HOW TO GROW A SABER-TOOTH: FELID CANINE GROWTH RATES

The evolution of the saber-toothed morphology in the upper canines of mammalian carnivores has evolved convergently at least four times, within marsupials, creodonts, nimravids, and felids. It has generally been assumed that due to their larger size saber-tooth canines took longer to grow than non-saber canines. This study explores the growth rates of saber-toothed canines among different taxa with the aim of determining whether rates were similar. To determine canine growth rates, stable oxygen isotope ratios were obtained from seven canines of five felid species including one extant and four extinct taxa. Those sampled include two non-saber-toothed species, Panthera atrox and P. leo, and three saber-toothed species, Homotherium serum, Smilodon fatalis, and S. gracilis. Ontogenetic samples were taken perpendicular to the tooth growth axis and across its entire length using a 0.3 mm carbide drill bit and a dental rotary tool. The stable oxygen isotope results show different rates of growth for the different species examined. The two Panthera species, P. atrox and P. leo, show growth rates of 2-3 mm/month. H. serum, a saber-toothed species, also shows a growth rate of 2-3 mm/month. The other saber-toothed species had faster growth rates: S. fatalis had a growth rate of 5-7 mm/month, while the rate of S. gracilis was about 6 mm/month. These results show that the saber-toothed morphologies, even within a single family, develop differently among taxa. This variation in growth rates among taxa further hints that the function of saber-toothed canines and behavior of the various saber-toothed taxa evolved for different reasons. Although absolute growth rates are generally difficult to determine for extinct taxa, this method has the potential to reveal insights about the metabolism, physiology, and even behavior of these unique extinct organisms.