Isotopic evidence of saber-tooth development, growth rate, and diet from the adult canine of Smilodon fatalis from Rancho La Brea

Abstract

The large size of canines in saber-toothed cats suggests implicitly that they took longer to grow than “normal” canines. If this were the case, then juveniles may not have been able to use them for hunting. Consequently, juvenile Smilodon fatalis, for example, may have had to remain in a social group and be fed by adults longer than is the case for modern large cats. Analysis of the stable carbon and oxygen isotope ratios within the enamel of the sabertooth provides a reasonably direct method to determine development and growth. Variation in the oxygen isotope ratios during the ontogeny of a particular tooth has been reported in many animals, and has been shown to correlate with temperature variation related to seasonality. Variation in the carbon isotope ratios typically reflects differences in diet, and can be used to determine if a carnivore preferred a prey that ate predominantly C 3 or C 4 plants. This study analyzed stable carbon and oxygen ( δ 13C, δ 18O) isotope ratios from sequential, closely spaced samples of enamel carbonate obtained from upper canines of S. fatalis. A sequential change in δ 18O through canine ontogeny for two specimens (which averaged approximately −4‰) is consistent with nearly 1 year being sampled. The results suggest that the canines of S. fatalis grew about 80 mm during a 12-month period, or about 7 mm/month, and the total duration of growth for this species was about 18 months. S. fatalis apparently achieved its long canine by combining the canine development strategies of extant lions and tigers: a quick growth rate, and growth over a long time. Mean carbon isotope values of −10.1‰ and −11.3‰ suggest consumption of animals eating a C 3 diet.