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A new study suggests flexibility may have helped brown bears survive while their close relative, the cave bear, disappeared.<\/p>\n
Rather than specializing for a single way of life, brown bears adapted their feeding anatomy as the climate changed.<\/p>\n
Evidence from the jaws <\/p>\n
The study was led by Anneke H. van Heteren at the Bavarian State Collections of Natural History (SNSB<\/a>). <\/p>\n Her team measured the lower jaws of nearly 200 bears, more than half of them fossils.<\/p>\n Of those jaws, 21 belonged to fossil brown bears<\/a>, the same species (Ursus arctos) that still roams parts of Europe today.<\/p>\n The rest were living bears and their close relatives, among them the plant-eating cave bear<\/a> and the meat-eating polar bear, which gave the team a wide spread of diets to measure against.<\/p>\n Mechanics of chewing<\/p>\n 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.<\/p>\n 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.<\/p>\n 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.<\/p>\n Two jaw designs kept returning<\/p>\n Until this study, no one had tracked the brown bear jaw across the Ice Age using its form rather than its chemistry.<\/p>\n Earlier attempts to read the diets of ancient bears leaned on chemical traces in the bones.<\/p>\n One analysis<\/a> of Siberian remains found that brown bears changed their diet as the ice sheets retreated.<\/p>\n Cold-period bears carried a longer row of cheek teeth and a chewing muscle set for more leverage. That build delivers extra grinding power.<\/p>\n 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.<\/p>\n 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.<\/p>\n Climate-driven changes in jaw anatomy<\/p>\n Two fossils make the contrast clear. One came from Le R\u00e9gourdou Cave in the Dordogne region of France and dates to roughly 243,000 years ago, a cold stretch of the Ice Age.<\/p>\n 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\u2019s warmer spells.<\/p>\n 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.<\/p>\n Warm-type and cold-type jaws turn up at different depths within the fossil record.<\/p>\n This indicates the brown bear jaw switched back and forth between the two builds many times as glaciers spread and withdrew.<\/p>\n Surviving the Ice Age<\/p>\n 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.<\/p>\n So the authors lean toward faster, reversible routes instead. One is phenotypic plasticity, where a young bear\u2019s jaw grows differently depending on what it spends its life chewing.<\/p>\n The other is epigenetic inheritance<\/a>, in which the activity of genes gets turned up or down and handed to offspring without any change to the DNA<\/a> sequence.<\/p>\n Why cave bears vanished<\/p>\n 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.<\/p>\n The cave bear had committed hard to a plant diet<\/a>, and it could not reverse that specialization when the climate lurched at the close of the Ice Age.<\/p>\n A separate study<\/a> of cave bear teeth argues the animal had lost the dietary flexibility that omnivores keep in reserve.<\/p>\n 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\u2019s long survival.<\/p>\n \u201cTheir ability to cope with such extreme climatic fluctuations likely played a decisive role in their evolutionary success,\u201d explains van Heteren.<\/p>\n A parallel for today<\/p>\n 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\u2019s European brown bear. <\/p>\n 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.<\/p>\n That has a bearing on the present, because brown bears face a fast-moving climate once more. That flexibility may still persist today.<\/p>\n One study<\/a> 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.<\/p>\n Evolution in action<\/p>\n What the fossils settle is plain enough. Europe\u2019s brown bears did not ride out the Ice Age<\/a> unchanged.<\/p>\n They rebuilt their jaws to match each swing of the climate, then shifted them back again.<\/p>\n That knack for reversible change now stands as a leading reason the brown bear outlasted its more rigid cousins.<\/p>\n It also gives conservationists a longer view of an animal that has already demonstrated an extraordinary capacity to adapt.<\/p>\n The study is published in Comptes Rendus Palevol<\/a>.<\/p>\n \u2014\u2013
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