Page last updated at 23:56 GMT, Tuesday, 20 May 2008 00:56 UK

Biggest rodent 'shrinks in size'

Artist's impression of Josephoartigasia monesi (Gustavo Lecuona)
The animal is still the largest rodent ever discovered

The largest rodent ever recorded might not have been as monstrous as was first suggested, a scientist has claimed.

A fossil skull belonging to the rodent Josephoartigasia monesi was uncovered in Uruguay, where the beast roamed 2-4 million years ago.

It was first thought to have weighed a whopping one tonne, but new estimates suggest the animal could have weighed as little as a third of that.

Details appear in the journal Proceedings of the Royal Society B.

Dr Virginie Millien, from McGill University in Montreal, Canada, said the mathematical models originally used to calculate the rodent's mass from its skull probably overestimated its body size.

"The problem of extrapolation is regarded as one of the largest sources of error and is known to cause overestimation of body mass," she explained.

"J. monesi is certainly the largest rodent ever described, but, based on these calculations, its body mass may have been as low as 350kg."

Broad range

The fossil was first described in January, by a team of Uruguayan researchers. They estimated the animal could have been about 15 times heavier than the largest living rodent.

The team took different parameters of the rodent's skull, fed them into the "allometric model" and obtained several body mass estimates for J. monesi.

I think that her methods probably are finer, but our conclusions are very similar
R Ernesto Blanco, Institute of Physics in Montevideo

These were between 468kg and 2,586kg - a very broad range - with an average value of 1,211kg.

Dr Millien decided to recalculate these size estimates, using a larger sample of rodents for comparison and making several adjustments to the mathematical method used by the original team.

Several of the estimates she obtained were smaller than the original calculations. But, as one of the authors of the original paper on J. monesi points out, some are larger.

In a formal reply, R. Ernesto Blanco, from the Institute of Physics in Montevideo, Uruguay, defended his team's results.

'Similar conclusions'

He pointed out that the mean value obtained by Dr Millien for the rodent's body size was 900kg, not far off the average estimate of 1,211kg calculated by his own team.

"In a problem with such large uncertainties, a 25% difference in values is not a big one," he wrote in Proceedings of the Royal Society B.

"Her methods probably are finer, but our conclusions are very similar."

He also criticised the emphasis placed by the McGill University researcher on the "low" figure of 350kg, when her analysis suggests the rodent's body mass could have been as great as 1,534kg, or one-and-a-half tonnes.

The giant animal's skull had lain in the Museum of Natural History in Montevideo for three years before being studied and identified as a new species.

It was recognised as a new creature by examining and comparing its teeth with other known species of Josephoartigasia.

Its incisors are extraordinarily large and researchers have speculated that the creature may have used the teeth to cut wood in a similar way to a modern-day beaver.

Gigantic fossil rodent discovered
16 Jan 08 |  Science/Nature

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