From Dragon to Cave Bear

In 1675, German physician Johann Paterson Hain published a detailed description of the bones of what he believed to have been a dragon. Such bones had been unearthed from German caves for centuries and identified as belonging to various mythical creatures, with unicorns perhaps the most popular - likely since one could sell supposed unicorn horns for their magical healing properties. Fortunately for us, his illustrations were highly accurate and we can identify them for what they really are.

Indeed, while naturalists continued describing such bones as belonging to fantastical animals into the 18th century, physician and rationalist Gregor Horst had beaten them to the punch, pointing out as early as 1656 that bones recovered from Unicorn Cave (yes, that is its actual name) looked remarkably like those of "bears, lions, and humans". Today, we can look at Paterson Hain's original illustrations and confirm that he had produced the first known published drawings of the bones of a cave bear.

The idea that the bones belonged to a bear had taken hold by the late 18th century, although Johann Friedrich Esper, for example, had, in 1774, decided that they belonged to regular polar bears, washed down from the Arctic in Noah's Flood. Then, in 1794, Johann Rosenmüller made the bones the subject of his doctoral thesis, for which he was awarded what we would now call a PhD. Following the then-new fashion for Linnean nomenclature, he gave the mysterious animal the scientific name it bears to this day: Ursus spelaeus - literally "cave bear" in Latin.

We now know that cave bears once lived across almost the whole of Ice Age Europe south of the glaciers, from Spain and southern England in the west to as far east as the Caucasus and northern Iran. The oldest fossils date back to around the Last Interglacial, about 130,000 years ago, which makes them relatively modern as fossil animals go. 

In fact, it's recent enough that we can do DNA analysis on some of the bones, and place cave bears into the ursine family tree in the same way that we can with living species. This shows that the cave bears were closely related to the brown bear, the widespread species that still lives in Europe today and that also includes the American "grizzlies". More recent analysis shows that cave and brown bears probably diverged over a million years ago, during one of the earlier Ice Ages. This is significant for a couple of reasons.

Firstly, it's long before the divergence between different subspecies of brown bear, and, more importantly, before brown bears split from polar bears. This shows that cave bears are, indeed, a distinct species, not just a subspecies. But secondly, since the oldest cave bear fossils are much younger than this, it shows that we are either missing a lot of early fossils, or that they descended from something else that lived in between. It's generally agreed that the latter is most likely correct, with the gap being filled by Deninger's bear (Ursus deningeri), which lived in Europe at around the right time and is sufficiently similar to cave bears that, for some of the 20th century, it was thought to be the same species.

Similar genetic testing has shown that, like polar bears, cave bears could crossbreed with brown bears at least on rare occasions. One estimate is that between 0.9% and 2.4% of the genomes of brown bears living in Europe are derived from cave bears - not wildly dissimilar to the amount of Neanderthal DNA in European humans today.

Physically, cave bears looked much like brown bears, but were, on average, significantly larger. That, however, is mainly because of the great variation in brown bear size, with relatively small individuals living in some areas, especially far inland. Cave bears had a more consistent size, broadly similar to that of modern Kodiak bears, the largest living brown bear subspecies. While it's difficult to weigh an animal you know only from the skeleton, one study put males at 400-500 kg (880-1,100 lbs) and females at 225-250 kg (500-550 lbs).

Undoubtedly, therefore, a cave bear would have been a fearsome animal to meet in the flesh, and a time traveller certainly would not want to anger or startle one. However, it's less clear that they would have tried to hunt you. Most ursine bears are, after all, omnivorous, and not predatory in the way that a big cat is. In the case of the cave bear, the shape of the teeth has long been thought to indicate a relatively herbivorous diet, and a 2011 study comparing the shape of the jaw to that of other bears showed that that, too, indicated they were herbivores. A study of their potential bite force also found that it was similar to that of a panda, implying that they chomped down on tough vegetation rather than meat.

A more detailed analysis of skull shape in 2014 indicated that the cave bear's diet was largely based on leaves, rather than, say, fruit. This does not, however, mean that they never ate meat at all, even if they did so less than modern brown bears do. Isotopic analysis of bones can help us determine what sort of food an animal ate in life, and cave bear bones are recent enough for us to do this sort of study on them... and the results are ambiguous.

A study on cave bears from Belgium found that they were as close to purely herbivorous as the researchers were able to measure. On the other hand, a study on Romanian cave bears found that they do appear to have eaten some meat, even if it wasn't a major part of their diet. It's possible, therefore, that the diet of cave bears changed with the circumstances, either of their geographic locality or of the climate at the time they died. Although likely mostly plant-based, their diet may also have shifted towards omnivory in the pre-hibernation period, as a means of acquiring more protein reserves.

Most fossil cave bears are either cubs or elderly individuals suffering from diseases or injuries that are visible in their bones. This suggests that they had few, if any, predators, being able to fend off most of the animals that could potentially have attacked them. Indeed, while they may have spent more of their lives in caves than brown bears typically do, most of them probably died while they were hibernating. Analysis of the skeletons of some of the younger bears suggests that they took 10 to 14 years to reach full size, longer than it does for brown bears. They probably lived for at least 20 years.

Cave bears went extinct around 22,000 BC, vanishing from France, Italy, and Poland more or less simultaneously. The last survivors seem to have held on in the Alps and in isolated limestone uplands with plentiful caves. There is still some debate around what caused the extinction given that brown bears, living in much the same areas, survived. The date corresponds, more or less, with an exceptionally cold few centuries within the Last Ice Age, so a drop in local plant productivity seems an obvious candidate... but it seems likely that they had faced similar challenges before over the course of their evolutionary history.

However, it's worth noting that modern humans were also becoming more common in Europe at the time, and ignoring human factors in the extinction of animals is rarely a safe bet. If we were responsible, it's noticeable that we didn't manage to do the same to the brown bear, but, in this respect, it may be relevant that cave bears had one significant disadvantage: they relied heavily on caves. 

Since humans at the time were also cave dwellers, it's plausible that they occupied the places that cave bears needed to hibernate. It's true that killing a hibernating cave bear would not have been a trivial matter - bears are not as fast asleep in winter as some other animals - but you'd only need to do it once to claim the cave. For thousands of years, bears and humans were at least in balance in terms of occupying available caves, but there s some evidence that the cave bear population began to decline around 40,000 years ago, just as modern humans supplanted Neanderthals in Europe. Brown bears, apparently more able to construct dens elsewhere, managed to avoid direct competition and survived where their ancient relative did not.

A combination of factors likely played a part. Climate change may have provided the killing blow for an animal that had a less varied diet than brown bears, but, even if it wasn't direct hunting, simple competition with humans may have led to long-term population decline that left them unable to adapt.

[Photo by Holger Uwe Schmitt, from Wikimedia Commons. Cladogram adapted from Kumar et al. 2017.]