Before the last Ice Age ended, Europe’s brown bears repeatedly reshaped their jaws as the climate swung between glacial cold and warmer periods.
The same two jaw forms appeared again and again, each better suited to a different environment.
A new study suggests flexibility may have helped brown bears survive while their close relative, the cave bear, disappeared.
Rather than specializing for a single way of life, brown bears adapted their feeding anatomy as the climate changed.
Evidence from the jaws
The study was led by Anneke H. van Heteren at the Bavarian State Collections of Natural History (SNSB).
Her team measured the lower jaws of nearly 200 bears, more than half of them fossils.
Of those jaws, 21 belonged to fossil brown bears, the same species (Ursus arctos) that still roams parts of Europe today.
The rest were living bears and their close relatives, among them the plant-eating cave bear and the meat-eating polar bear, which gave the team a wide spread of diets to measure against.
Mechanics of chewing
Rather than eyeball the bones, the researchers mapped dozens of three-dimensional reference points onto each jaw with a digitizing arm and a surface scanner.
The reference points clustered around the tooth row and the place where the masseter, the thick muscle that clamps the jaw shut for chewing, attaches to the bone.
That muscle does the heavy work of grinding. Where it anchors on the jaw sets how much leverage it has, so small moves in the attachment point change how hard a bear can crush tough food.
Two jaw designs kept returning
Until this study, no one had tracked the brown bear jaw across the Ice Age using its form rather than its chemistry.
Earlier attempts to read the diets of ancient bears leaned on chemical traces in the bones.
One analysis of Siberian remains found that brown bears changed their diet as the ice sheets retreated.
Cold-period bears carried a longer row of cheek teeth and a chewing muscle set for more leverage. That build delivers extra grinding power.
Their jaws matched those of brown bears living today in cold northern regions or at high elevations. The warm-period bears went the other way.
They had a shorter tooth row and a chewing muscle that surrendered some of that leverage, a lighter build similar to that of modern European brown bears.
Climate-driven changes in jaw anatomy
Two fossils make the contrast clear. One came from Le Régourdou Cave in the Dordogne region of France and dates to roughly 243,000 years ago, a cold stretch of the Ice Age.
The other came from Postes Cave in Extremadura, Spain, and dates to somewhere between 71,000 and 104,000 years ago, one of the Ice Age’s warmer spells.
The Spanish fossil carries the shorter tooth row and shorter muscle lever that mark a warm-climate bear. The two forms also pay no attention to geological age.
Warm-type and cold-type jaws turn up at different depths within the fossil record.
This indicates the brown bear jaw switched back and forth between the two builds many times as glaciers spread and withdrew.
Surviving the Ice Age
A jaw that changes creates a puzzle. Ordinary genetic evolution works over thousands of generations, far too slow to keep pace with a climate that rarely held steady for long during the Ice Age.
So the authors lean toward faster, reversible routes instead. One is phenotypic plasticity, where a young bear’s jaw grows differently depending on what it spends its life chewing.
The other is epigenetic inheritance, in which the activity of genes gets turned up or down and handed to offspring without any change to the DNA sequence.
Why cave bears vanished
Neither locks a bear in. Both can switch on and off within a generation or two, and both can run in reverse. That reversibility may be what decided who survived.
The cave bear had committed hard to a plant diet, and it could not reverse that specialization when the climate lurched at the close of the Ice Age.
A separate study of cave bear teeth argues the animal had lost the dietary flexibility that omnivores keep in reserve.
The brown bear kept that flexibility, went on eating whatever the season offered, and came through. That talent, the researchers argue, is central to the bear’s long survival.
“Their ability to cope with such extreme climatic fluctuations likely played a decisive role in their evolutionary success,” explains van Heteren.
A parallel for today
There is a large gap between the warm and cold fossil jaws. It runs about as deep as the difference between a modern grizzly and today’s European brown bear.
The variation is enough that the fossil animals might be better understood as a series of distinct climate-linked forms rather than one unchanging bear.
That has a bearing on the present, because brown bears face a fast-moving climate once more. That flexibility may still persist today.
One study of wild European populations found their diets are still steered far more by what mothers teach their cubs than by fixed genetics. This keeps their options open as conditions change.
Evolution in action
What the fossils settle is plain enough. Europe’s brown bears did not ride out the Ice Age unchanged.
They rebuilt their jaws to match each swing of the climate, then shifted them back again.
That knack for reversible change now stands as a leading reason the brown bear outlasted its more rigid cousins.
It also gives conservationists a longer view of an animal that has already demonstrated an extraordinary capacity to adapt.
The study is published in Comptes Rendus Palevol.
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