Figuring out how much a living animal weighs is relatively straightforward. But for many extinct mammals, all that’s left are bones. Now, researchers have come up with a new technique to use them to estimate the weight of creatures long gone. First, they make a detailed digital model of the bones in a lifelike arrangement. Then, they use a computer to digitally “shrink-wrap” the skeleton to come up with a rough idea of the creature’s volume. Finally, they estimate the creature’s weight using a volume-to-mass conversion based on 14 modern-day mammals ranging in size from the 90-kilogram red deer (Cervus elaphus) of Eurasia to the 2.7-metric-ton African elephant (Loxodonta africana). Interestingly, the tightest shrink-wrap doesn’t always produce the best weight estimate, the researchers report online today in Royal Society Open Science. For this study, the team used 3D scans of the skeletons of a woolly mammoth (Mammuthus primigenius) and a giant ground sloth (Megatherium americanum) that had been mounted in museums. The woolly mammoth (artist’s representation shown) probably weighed more than 3.6 metric tons, and the giant ground sloth likely tipped the scales at about 3.7 metric tons. The reliability of the new technique relies heavily on an accurate arrangement of bones, the researchers note, and the weight estimate is good only for that individual specimen. Further research will be needed to determine whether the new weight-estimating technique can be used for other large creatures such as dinosaurs, team members caution.