Saber-toothed tigers and dire wolves that lived in the last glacial period had surprisingly high rates of an inherited bone disease, which could reflect inbreeding as the ancient carnivores neared extinction around 10,000 years ago.
More than 6 percent of the thigh bones of tigers extracted from the La Brea tar pits in Los Angeles showed indentations and holes telltale of osteochondrosis, a prevalence at least six times that of modern mammalian species.
“I think there is no animal [species] today that it has a prevalence of 6 percent”, says Hugo Schmokel at IVC Evidensia Academy in Stockholm, Sweden. “In dogs, we are talking below 1 percent. In humans, it is clearly below 1 percent. So that’s incredibly high.”
Osteochondrosis occurs when small sections of growing bone fail to form, leaving holes that can cause pain and lameness. Although rare, the disease affects most mammalian species and tends to run in families or in specific breeds. Nine percent of border collies, for example, have osteochondrosis of the shoulders, while the disease is virtually nonexistent in many other dog breeds. Modern cats almost never develop osteochondrosis, although some cases were found in captive snow leopards that were genetically related to each other.
Schmökel, an orthopedic veterinarian specializing in cats and dogs, says he has always enjoyed looking at skeletons of ancient carnivores in natural history museums and eventually began to wonder if they had the same types of bone diseases as his modern patients.
he approached Mairin Balisi at the Raymond M. Alf Museum of Paleontology in Claremont, California, for access to the museum’s large collection of specimens from the tar pits. There, he closely examined 1,163 saber-toothed tiger leg and shoulder bones (Smilodon fatalis) and 678 bones from the legs and shoulders of dire wolves (Aenocyon dirus), then took X-rays of some of the bones.
Schmökel and his colleagues found that 6 percent of saber-tooth tiger femurs had osteochondrosis lesions. Most of the lesions were less than 7 millimeters across, but a third were up to 12 millimeters, although they were not yet considered large or serious. These injuries were likely too mild to cause pain or affect movement in most animals, Schmökel says.
As for the direwolves, the researchers found the majority of injuries to the shoulder joints, with a prevalence of 4.5 percent, comprised mostly of small lesions. But 2.6 percent of the wolf femurs also had osteochondrosis, and in these cases, most lesions were considered large, greater than 12 millimeters, but not serious.
“We often think of these things as new diseases related to domestication,” says Balisi. “But they are actually in old animals as well. That opens up a lot of new questions, I think.”
The bones span a wide range of dates, from about 55,000 to 12,000 years ago, shortly before the two species went extinct. It makes sense that high rates of an inherited disease would be tied to inbreeding as their populations dwindle, Balisi says, and he hopes to confirm this in the future.
“I think it’s only a matter of time before we can extract the DNA from the targets. And I wouldn’t be surprised if that reflects that these animals were becoming more and more inbred,” she says.
