How Europe’s Brown Bears Reshaped Their Jaws During the Ice Age

Before the last Ice Age came to an end, Europe’s brown bears repeatedly altered their jaws as the climate shifted between glacial conditions and warmer intervals.

The same two jaw types reappeared time after time, with each form better adapted to a particular environment.

New research indicates that this adaptability may have enabled brown bears to endure while their close cousin, the cave bear, died out.

Instead of becoming fixed around one lifestyle, brown bears adjusted the anatomy used for feeding as the climate changed.

Evidence preserved in the jaws

Anneke H. van Heteren of the Bavarian State Collections of Natural History (SNSB) led the study.

Her researchers examined the lower jaws of almost 200 bears, with fossils accounting for more than half of the sample.

Among these specimens were 21 fossil brown bears: the same species, Ursus arctos, that still inhabits parts of Europe.

The remaining jaws came from modern bears and closely related species, including the herbivorous cave bear and carnivorous polar bear, providing a broad dietary range for comparison.

The mechanics of chewing

Rather than assessing the bones by sight, the team used a digitising arm and surface scanner to plot dozens of three-dimensional landmarks on every jaw.

These landmarks were concentrated around the tooth row and the point at which the masseter attaches to the bone. The masseter is the powerful muscle that closes the jaw during chewing.

This muscle carries out most of the grinding work. Its attachment point determines its leverage, meaning that even slight shifts can alter how much force a bear can use to crush hard foods.

Two brown bear jaw designs repeatedly emerged

Before this research, nobody had followed changes in the brown bear jaw through the Ice Age by studying its shape rather than its chemistry.

Previous efforts to reconstruct ancient bear diets had relied on chemical signals preserved in bone.

One investigation of remains from Siberia showed that brown bears changed their diets as the ice sheets receded.

Bears from colder periods had a longer row of cheek teeth and a chewing muscle positioned to provide greater leverage. This arrangement produced more grinding force.

Their jaws resembled those of present-day brown bears from cold northern areas or high-altitude habitats. Bears from warmer periods showed the opposite pattern.

They possessed a shorter tooth row and a chewing muscle with reduced leverage, producing a lighter jaw similar to that of modern European brown bears.

Climate-driven changes in jaw anatomy

Two fossils illustrate the difference clearly. One was recovered from Le Régourdou Cave in France’s Dordogne region and is about 243,000 years old, dating from a cold phase of the Ice Age.

The other came from Postes Cave in Extremadura, Spain, and dates from between 71,000 and 104,000 years ago, during one of the Ice Age’s warmer periods.

The Spanish specimen has the shorter tooth row and shorter muscle lever associated with a warm-climate bear. Nor do the two forms follow geological age.

Jaws of both the warm and cold types occur at different levels in the fossil record.

This suggests that brown bear jaws repeatedly shifted between these two constructions as glaciers advanced and retreated.

Surviving the Ice Age

A changing jaw presents a problem: conventional genetic evolution takes thousands of generations, which is much too slow to track a climate that was seldom stable for long during the Ice Age.

The authors therefore favour quicker, reversible mechanisms. One possibility is phenotypic plasticity, in which a young bear develops a different jaw according to the foods it chews throughout its life.

The other is epigenetic inheritance, where gene activity is increased or reduced and then passed to offspring without altering the DNA sequence.

Why cave bears disappeared

Neither mechanism permanently confines a bear. Both can be activated or deactivated within one or two generations, and both are reversible. This capacity to reverse may have determined which species survived.

The cave bear had become strongly specialised for a plant-based diet and could not undo that specialisation when the climate shifted sharply near the end of the Ice Age.

A separate study of cave bear teeth suggests that the species had lost the dietary flexibility retained by omnivores.

Brown bears preserved that flexibility, continued eating whatever was available each season, and survived. The researchers argue that this ability was fundamental to the species’ long-term persistence.

“Their ability to cope with such extreme climatic fluctuations likely played a decisive role in their evolutionary success,” explains van Heteren.

A parallel for the present day

The difference between warm- and cold-period fossil jaws is substantial. It is roughly as pronounced as the gap between a modern grizzly and a present-day European brown bear.

The variation is great enough that the fossil animals may be more accurately viewed as a succession of distinct forms linked to climate, rather than a single unchanging bear.

This matters today because brown bears are again confronting a rapidly changing climate. Their flexibility may still be present.

One study of wild European populations found that their diets continue to be shaped much more by what mothers teach their cubs than by fixed genetics. This leaves their choices open as circumstances shift.

Evolution in action

The fossils make one point clear: Europe’s brown bears did not pass through the Ice Age without changing.

They reshaped their jaws for every climatic swing, then altered them again when conditions reversed.

This ability to change reversibly is now regarded as a major reason why brown bears outlasted their less flexible relatives.

It also offers conservationists a longer-term perspective on an animal that has already shown an exceptional ability to adapt.