The cave bear of Ice Age Europe was as big as any grizzly or polar bear alive today, weighing in at 1,500 pounds and standing some ten feet tall. Despite their fierce proportions, the bears, paleontologists have believed, ate mostly roots and berries. Their jawbones and teeth, recovered in caves throughout Europe, seem geared to plant grinding rather than flesh shearing. But Grant Hilderbrand, a zoologist from Washington State University, has always wondered how bears of such formidable bulk could have survived on protein-poor fare, when the smaller polar bear needs about 200 pounds of seal meat a day. Hilderbrand’s skepticism was well- founded. He has discovered good evidence that cave bears were probably omnivores with a decided taste for meat.
Hilderbrand looked at the ratio of two different forms of nitrogen in the bones of cave bears. Nitrogen is a fundamental building block of plant proteins, and plants preferentially absorb the heavier type of nitrogen from rainwater and soil. Herbivores accumulate and concentrate the nitrogen from plants in their tissues, as do the predators of herbivores. The heavy nitrogen bias becomes more pronounced at every step of the food chain. If cave bears survived on plants alone, the nitrogen levels in their bones should match those of herbivores.
Hilderbrand got fragments of cave-bear bones from the National Museum of Natural History in Washington, D.C., the American Museum of Natural History in New York, and the Bulgarian Museum of Natural History in Sofia. He compared the nitrogen content of these fragments with blood samples from 11 captive brown bears fed diets ranging from all greens to all meat. The bears on meaty diets had blood nitrogen ratios that matched those found in the cave-bear bones.
By learning more about the dietary needs of past and present bears, Hilderbrand hopes he and his colleagues can determine the most suitable environments for the reintroduction of endangered species like the grizzly. It’s not clear, for example, whether grizzlies can be reintroduced into some parts of their former range where prey are not as plentiful today as in the past. If biologists underestimate the amount of meat a bear population needs, they may wrongly assume that an ecosystem will support more bears than it really can. Says Hilderbrand, Big bears have a tough time making it on a strictly vegetarian diet.