His feat, loosely billed as a record, has also alarmed the scientists, who say it shows that coastal islands cleared of rats can easily be reoccupied. Mick Clout, Professor of Conservation Ecology at the University of Auckland, said yesterday Razza was taken to Motuhoropapa, a flyspot east of Auckland, for a study on how a rat behaves when alone on an island. "We assume most rat invasions begin with one or two rats coming ashore, probably from ships," Professor Clout said.

Researchers wanted to know how hard it would be to spot a single invader, and how difficult it would be to capture. Razza had a small radio transmitter attached and was set free on the island. Scientists intended to recapture him within eight weeks, but Razza gave a new meaning to "rat cunning". He avoided all the scientists' traps, and after 10 weeks his radio signal failed. "It would be fair to say that at that point we were worried," Professor Clout said. The Conservation Department was also worried, as the island had been cleared of rats.

Two weeks after the transmitter failed a woman reported finding rat droppings on Otata, a rat-free island 400 metres from Motuhoropapa. DNA tests confirmed they were Razza's. "To our knowledge this is the first record of a rat swimming across open sea, and it's often quite rough water," Professor Clout said. "We assume he did it deliberately, but who knows what was in his mind?"

He speculated that Razza may have wanted female company. A trap on Otata finally ended Razza's four months of freedom, and his life. His adventures are detailed in today's Nature.