Feed him the food. Then unlock the chain. You may then get chain, which is a treasure item.

The bear then becomes your friend. If you type get bear, then he follows you around. This becomes quite handy on the way back over the Troll Bridge. When the troll pops up demanding a treasure again, throw bear! The bear will chase the troll away and then you can cross unmolested.

If you don't throw or drop the bear before you actually cross over the bridge, though, the combined weight of the bear and yourself will cause the bridge to collapse.