Aching bones: Ice Age saber-toothed cats and dire wolves also suffered from pathological joint wear and tear, known as osteochondrosis. As a result, bone cells and cartilage in the joints increasingly perish and cause arthrosis. Fossil analysis shows that up to six percent of Ice Age predators suffered from this partially genetic disease. This may indicate that incest was rampant in the declining populations of these cats and wolves at the time.
Osteochondrosis dissecans is a joint disease that can affect humans and animals alike. A circulatory disorder initially leads to the death of bone cells in the area of the joint. As a result, the overlying cartilage does not receive enough nutrients and also dies, resulting in progressive joint wear and tear. In addition to overloading the joints, risk factors for osteochondrosis are also genetic factors.
While such cases of osteochondrosis have been well researched in humans and domestic animals, the prevalence among wild animals is still largely undocumented.
History in the Ice Age
Hugo Schmökel of the Swedish Evidensia Academy and his colleagues have now extended osteochondrosis research not only to the field of wild animals, but also to the past. In a large-scale study, they examined over 1,000 fossil bones of saber-toothed cats (Smilodon fatalis) and over 500 direwolves (Aenocyon dirus) for signs of joint wear. Osteochondrosis typically leaves ring-shaped defects on the affected bone sections, which can be seen on x-rays and sometimes even with the naked eye.
The predators studied lived in the Ice Age 55,000 to 12,000 years ago and met their end in the La Brea Tar Pits near Los Angeles. Cats and wolves unwittingly followed them to their deaths, lured by large prey that had once gotten stuck in the asphalt pits there. Their bones have been preserved in large numbers to this day.
Joint problems apparently widespread
And these bones now reveal that both saber-toothed cats and direwolves apparently once suffered from osteochondrosis. In the Ice Age cats, Schmökel and his team found characteristic bone defects in six percent of the examined knee joints. These were mainly small defects with a diameter of less than seven millimeters. At the time the predators died, five knee joints had already developed mild arthrosis.
In the case of the direwolves, 4.5 percent of the shoulder and 2.6 percent of the knee joints were marked by osteochondrosis, as the researchers report. While the defects on the shoulders were mostly small, Schmökel and his colleagues found some larger marks with a diameter of more than twelve millimeters on the knee joints.
Incest as a trigger?
The researchers themselves are surprised by this unexpectedly large number of prehistoric osteochondrosis cases. “The prevalence of six percent of affected knee joints in the Smilodon fatalis population is high compared to humans with a prevalence of 0.015 to 0.029 percent and domestic dogs with a prevalence of 0.05 percent,” explain Schmökel and his team. The only metric that has a modern equivalent is the direwolves’ shoulder defects. They also occur in similarly high numbers in some overbred dog breeds such as the Border Collie and the Greater Swiss Mountain Dog.
This association could also explain why osteochondrosis was so common in the Ice Age predators studied. “Because osteochondrosis is a developmental disease with a genetic basis, its occurrence may indicate a genetic response to environmental stress, inbreeding, or other broader factors,” the researchers report. Both saber-toothed cat and direwolf were close to extinction between 12,000 and 10,000 years ago. According to Schmökel and his colleagues, shrinking populations could have led to more inbreeding and thus to more osteochondrosis cases.
Family cohesion ensured survival
How badly saber-toothed cats and direwolves once suffered from the symptoms of their joint problems paleontologists cannot say. But even if the osteochondrosis affected the predators’ hunting abilities, it probably didn’t mean certain death. Schmökel and his colleagues assume that a direwolf with osteoarthritis was probably supported by the other members of his pack.
In the case of sick saber cat kittens, this role may have been fulfilled by the mother, who cared for the little ones for several years. (PLoSONE, 2023; doi: 10.1371/journal.pone.0287656)
Source: PLoSONE