What evidence suggests that inbreeding and mobility impairments contributed to the extinction of saber-toothed cats?

What the evidence shows

Fossils and studies point to a striking pattern: saber-toothed cats show signs of both inbreeding and body problems that would have made moving and hunting harder before they disappeared [1][32][50][66]. Population declines appear to have led to a spike in spinal deformities and rare tumors [33][49].

Spinal disease in the fossil record

• Researchers examining saber-toothed cat fossils from the La Brea Tar Pits found a wide range of spinal deformities, including serious developmental abnormalities and widened holes in vertebrae that indicate spinal nerve tumors [2][51][52][53]. Hundreds of deformities included fused spinal bones and abnormally split shapes [55]. Three cats had vertebrae with cavernous hollows caused by slow-growing tumors on the spinal nerves [56].
• The tumor rate was unusually high: about 353 cases per 100,000 Smilodons [8], roughly 1,000 times higher than the very rare rate in humans [14][57]. Two of the tumor-bearing fossils date to 12,000 and 13,000 years ago, shortly before the species went extinct at the end of the Pleistocene [19][62].
• Other skeletal evidence includes fused vertebrae from arthritis [29][37], ossification between vertebrae [30][38], broken legs, dislocated hips, bone infections, and back injuries [31][36], and bone damage from the teeth of other saber-toothed cats [39]. Fossil bones can also show evidence of arthritis, cancer, nutritional stress, and fractures [28][35].

Why researchers think inbreeding was involved

• The high prevalence of developmental abnormalities led the team to suspect the species had become inbred, though they noted it cannot be proven without genetic analysis [9]. As populations shrank, fewer mates were available, so harmful genetic variants could become concentrated and increase the chance that kittens inherited health problems [4][9].
• Researchers suggest the diseased spines may have been caused by inbreeding during the species’ final phase, when cat populations were waning [13][54]. One researcher said it makes sense that high rates of an inheritable disease would be tied to inbreeding as populations declined [47].
• Similar spinal deformities are found in inbred gray wolf populations today [10][18][61]. Other now-extinct mammals, including mammoths, woolly rhinos, and giant deer, also show signs of inbreeding right before their disappearance [20].
• A 2023 study described high rates of bone disease in saber-toothed cat limbs that may also have been caused by inbreeding [21][63]. Another analysis found that more than 6% of thigh bones from La Brea showed osteochondrosis, an inheritable bone disease, at least six times the prevalence in modern mammals, possibly reflecting a dwindling gene pool as they neared extinction [42][43][44].
• Genetic evidence is limited but consistent: saber-toothed cats had low mitochondrial genome diversity, supported by low genetic diversity between short mitochondrial DNA fragments [34].

How mobility problems might have hurt them

• Spinal nerve tumors cause significant pain and reduced limb function, and could have made cats particularly vulnerable to the lure of prey animals stuck in tar pits [5]. In advanced cases, the nerve and affected limb eventually lose function, making hunting very difficult [7]. Pain and difficulty hunting might have led sick cats to take greater risks and get trapped [6][11].
• Saber-toothed cats likely hunted by bracing on their hind legs, pulling back, and launching at their prey, then grappling with heavy animals like bison, which would have stressed the lower back [16][59]. So spinal disease would have been especially damaging for them [15][59]. Sick cats may have sought easy prey and then become trapped in the tar too [17][60].
• A deformed Smilodon pelvis from La Brea was identified as congenital hip dysplasia, a condition that would have been extremely problematic for a hunter that needed strong hind limbs and healthy hips [23][24][26]. This individual was hobbled since kittenhood and could never have hunted or defended territory on its own [25]. Other Smilodon fossils show healed grievous injuries that likely would have meant starving without social support [27].
• The bone disease osteochondrosis can cause pain and limping, but in most of the La Brea animals the lesions were probably too mild to cause pain or affect movement, according to one researcher [45][46]. That is an important caveat to the mobility argument.

What is still uncertain

The inbreeding explanation rests mostly on skeletal patterns and comparisons with living inbred animals, not yet on direct ancient DNA: scientists have yet to pull usable DNA from La Brea’s saber-toothed cat fossils [65]. Researchers also caution that the elevated tumor rate found in the La Brea collection may not apply more broadly to all saber-toothed cats [58]. So the evidence suggests a likely role for inbreeding and mobility impairment, but it does not prove those factors alone caused extinction [9][58][65].