From a Twelfth-Century description of the land of the Rūs:
The beaver is a wonderful animal. It lives in the great rivers and builds houses on land, at the edge of the water. It makes a kind of high platform for itself and to the right another, less high, for its wife and to the left another, for its children. Below, there is a place for its slaves. The house has a door which gives on to the river and another, higher up, on to the land. Sometimes, it eats the wood known as khalanj; at other times it eats fish. Some beavers are jealous of others, and make them prisoners.
Those who trade in those lands and through the country of Bulghār have no trouble in distinguishing the fur of slave beavers from those of the masters. This is because the slave beaver cuts the wood of the khalanj and other trees with its teeth, and as it gnaws them, they rub its sides and its hair falls off right and left. Hence they say, ‘This pelt is from the servant of the beaver.’ The fur of the beaver who owns slaves, on the other hand, is perfect.
— Abū Hāmid al-Andalusī al-Gharnātī, The Travels of Abū Hāmid al-Andalusī al-Gharnātī, 1130-1155, translated by Paul Lunde and Caroline Stone