Ancient saber-tooth cats may have been more social than we thought

Computed tomography reveals hip dysplasia in the extinct Pleistocene saber-tooth cat Smilodon

Mairin Balisi, Abhinav Sharma, Carrie Howard, Christopher Shaw, Robert Klapper, Emily Lindsey

Summarized by Nicole Christensen, a senior geology major at the University of South Florida. She went to community college for her Associates’ degree, and initially started working on a degree in Environmental Engineering before pursuing geology instead. She likes many different fields in geology and isn’t yet sure what she would like to specialize in, but she knows she’d like to get her Master’s degree someday. In her spare time, she likes to play music, draw, and knit.

What data were used? The pelvis (hip) and right femur (upper leg bone) of a saber-tooth cat were found in the Rancho La Brea asphalt seeps in California. The pelvis was an unusual find, as it is asymmetrical, with a build-up of bony growth on the right side. The disfigured right hip socket is shallow and oval-shaped, rather than the circular shape of a healthy socket. This disfigurement was originally thought to be due to an infection when the animal was alive; however, further disfigurement is seen on the later-found matching right femur, which bears a flat head instead of a typical rounded one, to fit the shallow hip socket.

Methods: All fossils were collected from the same deposit to limit the range of time represented in the study. Dr. Balisi and her team then observed the surface of the pelvis and femur, comparing their characteristics to other deformed bones from different specimens of the same species. A 3D scan was taken using an Artex Space Spider, which is a type of 3D scanner that produces high-resolution color scans. They then used CT scans to capture images of the internal structure of the bones and allow the 3D models to be cross-sectioned. The cross-sections allowed them to determine cause of the deformations. Since the bone lacked fractures, calluses, or healing markings and the bones did have the presence of arthritic degeneration, researchers were able to narrow down the causes, explained in the results. In order to estimate body size, they measured the pelvis length and femur circumference in question, as well as other deformed and non-deformed femurs and pelvises of other specimen from the same location as a comparison. 

Four different angles of the deformed pelvis on white background, each to show off the extent of the deformation clearly. The right hip socket where the femur would fit is much shallower and larger than the non-deformed left hip socket. The left hip socket has a ridge to keep the femur in place, which the right hip socket is missing. The pelvis is asymmetrical; the right side is smaller with thick bony growths, while the right side is smooth.
Figure 1: The recovered deformed pelvis of a saber-tooth cat. A) The right-side view, showing deformation of the hip socket. B) The left-side view, showing a non-deformed hip socket, but with extra bone growth around the edge. C) The top view and D) bottom view, showing that the pelvis is asymmetric.

Results: The deformations seen in the femur and pelvis, shown in Figure 1, are consistent with hip dysplasia, a condition that the Smilodon in question was born with, rather than due to an injury later in life. The femur wasn’t fully developed, and so it didn’t properly fit into place against the pelvis. This would have led to pain when walking, causing the cat to not bear weight on its right leg. Researchers ruled out possibilities that the cat could have had an injury, infection, or degeneration over time. If the cause was injury or infection, the femur and pelvis would have been fully developed before the degeneration began, and so the top of the femur would not have been affected. However, the femur of this study has a deformed head, indicating the hip socket was not properly developed. In the same area that the studied pelvis and femur were found, there were several other pelvises that showed signs of deformation similar to the pelvis of this study.

Why is this study important? Saber-tooth cats are ambush predators. They wait until the right moment to leap at their prey and then drag it to the ground. It would have been very difficult for one to reach adulthood without being able to use one of its hind legs effectively. Based on the size of the pelvis and femur, the studied Smilodon lived to adulthood. Hip dysplasia begins to affect animals from a young age. This means it is likely that saber-tooth cats lived in communities, which provided both food and protection from predators to those who could not take care of themselves.

The big picture: Evaluations of bones and their markings can lead to discoveries about the lifestyle and behavior of extinct animals, even ones as well-known as Smilodon. Many living species of cats do not show social behavior, making this an advancement in understanding the behavior of Smilodon. Modern technology, such as CT scans, can bring about new methods of evaluating bones past their surface appearance. In this study, CT scans showed evidence of degenerative arthritis in the right hip socket, as well as markings on the top of the femur. Both of these indicate the deformation results from hip dysplasia. CT scans could also build a paleopathology dataset for reference in future studies. 

Citation: Balisi, M. A., Sharma, A. K., Howard, C. M., Shaw, C. A., Klapper, R., & Lindsey, E. L. (2021). Computed tomography reveals hip dysplasia in the extinct Pleistocene saber-tooth cat Smilodon. Scientific reports11(1), 1-12.