DID SABER-TOOTH KITTENS GROW UP MUSCLEBOUND? A STUDY OF POSTNATAL LIMB BONE ALLOMETRY IN FELIDS FROM THE PLEISTOCENE OF RANCHO LA BREA

Previous studies have demonstrated that the Pleistocene saber-toothed cat Smilodon fatalis had many forelimb adaptations for increased strength, presumably to grapple with and subdue prey. The Rancho La Brea tar pits yield large samples of juvenile limb bones forming a growth series that allow us to examine how Smilodon kittens grew up. Almost all available juvenile limb bones were measured, and reduced major axis fits were calculated to determine the allometric growth trends. Contrary to expectations, Smilodon kittens show the typical pattern of growth found in other large felids (such as the Ice Age lion, Panthera atrox, as well as living tigers, cougars, servals, and wildcats) where the limb grows longer and more slender faster than they grow thicker. This adaptation is thought to give felids greater running speed. Smilodon kittens show no evidence of more robust forelimbs in their growth history. Even though their adult bones have features such as thickened cortical bone or other adaptations that indicate stronger forelimbs, Smilodon kittens do not grow with the predicted pattern of more robust forelimbs compared to other felids. Instead, the front limbs of saber-tooth kittens start out robust compared to other felids, and do not change the ancestral felid growth pattern. Apparently, the growth of felid forelimbs is highly canalized and ontogeny is tightly constrained, so they do not change the slope of their growth curve, but start out robust and maintain the standard felid pattern.