More than 3,000 Smilodon vertebrae from Los Angeles' La Brea Tar Pits yielded hundreds of spinal deformities, including three cats with a normally rare nerve-sheath tumor linked to enlarged hollows in lower-back bones.
The researchers estimate that tumor rate was about 1,000 times higher than in modern humans, pointing to possible inbreeding as saber-toothed cat populations shrank near extinction.
Two of the three tumor-bearing fossils date to 12,000 and 13,000 years ago, shortly before Smilodon vanished at the end of the Pleistocene.
Painful back disease may also have made hunting large prey harder and pushed weakened cats toward easier targets, increasing the chance they became trapped in asphalt.
The findings add to a 2023 report of high limb-bone disease in Smilodon, but scientists say DNA from La Brea fossils is still needed to confirm inbreeding directly.