The heads of beavers, large rodents known for building dams, are their own kind of highly complex dam structure, with various retractable walls that let water in or keep it out. They can close valves in their nostrils and ears and a special membrane over their eyes; their epiglottis, the flap that stops water entering the lungs, is inside their nose instead of their throat; they use their tongue to shield their throats from water; and their lips to shield their mouths – their lips can close behind their front teeth. Their teeth are rust-orange, because they are strengthened with iron.
Their back feet are webbed like a duck’s; on land, their front feet act like hands, digging, grasping and carrying things from the riverbed to the surface – rocks, for example, tucked under their chins and cradled by their arms. When they swim, they do so while holding their front paws to their chests, like a severe governess in a Victorian novel, or a child pretending to be a rabbit. They prefer to carry branches in their teeth, like dogs. The biggest beavers weigh 50kg.
