### Rhinoceros weight problems

As the largest land mammal that has ever lived, there is much fascination with maximum body size and weight estimates for extinct indricotheres. Most often you see sources parroting each other around a common estimate of 20 metric tonnes (20,000 kg), although Alexander (1989) estimated 34 tonnes and Savage and Long (1986) quoted 30 tonnes. Prothero and Schoch (2002) followed Alexander (1989) and used the number of 34 tonnes, and there have been a wide range of estimates all over this range. As Fortelius and Kappelman (1993) showed, these estimates came from some questionable assumptions and extrapolations by Osborn, Granger, and Gregory based on their partial skeleton. That skeletal reconstruction is a composite, a chimaera based on bones from individuals of at least four different size classes, so it is not a very reliable source for the body size of an actual animal.

Most of the weight estimates are based on the combined head and body length (HBL) of the animal, which may be exaggerated if Granger and Gregory’s (1935, 1936) reconstruction drawn by Helen Ziska is inaccurate. As Fortelius and Kappelman (1993, Table 1) showed, different authors estimated a wide range of HBL values, from Gromova’s estimate of 740 cm, to Granger and Gregory’s (1936) maximum of 870 cm, which drops to 621 cm if you use just the smallest individuals. Using the weight equations of Damuth (1990) for converting HBL to mass, this gives weight estimates from as low as 8.4 tonnes (in the smallest specimens) to 24 tonnes in the largest specimens.

Fortelius and Kappelman (1993, Table 2) looked at a spectrum of other methods to estimate body size, based on measurements of the skull. The measurements of skull dimensions gave a range of masses form 7-16 tonnes, with a single high estimate of 19.8 tonnes, which is probably an overestimate based on the creatures on which this estimate is based. Using the teeth as an estimator gives values in the range of 5-15 tonnes, although indricothere teeth seem unusually small compared to the size of their skulls and bodies. Estimates based on upper limb bones (humeri and femora) gave a range of 5 to 15 tonnes. Overall, their estimates placed most specimens of indricotheres in the 9-15 tonne range, with only a few specimens giving values as high as 20-24 tonnes.

Gingerich (1990) developed his own method of estimating body mass based on various limb bone measurements. He obtained weight estimates of about 9 tonnes for smaller indricotheres, and about 14-15 tonnes for the largest specimens. As he pointed out, however, there are serious problems with any of these estimates. Not only is it difficult to get accurate predictions based on just a few bones from four different size classes, but indricotheres had very differently proportioned limbs compared to any other living mammal.

Once we drop the outrageous overestimates of the 30 tonne range and use a more realistic 15-20 tonne estimate for the biggest indricotheres, the huge difference in size between indricotheres and elephants vanishes. Rather than “weighing as much as five elephants” as is often quoted, indricotheres were typically in the size range of the largest elephants and mammoths. The largest living elephant specimen ever measured was bull elephant killed in Angola in 1955 and now on display in the Smithsonian, which weighed 10 tonnes, but most elephants weigh about 5-6 tonnes (Prothero and Schoch, 2002, pp. 182-183). However, Fortelius and Kappelman (1993) pointed to examples of limbs of the mammoth *Mammuthus trogontherii* and the *Deinotherium* (with its downward-flexed tusks) that suggest weights of 13 tonnes to possibly as high 20 tonnes (Christiansen, 2004). Saarinen (pers. commun., 2011) looked at limb bones of *Deinotherium* from European museums, and estimated range of weights of 7.4-17.4 tonnes, with a mean of 11.2 tonnes. Most fossil proboscideans give weight ranges between 3-10 tonnes, which would make them somewhat lighter than indricotheres. Thus, we should be cautious about calling indricotheres the “largest land mammals that ever lived.” They were certainly taller than modern elephants or other living mammals, but their mass was roughly equivalent to the largest mammoths and deinotheres.

These weight estimates are not just a piece of trivia to pop into a textbook, but have implications as well. A number of ecologists and physiologists have speculated on the maximum body size that mammals can obtain, based on constraints due to metabolic factors. Economos (1981) was the first to do this, using mathematical estimates of the metabolic costs of gravity. Based on this method, he concluded that a land mammal cannot theoretically reach sizes of much greater than 20 tonnes, which seems to correspond to our upper limit for largest indricotheres and mammoths. Clauss et al. (2003) looked at the nutritional constraints of large body size, particularly in contrasting the relatively efficient foregut-fermenting ruminants (which tend not to grow to huge sizes) versus the inefficient hindgut fermenters like elephants, rhinos and hippos. Based on these constraints, they found that Fortelius and Kappelman’s (1993) estimate of 11-15 tonnes for indricotheres was more consistent with digestive constraints than the higher estimates of 20 tonnes or greater.

Thus, we must be careful when quoting old numbers from early authors about the weights of extinct creatures. Indricotheres probably weighed only in the 10-15 tonne range, and maxed out at 20 tonnes on the largest individuals. It is very unlikely that there were any in the 30-35 tonne range, as is so often cited.

#### References

- Alexander, R. M. 1989.
*Dynamics of Dinosaurs and Other Extinct Giants*. Columbia University Press, New York, New York. - Christiansen, P. 2004. Body size in proboscideans, with notes on elephant metabolism.
*Zoological Journal of the Linnean Society*140:523–549. - Clauss, M., R. Frey, B. Kiefer, M. Lechner-Doll, W. Loehlein, C. Polster, G. E. Rössner, and W. J. Streich. 2003. The maximum attainable body size of herbivorous mammals: morphophysiological constraints on foregut, and adaptations of hindgut fermenters.
*Oecologia*136:14–27. - Damuth, J. 1990. Problems in estimating body masses of archaic ungulates using dental measurements, pp. 229–253, in Damuth, J., and B. J. MacFadden, (eds.),
*Body Size in Mammalian Paleobiology*. Cambridge University Press, Cambridge. - Economos, A. C. 1981. The largest land mammal.
*Journal of Theoretical Biology*89:211–215. - Fortelius, M., and J. Kappelman. 1993. The largest land mammal ever imagined.
*Zoological Journal of the Linnean Society*108(1):85–101. - Granger, W., and W. K. Gregory. 1935. A revised restoration of the skeleton of
*Baluchitherium*, gigantic fossil rhinoceros of central Asia.*American Museum Novitates*787:1–4. - Granger, W., and W. K. Gregory. 1936. Further notes on the gigantic extinct rhinoceros
*Baluchitherium*from the Oligocene of Mongolia.*Bulletin of the American Museum of Natural History*72:1–73. - Prothero, D. R., and R. M. Schoch. 2002.
*Horns, Tusks, and Flippers: The Evolution of Hoofed Mammals.*Johns Hopkins University Press, Baltimore, Maryland, 309 pp. - Savage, R. J. G., and M. R. Long. 1986.
*Mammal Evolution: An Illustrated Guide*. Facts on File Publications, New York, New York.

Fascinating article. Just curious, though: wouldn’t there be a fairly straightforward correlation between cross-sectional area of limb bones and body mass?

I believe it’s been tried, but the correlation is no stronger than any other. PLUS cross-sectional area scales with a power of 2 up to a point, then is affected by allometric constraints since mass increases by a power of 3.