Why Are Caves Full of Bear Bones? An Animal That Died Where It Slept

The animal used caves for winter dormancy, returning to the same sites across many generations, and a proportion of individuals entering dormancy in poor condition died during it without ever leaving. Because the same caves were used for tens of thousands of years, those individual deaths accumulated into deposits containing the remains of thousands of animals in a single site. Cave conditions preserve bone well, being stable in temperature, sheltered from weathering and frequently alkaline from limestone, so the material survives in excellent condition. The result is that this species is known from more complete remains than almost any other extinct large mammal, including specimens of every age from unborn cubs to the very old.

The remains support a detailed reconstruction:

• •Larger than a modern brown bear, with males substantially heavier
• •A domed forehead and a steep facial profile, unlike a brown bear
• •Broad flat molars adapted for grinding plant material
• •Reduced premolars, which is a diagnostic feature
• •Powerful forelimbs and a build suited to strength over speed
• •Living across Europe and into Asia from around three hundred thousand years ago

The diet has been reconsidered substantially and the evidence is unusually direct. Tooth shape suggested plant eating from the beginning, since the broad grinding molars resemble those of a herbivore rather than of a carnivore. Chemical analysis of the bones, examining the ratios of nitrogen and carbon isotopes that differ between plant and animal foods, has confirmed a diet overwhelmingly of plants across most populations, comparable to that of a modern panda or a heavily vegetarian brown bear. Some populations show signatures indicating more animal material, so the picture varies regionally. Microscopic wear patterns on the teeth support the plant interpretation. That specialisation is central to explaining what happened to the species.

The sheer quantity of material allows questions about individual lives that are impossible for most extinct species. Growth layers in teeth record the age of an animal at death and the season in which it died, and the distribution shows that deaths concentrated in late winter, which is what would be expected if animals were dying during dormancy after exhausting their reserves. Cub mortality was high, with the remains of very young animals abundant. Healed injuries, arthritis, tooth wear and dental disease are all recorded, so individual medical histories can be reconstructed. Ancient DNA has been recovered from many specimens and from cave sediments without any bone at all, which has established population structure, decline and relationships to living bears.

Extinction occurred around twenty four thousand years ago, at the coldest part of the last glaciation, and the explanations under discussion are the familiar pair. Climate is the leading candidate, since a large animal dependent on plant material faced shrinking vegetation as conditions deteriorated, and populations declined progressively over the preceding tens of thousands of years rather than collapsing suddenly, which genetic evidence from ancient DNA has now established. Human pressure is the other, through competition for the caves themselves, which people also used, and through hunting, for which there is direct evidence in the form of butchery marks on bones and projectile injuries. The current position gives most weight to climate with humans contributing.

The animal hibernated in the same caves across many generations, and those dying during dormancy accumulated into deposits holding thousands of individuals, preserved in stable alkaline conditions. Broad grinding molars, isotope chemistry and tooth wear all indicate a diet overwhelmingly of plants. Extinction around twenty four thousand years ago followed a long decline recorded in ancient DNA, attributed mainly to climate with human pressure contributing.