The bears digestive track. I went to Alaska for a week a while back and we actually learned about how the local bears eat. Eventually a bear's canines (or whatever teeth they use for meat tearing) will wear down to stubs making it impossible for them to eat meat. When this happens they will start to exclusively eat plant matter. Unfortunately this is also what ends up killing them. A young bear typically eats plant matter to slow down their digestive track for the winter. But in the case of an older bear they eat so much that their digestion literally backs up until it kills them