When we think about the mammoth, we picture the 3-meter-high, 6-ton beast roaming northern Europe in imposing herds, fending off human hunters with their dangerous tusks. We don’t imagine genetically deformed creatures stumbling through a doomed landscape, going to desperate lengths to stay alive in a rapidly changing world. But now, an unusual feature on some mammoth fossils dredged from the North Sea suggests that inbreeding may have hastened the mammoth’s extinction 10,000 years ago.
The strange feature in question is a round, flat area that researchers were surprised to find on a mammoth neck vertebra from the North Sea. This meant that the neck bone once had a small rib attached to it, a rare abnormality that can point to other skeletal problems. When neck ribs—also called cervical ribs—occur in humans, for example, 90% of affected individuals die before they reach adulthood, not because of the rib itself, but because the condition occurs alongside other developmental problems. Neck bones may be fused together, for example, or bones in the lower back may fail to solidify. The condition is also associated with chromosome abnormalities and cancer.
Curious as to how widespread the neck rib abnormality might have been among North Sea mammoth populations, researchers led by paleontologist Jelle Reumer of the Natural History Museum Rotterdam in the Netherlands combed local museum collections looking for rib facets on mammoth neck bones that had been dredged from the North Sea. They found them in three of nine cases , the researchers report today in PeerJ. "This seemed [to be] an extremely high incidence," Reumer says. A similar search among modern-day elephant bones in museums, for example, revealed that only one out of 21 individuals had a cervical rib.
"Cervical ribs indicate there has been a disturbance of early pregnancy," says paleontologist Frietson Galis of the Naturalis Biodiversity Center in Leiden, the Netherlands, who worked with Reumer on the analysis. A neck rib could be a sign that the mother suffered harsh conditions, like disease or famine, while pregnant, or it could be a sign of genetic trouble due to inbreeding. In the case of the mammoths, Reumer and Galis suspect both.
Abnormalities fit in with Reumer's preferred explanation for mammoths' extinction, in which climate change  fragmented their habitat, separating pockets of the animals from each other. In such small populations, inbreeding ensued, and the loss of genetic variation left the animals with few defenses against new onslaughts from parasites, disease, and human hunting. Galis describes the vicious cycle of inbreeding and vulnerability as an "extinction vortex." Still, because the cervical rib-sporting mammoth vertebrae weren’t part of complete skeletons, the researchers couldn’t tell if their bearers suffered other deformities.
Eleftheria Palkopoulou, a geneticist who has analyzed mammoth DNA but was not involved in the new study, says this scenario fits with her research that shows mammoth populations shrank about 20,000 years ago. She notes that genetic analysis could determine if inbreeding was truly occurring in the last mammoth populations, and that such studies are "now becoming technologically possible."
Paleontologist Daniel Fisher of the University of Michigan, Ann Arbor, is skeptical, however, noting that the study included only a small number of mammoths and that inbreeding could be purely a result of dwindling population rather than a cause. Still, he says, "there's no question that [the neck rib] represents some interesting natural history."
"It's a fascinating idea," says Ross MacPhee, a paleontologist at the American Museum of Natural History in New York City, who was not involved in the study. Still, he notes that the researchers relied on the bones' approximate ages, which span a period of 20,000 years, but didn't perform the radiocarbon dating that could pin down the abnormalities to the very last years of the mammoths’ reign. Without that precision, it's impossible to know if the abnormalities occurred only in a population that was in decline, or if they persisted for tens of thousands of years as an unusual but harmless feature. "I'm just not convinced” that the neck ribs are a smoking gun, he says.