The extinct cave bear may have died off because of their evolved inability to chew meat due to their large sinuses, a new study suggests.
US researchers say that a cooling climate forced cave bears to develop large paranasal sinuses – the air-filled spaces within the bones of the skull – to allow for greater metabolic control during hibernation.
That evolution, in turn, changed the shape of cave bears’ skulls, potentially compromising their ability to chew.
Cave bears were a species of bear (Ursus spelaeus) that lived in Europe and Asia that went extinct about 24,000 years ago.
Previous research has shown that cave bears were mainly herbivorous but, even during cooling periods when vegetation waned, they didn’t change their diet.
This dietary inflexibility – likely caused by their evolved inability to chew meat properly – led to cave bears’ demise, the team argue.
An artist’s impression of the European cave bear, which ranged in weight from around 400 to 1,000 kg (about 880 to 2,200 pounds)
The well-developed sinus system in the now extinct cave bear (top left) is associated with uneven mechanical stress distributions in biting simulations (bottom left). The much less developed sinus system in living bears, for example the sun or honey bear, (top right) allows mechanical stress to distribute evenly over the forehead region, as seen in the biting simulation (bottom right)
‘Our study proposes that climate cooling probably forced the selection of highly developed sinuses, which in turn led to the appearance of the characteristic domed skull of the cave bear lineage,’ said study author Dr Alejandro Pérez-Ramos from the Universidad de Malaga.
‘Through the use of new techniques and virtual methods, we propose that large sinuses were probably selected in cave bears in order to be able to hibernate for longer periods with very low metabolic costs.’
The cave bear was named as such for its its habit of inhabiting caves, where its remains are frequently preserved.
In European caves, the remains of more than 100,000 cave bears have been found, including in England, Germany, Russia and Italy.
European cave bears were comparable in size to the largest modern-day bears, with weights ranging from around 400kg to 1,000kg.
A cave bear (Ursus spelaeus) skeleton. It is likely that most cave bears died in the severe glacial winters
Two main theories of how and why they died out have been proposed – a human-driven decline, either by competition for resources or direct hunting, and a demise in population sizes as a result of the climatic cooling that caused vegetation to wane.
Previous research shows that cave bears were primarily herbivorous at least from 100,000 to 20,000 years ago.
But even during cold snaps when vegetation productivity waned, cave bears didn’t change their diets.
The researchers propose that both dietary inflexibility and competition with humans for cave shelters is what led to their extinction.
The researchers analysed 3D computer simulations of the cave bear’s biting mechanisms and the resulting stresses on the skull, compared with today’s bear species, such as the sun bear (Helarctos malayanus).
They found that cave bears had large sinuses that shaped their skulls in such a way that they couldn’t chew effectively with their front teeth, which would have been crucial when eating meat.
The development of large sinuses caused the bears’ cranial dome to expand upward and backward from the forehead, changing the geometry of the bear’s skull.
An expanded sinus system resulted in less structural support to resist the physical forces generated by chewing.
The cave bear head was very large, and the jaws bore distinctive teeth, which suggests that the animal was largely vegetarian
‘This geometrical change generated a mechanically suboptimal cranial shape, with a very low efficiency to dissipate the stress along the skull, particularly when biting with the canines or carnassials, the teeth most often used by predatory mammals,’ Dr Pérez-Ramos said.
Ultimately, in a cruel ironic twist, developing larger sinuses to survive hibernation may have resulted in their eventual extinction.
‘Being able to stay alive during the coldest periods would have been equally important to human and bear alike,’ said professor Jack Tseng of pathology and anatomical sciences at the University at Buffalo
‘The success or demise of prehistoric megafauna, such as cave bears, provide crucial clues as to how humans may have out-competed and out-survived other large mammals during a critical time for the evolution of our own species.’
The study has been published in journal Science Advances.
