Today’s scientific disappointment: a giant rodent thought to weigh over 1,000 kilograms that once rampaged along the South American coast was not actually that big, according to a Canadian researcher.

Earlier this year the Proceedings of the Royal Society B carried a paper detailing the monster rodent Josephoartigasia monesi, based on a 53 centimeter-long skull discovered in Uruguay. Researchers Andrés Rinderknecht and Ernesto Blanco said their beast was a fair bit larger than the previous king rodent, the weedy 700 kg Phoberomys pattersoni.

But in this week’s Proc B, Virginie Millien takes issue with this claim, saying the body mass of J. Monesi “may have been overestimated” (I’ll link to the papers when they’re published online).

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

The problems she has are quite technical and centre on the method used by Rinderknecht and Blanco, which involves looking at the size and shape of living animals and then applying relationships discovered to extinct species . Millien takes issue with the reference dataset used in the original study, the statistical analysis, and the way the data was presented.

In a response running alongside Millien’s paper, Blanco says her work improves some points of the original paper but that it is not possible to exclude masses of 1000 kg. The response adds:

Our technical conclusions (leaving aside all the media simplifications and speculations about body length [sorry about that – ed.]) is that J. monesi’s body mass was probably between 468 and 2586 kg, a very broad range of error. However, Millien’s (in press) first paragraph highlighted only our largest estimations, ‘J. monesi was estimated to have weighed 1211 kg on average, and perhaps as much as 2584 kg’.

More coverage

Biggest rodent ‘shrinks in size’ – BBC

Image: Royal Society