The fearsome saber-toothed cat, Smilodon fatalis, roamed North and South America from about 700,000 years to 11,000 years ago. Its daggerlike canine teeth, which protruded up to 18 centimeters from its upper jaw, could easily shred any bison, camel, or other prey that crossed its path. New research, published online today in PLOS ONE, helps explain why. Using a new technique that combines isotopic analysis with x-ray imaging, scientists scanned Smilodon skulls from the large collection of saber-tooth fossils at the La Brea Tar Pits in Los Angeles. They found that the permanent canines of S. fatalis grew at a rate of 6 millimeters per month, about twice as fast as an African lion’s teeth (and almost twice as fast as human fingernails). Moreover, the study showed, young saber-tooths had fully grown canines when they reached about 3 years of age, meaning that these ferocious cats—which many researchers think were social animals—were ready at an early age to hunt with the pack.