Tens of thousands of years ago, the caves of central Europe were full of two species: the aptly named cave bears and cave lions. Even bigger than their modern counterparts, these ancient monsters were locked in deadly battle for survival.
Cave lions were particularly huge, about 25% bigger than modern African lions. While modern bears do hibernate in caves, that's pretty much all they do in there. Cave bears, on the other hand, spent most of their lives inside. The fossil record reveals their grim struggles, as lone cave lions would venture into the bears' homes looking for food.
It's not always easy to get that much insight into creatures that died out 27,500 years ago, but cave bear dens have proven spectacularly good at preserving evidence of these ancient struggles. For instance, three cave lion skeletons were found about a half-mile into the massive Ursilor cave in Romania, so named for its cave bear inhabitants, and a whopping 13 cave lion skeletons have been discovered in Germany's Zoolithen cave. Both of these hoards were found deep into cave bear territory, meaning the cave lions had ventured inside to hunt these massive creatures.
Almost all the cave lion skeletons found were those of older males, which reinforces the idea that this was a hunting party - or at least it was the adult males that did most of the fighting, and so were the most likely to leave their skeletons in the cave. Researchers believe that the cave lions had to go after cave bears once their earlier food sources, the woolly mammoth and woolly rhinoceros, died out.
Unlike their modern counterparts, cave bears were herbivorous and so would not have eaten any of the meat that the dead cave lions represented. Instead, they would have simply gone about their business, trampling the cave lion bones into the cave floor and, eventually, into the fossil record.
While these ancient ancestors of bears and lions were pretty evenly matched, subsequent evolution has made this fight decidedly more one-sided, perhaps in part because bears and lions no longer share a habitat as they did tens of thousands of years ago. Modern lion skulls are thin enough that one decent blow from a bear will crush it, killing the lion instantly. California settlers during the Gold Rush found this out the cruel way, as they organized fights between bears and various other animals as a rather sick way to pass the time, eventually shipping in African lions in a futile attempt to give the bears a challenge. For more on this rather sordid affair, I direct you to this conversation with Britain's leading intellectuals.