation.
It is done at night from a boat, explained Botha. "We drift quietly among the crocodiles, and the one selected according to size is blinded with a strong spotlight.
"Using a long pole, a steel-cable noose is slipped over its head. The crocodile dives down, tightening the noose around its head and dislodging the cable from the pole. We let it struggle for about five minutes to tire sufficiently for us to drag it onto the shore.
"The first priority is to get its head under control and to tie its jaws shut with duct tape. We also tie the feet. It takes at least four people to do the job. The important thing is not to get head-butted or hit by its tail, especially when it is pulled from the water. We don't use tranquilizer. We don't want to risk the crocodile getting attacked by others while recovering from sedation.
"Four holes are drilled into the bony scales behind the head and surgical-steel wire is used to attach the cylinder. It takes about ten minutes to complete the operation and for the crocodile to be set free again," said Botha.
Martin Haupt of African Wildlife Tracking, supplier of the tracking device, explained that both the GPS and cell phone operate from a 3.6-volt lithium battery which will give five readings a day for two years. To save power, the GPS/GSM unit only switches on when sending and receiving information at set times. Settings can be adjusted via the scientist's PC or cell phone.
The unit also has an on-board logger that can store 2,000 position readings. Therefore no readings are lost while the crocodile is out of GSM range.
The tracking project is part of a wider research program at the Flag Boshielo lake which has also involved National Geographic Television host Brady Barr. With Botha's assistance, Barr got really close to crocodiles by using several innovative gadgets. He did this for the National Geographic Television series Reptile Wild and Croc Chronicles.
Botha said one gadget Barr used was a foam-rubber crocodile mask which he put over his head. "It looked so much like the real thing that one big crocodile saw it as competition and threatened to attack it. We had to get Brady very quickly out of the water."
They also tried a remote-controlled boat with a camera mounted, but it was not too successful. For some reason it scared off the crocodiles.
The worst fate suffered, said Botha, was last year by a small remote-controlled vehicle placed on a sandbank where the crocodiles like to lurk. It was not long before a big crocodile burst from the water and caught it between its jaws. "You just saw wheels flying. I found the broken camera two months later in the mud when the lake had gone down somewhat."
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