PT - JOURNAL ARTICLE AU - Mairin A. Balisi AU - Abhinav K. Sharma AU - Carrie M. Howard AU - Christopher A. Shaw AU - Robert Klapper AU - Emily L. Lindsey TI - Computed tomography reveals hip dysplasia in <em>Smilodon</em>: Implications for social behavior in an extinct Pleistocene predator AID - 10.1101/2020.01.07.897348 DP - 2020 Jan 01 TA - bioRxiv PG - 2020.01.07.897348 4099 - http://biorxiv.org/content/early/2020/01/07/2020.01.07.897348.short 4100 - http://biorxiv.org/content/early/2020/01/07/2020.01.07.897348.full AB - Reconstructing the behavior of extinct species is challenging, particularly for those with no living analogues. However, damage preserved as paleopathologies on bone can record how an animal moved in life, potentially reflecting patterns of behavior. Here, for the first time, we use computed tomography (CT) to assess hypothesized etiologies of pathology in a pelvis and associated right femur of an adult Smilodon fatalis saber-toothed cat, one of the best-studied mammal species from the Pleistocene-age Rancho La Brea asphalt seeps, Los Angeles, California. The pelvis exhibits massive destruction of the right acetabulum that previously was interpreted, for nearly a century, to have resulted from trauma and infection. We evaluated this historical interpretation using CT imaging to supplement gross morphology in identifying symptoms of traumatic, infective, or degenerative arthritis. We found that the pathologic distortions are inconsistent with degenerative changes that started only later in life, as in the case of infective or traumatic arthritis. Rather, they characterize chronic remodeling that began at birth and led to degeneration of the joint over time. These findings suggest that this individual suffered from hip dysplasia, a congenital condition common in domestic dogs and cats.The individual examined in this study reached adulthood (at least four to seven years of age) but never could have hunted properly nor defended territory on its own. As such, this individual, and other critically pathologic Smilodon like it, likely survived to adulthood by association with a social group that assisted it with feeding and protection. The pathologic specimens examined here in detail are consistent with a spectrum of social strategies in Smilodon supported by a predominance of previous studies. This application of a relatively new and interdisciplinary technique to an old question therefore informs the longstanding debate between social and solitary hypotheses for the behavior of an extinct predator.