The etymology of the name Bern is uncertain. According to the local legend, based on folk etymology, Berchtold V, Duke of Zähringen, the founder of the city of Bern, vowed to name the city after the first animal he met on the hunt, and this turned out to be a bear. It has long been considered likely that the city was named after the Italian city of Verona, which at the time was known as Bern in Middle High German. As a result of the find of the Bern zinc tablet in the 1980s, it is now more common to assume that the city was named after a pre-existing toponym of Celtic origin, possibly *berna "cleft". The bear was the heraldic animal of the seal and coat of arms of Bern from at least the 1220s. The earliest reference to the keeping of live bears in the Bärengraben dates to the 1440s.
What is the earliest that they kept live bears in the Bärengraben?
1440s.