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Last Updated: Wednesday, 19 October 2005, 19:21 GMT 20:21 UK
Fugitive rat sets distance record
Brown rat, Nature Picture Library
Rattus norvegicus is now a globally distributed pest (Image: NPL/Susan McMillan)
The swimming ability of a rat which crossed open sea to find new territory has impressed New Zealand scientists.

The rodent had been radio tagged and its movements tracked by researchers to learn more about pest species and how they invade small islands.

The rat was released on the uninhabited island of Motuhoropapa but refused to be captured at the project's end.

The NZ team tells Nature magazine the animal finally turned up on the nearby Otata Island - a mighty swim of 400m.

James Russell, from the University of Auckland, and colleagues think this may be the longest distance recorded for a rat swimming across open sea.

"Norway rats can supposedly swim up to 600m but, to our knowledge, this is the first record of a rat swimming hundreds of metres across open water," they write.

In total, the rat was free for 18 weeks. It was eventually killed in a trap baited with penguin meat.

Invasive species are second only to habitat loss as the major driver of extinctions.

Noises Islands (Jo Hoare Victoria University)
Islands in the New Zealand's Hauraki Gulf are close to each other
Rodents, in particular, have wrought havoc to many small islands around the world. They prey on native birds, hunting their chicks and eggs for food, and destroying nests. They also compete with native species for essential seed, insect and plant food.

Eradicating them once they have established themselves has proved extremely difficult. This particular rat evaded a diverse arsenal of traps, baits and even sniffer dogs.

The team says that rodent pests are even harder to eliminate when they occur in small numbers, perhaps because of the lack of competition for food.

Keeping on top of the invading aliens is a constant battle for the New Zealand authorities.

The uninhabited and forested Noises Islands (Motuhoropapa and Otata) off northeast New Zealand were reinvaded by Norway rats at least six times between 1981 and 2002.

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