The largest among the smallest: the body mass of the giant rodent Josephoartigasia monesi.

Body mass in extant rodents covers more than four orders of magnitude, from a few grams up to 40 kg on average in the capybara (Hydrochaeris hydrochaeris; Silva & Downing 1995). However, rodents have been much bigger in the past. The South American fossil rodent, Phoberomys pattersoni (Sanchez-Villagra et al. 2003), found in northwestern Venezuela (Mones 1980; Horovitz et al. 2006) had an estimated body mass of 436–741 kg (Sanchez-Villagra et al. 2003). Most recently, Rinderknecht & Blanco (2008, hereafter R&B 2008) described a giant fossil rodent from the Plio–Pleistocene of Uruguay. The new species Josephoartigasia monesi was estimated to have weighed 1211 kg on average, and perhaps as much as 2584 kg; a value well outside the range of size of any previously described rodent. However, a closer examination of R&B’s (2008) methods suggests that the body mass of J. monesi may have been overestimated. One approach that is commonly used to estimate body mass in extinct species is to calculate the allometric relation between a character and body mass in a number of recent species. This relation is then applied to extinct species to estimate their body mass. This procedure is in principle very straightforward, but it has a number of limitations (Smith 1980, 1996; Schmidt-Nielsen 1984; Damuth & MacFadden 1990; Egi 2001; Reynolds 2002). In particular, I have identified four main issues that place doubt on the body mass estimate of R&B (2008): the reference dataset; the choice of the character; the statistical analysis; and the presentation of the data. The reference data used by R&B (2008) contain a rather small sample size. Ideally, the sample size should be large enough to allow as much confidence in the results as possible. Their data are a mixture of intraand interspecific measurements taken on 13 individuals representing nine species of hystricognath rodents, all close living relatives to Josephoartigasia. Because the allometric model used is an evolutionary allometric model at the level of the infra-order, the dataset should only contain interspecific data (using a mean value when several individuals from the same species are measured) as opposed to a static allometric model based on intraspecific data (Klingenberg & Zimmermann 1992). Thus, the actual sample size is 9 in R&B (2008). The use of a species mean also reduces the possibility of giving too much weight to a single individual such as the small 19 kg capybara in table 1 of R&B (2008). Second, the reference dataset should cover the widest range possible. This is especially true for fossil species that fall well outside the