Detentus is known to live only in two areas of Manus Island.

It is an “island giant”, according to Prof Flannery, larger than almost any rat in Melanesia. A typical detentus weighs nearly half a kilogram and has very coarse fur and a short tail.

Over millennia of isolation on Manus, detentus has adapted to conditions. It has powerful front incisors but small molars, suggesting it uses its front teeth to break open hard nuts.

Before confirming detentus existed, Flannery said scientists had suspected there was a large rat endemic to the island. He said it had been exciting and “an immense privilege” to be able to discover and name the new species. “I’ve been looking for this rat for 30 years,” he said.

Labelling the detentus a new species was based on three specimens collected on Manus Island between 2002 and 2012.

The specimens were compared with subfossil specimens from the Pamwak archaeological site on Manus, which confirmed the species as a long-term resident of Manus.

Flannery suggested the name ‘detentus’ to his scientific colleagues, who immediately agreed.

“It’s not very often as a scientist that you get to make a statement like this,” he said, “but I wanted to express my sympathy and my solidarity for the people held in the detention centre on Manus. I wanted to say to them, in this small way, ‘You are not forgotten.’”

Sightings of the detentus are rare and it is possibly seriously endangered. Residents report it has previously been seen across Manus and sometimes on adjacent Los Negros Island but efforts by zoologists to find further evidence of it have been fruitless.

“That really rang alarm bells for us, that it couldn’t be found by people who are expert in this field,” Flannery said. “Perhaps in times past it was common, but it’s rare now.”

Detentus might be threatened by its forest habitat being converted to farmed land, feral cats or by the spread of introduced rat species which may carry disease.