Why is this adorable Beaver rubbing his belly?

The Beaver is waterproofing its fur! At the base of the tail are both a pair of inverted castor sacs and a pair of anal glands. The Beaver uses its front feet to get oil from the anal glands and then rubs it all over his body. Beavers are constantly grooming and oiling their fur not only to waterproof, but also to remove debris. The sticky yellowish substance from the castor sacs is used for an entirely different purpose, to mark their territory and to identify each other.