Using a single bone to estimate the weight of an extinct animal, like we did last month, beats consulting the Psychic Hotline, but not by much. Scientists usually start with a weight-bearing bone like the femur. Unfortunately, fossil skeletons are usually incomplete and the bones fragmentary. When nothing else is available they look to tooth dimensions, jaw length, the thickness of a bone’s cortex, even the width of a joint (Scott, 1990). . . the equations are as numerous as the techniques for measuring the bones or the pieces thereof. (photo borrowed from)

In theory, femurs are simply columns supporting a TBD weight, subject to forces of tension and compression that engineers have understood for 2,000 years. Distal limb bones share the load in uncertain ways and can mislead–cranial measurements even more. Measurements directly related to the bone’s weight-bearing capacity (e.g. cross-section) promise the best results, but how do you measure something as irregular as a sloth femur? Femur length brings a much higher margin of error (Scott, 1990).

Our formula, like many others, is based on correlations derived from ungulates, for which there is a wide sample, but is that baseline relevant to ground sloths? The standard estimating error is 42%, applied to other ungulates. In other words, if our sloth were a moose, and we knew for sure ground sloths were strictly quadrupedal, didn’t have large weight-bearing tails, and their mass was distributed like the average mammal (i.e. 60% on the forefeet and 40% on hind feet, Alexander, 1985), then we could be 95% sure our sloth weighed between 453 and 5,205 pounds.

A baseline drawn from living Xenarthrans is hopeless for narrowing the estimate. We’re trying to cross a chasm of million years of separate evolution, significant disparities in habitat and life-style, and a huge size difference. . . . We might average estimates derived from other bones, but given the scarcity of fossils, the chance of finding that same combination of bones at another site is virtually nil. Comparing different animals using different bones would only increase the expected error. For all of its flaws, Greg’s methodology is still the best option. (sloth photo borrowed from)

Why bother when the potential for error is obviously so great? Paleoecologists can draw a wide range of fundamental conclusions from an estimate of an animal’s mass including metabolic rate, food intake, foraging time, forage quality and retention time, home range size, social patterns, population density, gestation period, litter size, life span, etc. (Peters, 1983; Schmidt-Nielsen, 1984). These are especially insightful for megaherbivores (mass > 1 ton), like ground sloths, with their special challenges and opportunities (Owen-Smith, 1988).

Until we invent a time machine there’s no way of determining how accurate the 2,829 pound-estimate is for our adult. Greg’s number feels right, but its greatest value may be in comparing this specimen to other Megalonyx specimens, and not in the absolute number. The best answer to the query, “How much did it (they) weigh?” may be, “about as much as a small elephant.” Disappointingly imprecise, I know, but as clear a picture as we have . . . . Dave

References

Alexander, R. M. 1985. Mechanics of posture and gait of some large dinosaurs. Zoological Journal of the Linnean Society 83: 1-25.

Owen-Smith, R. N. 1988. Megaherbivores: The influence of very large body size on ecology. Cambridge University Press, Cambridge.

Peters, R.H. 1983. The ecological implications of body size. Cambridge University Press, Cambridge.

Scott, K. 1990. Postcranial dimensions of ungulates as predictors of body mass. In Body Size in Mammalian Paleobiology: Estimation and Biological Implications. J. Damuth and B.J. McFadden (eds.). Cambridge University Press, Cambridge.

Schmidt-Nielsen, K. 1984. Scaling: Why is animal size so important. Cambridge University Press, Cambridge.