About 25 percent of the weight of living creatures' bones is collagen, a protein that contains carbon and nitrogen atoms. Multiple versions of these atoms, or isotopes, exist in the bones, Bocherens explained.
"The ratio of these isotopes for a given chemical element depends on the type of food consumed by the animal during its lifetime," he said.
The number and types of carbon and nitrogen isotopes found in collagen can reveal whether an ancient creature ate mostly leafy plants, tubers and legumes, meat, or fish.
Collagen also contains a radioactive isotope of carbon, which decays at a constant rate and can be used for dating the remains.
The researchers surveyed about a hundred bones in the cave and found that about a quarter of them still contained collagen.
An isotope analysis of those bones showed that the cave bears were in fact vegetarians and that they died in the cave around 30,000 years ago, the same time the paintings were made.
"The new results confirm that cave bears and humans were not competing for food, as the first ones were vegetarians, whereas the second ones were mostly meat-eaters," Bocherens said.
"It shows also that some of the bear skeletons were in the cave when the paintings were made," suggesting humans moved in after the resident bears died.
But not all researchers agree with the findings.
Paul Pettitt is a researcher from the Department of Archaeology at the University of Sheffield in England who was not involved in the study.
"The only direct link between the art and bears are the bear-claw marks superimposed on some of the art," he said.
"But we obviously do not know the time at which these bears scratched the walls."
In terms of the chemistry and dietary interpretations, the work is robust, Pettitt said.
"But we have to remember that the number of bears actually dated represent a small subsample of all the bears at the cave," he said.
"So this only gives us a glimpse that bears were in the cave as early as 38,000 years [ago] and as late as 27,000 years [ago]. Given their abundance, they were probably around before and or after this, too."
For the future, the researchers hope to expand their work to include genetic studies of the cave bear bones. The scientists could perhaps link their findings to other prehistoric sites with evidence of late Neandertals and early modern humans.
"One goal is to document the possible impact of changes in human behavior on cave bears around this time," Bocherens said.
Drawings of bears on the cave walls also have the researchers wondering if the animals had some sort of spiritual meaning for early humans.
"Chauvet Cave is a key site for understanding the beginning of art in Europe in the context of Neanderthal replacement by modern humans," Bocherens said.
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