Don't get too distracted by those teeth. It turns out the saber-tooth tiger's (Smilodon fatalis) real strength is in its arms. Researchers x-rayed fossils of saber-tooth limbs (left, inset) and compared them with the bones of 29 other cat species, including the extinct American lion and a present-day tiger (middle and right, inset). As they report  online today in PLoS ONE, saber-tooth forelimbs have an outer layer of dense bone that is, given their leg length, about 15% thicker than expected. The researchers suggest that the saber-tooth developed this strong shell by pinning down struggling prey with its arms. And that helped it get its next meal without risking damage to those long pearly whites.