I have long been fascinated by the homes that animals construct. Over the years I have contemplated the nests of hundreds of different species—including ants, termites, wasps, birds, fish and mice—by poking and prodding nests in the wild, manipulating them in the laboratory and reviewing the work of other scientists. I have dug holes meters deep, trying to find the bottoms of ant nests. I have snorkeled over bluegill fish, watching them excavate and tend to their dish-shaped nests. As a boy, I even tried to swim up into a beaver lodge.