If you are a fan of the movie The Princess Bride (or if you read my previous post on Neoreomys), you know that R.O.U.S. stands for Rodents Of Unusual Size.

But did you know that R.O.U.S. really did exist? Moreover, they were much, much bigger than the creepy-looking animals depicted in the movie.

The current record-holder for world’s largest rodent is Josephoartigasia monesi from Uruguay, a species of uncertain Pliocene or Pleistocene age. It made a big splash when the species was named in 2008, and rightly so; its nearly complete skull measures some 20” in length (53 cm), similar to a that of a small horse.

But how big was J. monesi overall, and how much did it weigh? That is more difficult to determine. Its describers estimated is body mass at more than a ton (2,000 lbs or 1,000 kg), but other researchers have suggested it was much smaller (though still giant for a rodent), perhaps weighing as little as 770 lbs (350 kg). There are several reasons why coming up with a plausible estimate is difficult.

Most importantly, we are dealing with a case of extrapolation: trying to estimate something beyond the range of our comparative sample. No rodents alive today are anywhere close to the size of J. monesi. The largest rodent, the capybara, weighs only about 110 lbs. (50 kg), and its skull is about half the size of that of J. monesi (9.5” or 24 cm). Therefore, we don’t know how big the skull of a 1-ton rodent “should be.” Maybe rodent skulls get proportionately bigger (or smaller) as rodents get larger? If so, this can result in over- or under-estimates of body size in extinct species. This is a challenge no matter what measurement we are using as the basis for our estimate.

Another difficulty with J. monesi is that it isn’t closely related to the largest living rodents. It is more closely related to chinchillas than capybaras, and the sole living member of its family is an elusive tropical forest-dwelling species known as the pacarana. With only a single close living relative, it isn’t possible to know how body proportions change with overall body size in rodents like J. monesi.

Would having limb bones of J. monesi help to estimate its size more accurately? Maybe, maybe not. The current silver medal holder for largest rodent is a geologically older species from Venezuela called Phoberomys pattersoni. Unlike J. monesi, P. pattersoni is known from many bones of its skeleton and only a few fragments of its skull. Based on these materials, its describers concluded that it weighed some 1,500 lbs. (700 kg). But this estimate also has been criticized as too large; 500-600 lbs (220-280 kg) may be a more likely range. Whether dealing with limb bones or teeth and skulls, extrapolation is still a problem, and this can be exacerbated by a lack of close living relatives. (P. pattersoni is even worse off than J. monesi in this regard; its family – Neoepiblemidae – has no living members.)

So what is a paleontologist to do? One option is to step back and focus on the forest rather than the trees.

Mammals and other animals basically have the same density as water. As a result, if you can estimate the volume of an animal, you can estimate its mass pretty well. This can be tough to do for an extinct animal, but head-body length is a good alternative, and that can be estimated from a skeleton.

Let’s use P. pattersoni as an example. Based on the image below, it had a head-body length of nearly 7 feet (2.1 m). If we plug this value into several equations for calculating body mass from head-body length (based on different types of modern mammals), its mass is estimated at 170-230 kg, a far cry from the original estimate of 700 kg and much closer to lower estimates of other researchers. Working backwards, we would expect a typical 700 kg animal to be nearly 10 feet long (3 m), nearly half again as long as our very rough head-body length estimate for P. pattersoni.

Is it possible that P. pattersoni was a stocky animal and actually did weigh 700 kg? Certainly. But science is based on probabilities, and that is a less likely alternative based on current evidence. For my money, 200-250 kg is a better bet than 700 kg for the body mass of P. pattersoni.

How about J. monesi? We don’t have a skeleton for that species, but we do have a well-preserved skull. If we assume its head-body length was 4.5-5x its skull length (a typical range for many mammals), its estimated body mass is 230-460 kg rather than 1,000 kg. Maybe J. monesi weighed a ton, but its skull isn’t the size of a typical one-ton mammal. As noted above, it is about the size of a horse. Horses typically weigh 200-350 kg.

These calculations are not based on precise measurements, but what they lack in precision, they probably make up for in accuracy. And accuracy is what is important.

As a paleobiologist, I would say that P. pattersoni and J. monesi were tapir-sized or bear-sized rodents rather than cow-sized ones. Although this may seem like a demotion in how these species are often portrayed, it shouldn’t detract from the appeal of these truly remarkable Rodents Of Unusual Size!

References and Further Reading:

• Croft, D. A. 2016. Horned Armadillos and Rafting Monkeys: The Fascinating Fossil Mammals of South America. Indiana University Press, Bloomington, Indiana, 320 pp.
• Damuth, J. 1990. Problems in estimating body masses of archaic ungulates using dental measurements; pp. 229-253 in J. Damuth and B. J. MacFadden (eds.), Body Size in Mammalian Paleobiology: Estimation and Biological Implications. Cambridge University Press, Cambridge, United Kingdom.
• Hopkins, S. S. B. 2018. Estimation of body size in fossil mammals; pp. 7-22 in D. A. Croft, D. F. Su, and S. W. Simpson (eds.), Methods in Paleoecology: Reconstructing Cenozoic Terrestrial Environments and Ecological Communities. Springer Nature, Cham, Switzerland.
• Millien, V. 2008. The largest among the smallest: the body mass of the giant rodent Josephoartigasia monesi. Proceedings of the Royal Society B: Biological Sciences 275:1953-1955.
• Millien, V., and H. Bovy. 2010. When teeth and bones disagree: body mass estimation of a giant extinct rodent. Journal of Mammalogy 91:11-18.
• Perea, D., A. Rinderknecht, M. Ubilla, E. Bostelmann, and S. Martínez. 2013. Mamíferos y estratigrafía del Neógeno de Uruguay; pp. 192-206 in D. Brandoni and J. I. Noriega (eds.), El Neógeno de la Mesopotamia Argentina. Asociación Paleontológica Argentina, Publicación Especial 14, Buenos Aires.
• Rinderknecht, A., and R. E. Blanco. 2008. The largest fossil rodent. Proceedings of the Royal Society B: Biological Sciences 275: 923-928.
• Sánchez-Villagra, M. R., O. Aguilera, and I. Horovitz. 2003. The anatomy of the world’s largest extinct rodent. Science 301:1708-1710.