Until the end of the last ice age, brown bears in Europe repeatedly changed the shape of their jaws in response to climate fluctuations, which alternated between glacial cold and warmer periods. The same two jaw forms emerged again and again, each better suited to different environmental conditions.
A new study suggests that this flexibility may have helped brown bears survive, while their close relative, the cave bear, went extinct. Instead of specializing in a single lifestyle, brown bears adapted their feeding anatomy to shifting climates.
The research was led by Anneke H. van Heteren from the Bavarian State Collections of Natural History (SNSB). The findings were published in the journal Comptes Rendus Palevol.
Her team measured the lower jaws of nearly 200 bears, more than half of which were fossils. Among these jaws, 21 belonged to fossil brown bears, the same species (Ursus arctos) that still inhabits parts of Europe today.
The remaining animals were living bears and their close relatives, including the herbivorous cave bear and the carnivorous polar bear, providing the team with a wide range of dietary comparisons.
Rather than assessing the bones visually, the researchers placed dozens of three-dimensional reference points on each jaw using a digital manipulator and a surface scanner. The reference points were concentrated around the tooth row and the attachment site of the masseter muscle—a thick muscle that compresses the jaw for chewing—to the bone.
This muscle does the primary work of grinding. Its attachment point on the jaw determines the leverage, so small changes in the attachment site affect how strongly the bear can break down tough food.
Before this study, no one had tracked the brown bear’s jaw across the ice age using its shape rather than its chemical makeup. Earlier attempts to determine the diet of ancient bears relied on analyzing chemical traces in bones.
An analysis of Siberian bear remains showed that brown bears changed their diet as the glaciers retreated.
Bears living during cold periods had a longer row of chewing teeth and more developed masseter muscles for greater leverage. This structure provided extra grinding power.
Their jaws resembled those of brown bears living today in cold northern regions or at high altitudes. Conversely, bears from warmer periods had jaws that were similar to each other.
They had a shorter tooth row and a masseter muscle that somewhat reduced leverage, along with a lighter build, similar to that of modern European brown bears.
Two fossils vividly illustrate this contrast. One was found in the Le Regourdou cave in the Dordogne region of France and dates to about 243,000 years ago, a cold period of the ice age.
The second discovery was made in the Postes cave in Extremadura, Spain, and dates to between 71,000 and 104,000 years ago, one of the warmest periods of the ice age.
The Spanish fossil has a shorter tooth row and a shorter muscle lever, characteristic of a bear living in a warm climate. Moreover, these two forms are independent of geological age.
Warm and cold-type jaws appear at different depths in the fossil record. This suggests that the brown bear’s jaw repeatedly switched between the two body types as glaciers advanced and retreated.
The jaw change poses a puzzle. Typical genetic evolution occurs over thousands of generations, which is too slow to keep up with a climate that rarely remained stable for long during the ice age.
Therefore, the authors lean toward faster and reversible pathways. One is phenotypic plasticity, where a young bear’s jaw grows differently depending on what it chews over its lifetime. Another type of inheritance is epigenetic, where gene activity is increased or decreased and passed on to offspring without altering the DNA sequence.
Neither locks the bear into a fixed state. Both can be switched on and off within one or two generations, and both can work in reverse. Perhaps this reversibility determined who survived.
The cave bear stuck firmly to a plant-based diet and could not change this specialization when a sharp climate shift occurred at the end of the ice age. A separate study of cave bear teeth argues that the animal lost the dietary flexibility that omnivores keep in reserve.
The brown bear retained this flexibility, continued to eat what the season offered, and survived. The researchers claim that this talent is key to the bear’s long life.
“Their ability to cope with such extreme climate fluctuations likely played a crucial role in their evolutionary success,” explains van Heteren.
There is a large gap between the warm and cold fossil jaws. It is roughly equal to the difference in jaw depth between a modern grizzly and a European brown bear. The differences are enough that the fossil animals might be better seen as a series of climate-linked forms rather than a single unchanging bear species.
This has relevance for the present, as brown bears again face rapidly changing climate. This flexibility may persist today.
One study of wild European populations showed that their diet is still determined much more by what mothers teach their cubs than by fixed genetics. This leaves them with a wide range of options depending on changing conditions.
The results of the fossil study are clear enough. European brown bears survived the ice age not without change. They rebuilt their jaws, adapting to each climate shift, and then returned them to their original state.
This ability for reversible change is now one of the main reasons why the brown bear outlived its more conservative relatives. It also allows conservationists to view the animal from a longer-term perspective, considering its exceptional adaptability.
About the Author
