Giant Cave Bears' Sinus Cavities Possibly Sent Them Extinct

Caves across Europe and northern Asia are littered with the bones of dead cave bears, but why did they die out? The answer may lie in the gaps in their skulls. Horia Bogdan/Shutterstock

Human pre-history is filled with terrifying creatures our ancestors had to contend with, but even among these, cave bears (Ursus spelaeus) stand out, having sometimes reached a tonne in weight. For all their fearsome defenses, new evidence suggests their paranasal sinuses, the air-filled cavities at the front of the skull, may have been their undoing.

Cave bear fossils are relatively common across Europe and Asia because so many died while hibernating. Yet we know less than we would like about them, including their diet. They have been suggested as everything from powerful predators to cowardly scavengers or gentle herbivores.

“Knowing the feeding behavior of the cave bear is not a trivial aspect," Dr Borja Figueirido of Spain's Universidad de Malaga said in a statement

The cause of their extinction is also under debate with the two usual suspects – climate change (in this case, a particularly cold part of the ice age) and human competition. In Science Advances, Figueirido and Dr Jack Tseng of the University of Buffalo have brought these together, proposing the bears' were made vulnerable by their need to sleep through longer winters.

Large paranasal spaces can be used to hold nitrogen oxide, which hibernating animals use to depress their metabolism – more space allows greater metabolic control. As a consequence, U. spelaeus sported some impressive holes in its skull, larger than surviving bear species the authors compared them against. In the case of U. spelaeus' even larger Eastern European cousin U. ingressus, these reached 60 percent of the skull volume, but it seems in the long run these holes in their head were not what they needed at all.

"Mechanically speaking, being 'thickheaded' may not be a bad thing because more bone means more structural strength," Tseng said. "However, our findings support the interpretation that requirements for sinus system function in cave bears necessitated a trade-off between sinus development and skull strength." 

Adding large sinuses helped cave bears slow their metabolism during hibernation, but it reduced their chewing force, reducing their dietary flexibility and probably leading to their demise. Alejandro Pérez-Ramos

The authors used the emerging field of biomechanical modeling to estimate the strength of the former bears' jaw muscles. They concluded that as the bears developed bigger sinuses to last the ice age, they gave up some of their chewing capacity, restricting their diet.

This inevitably made the bears more vulnerable to anything that affected their food supply. If the climate changed again or humans started competing with them for food, they were unable to fall back on alternative sources of nutrition. No doubt human competition for caves, a popular explanation for their demise, bears some of the blame, but U. spelaeus may have gotten stuck at an evolutionary dead end.

Tseng thinks identifying what led to the demise of other large mammals can help us understand what allowed our ancestors to survive in the same conditions. That, in turn, could be helpful in learning what is essential for future success in a changing environment.

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