Yank, the protagonist ofThe Hairy Ape, is obsessed with the concept of "belonging," since it gives him security in a larger world. Yank is strong, because of his job shoveling coal, and believes that a person's usefulness and place in the world determines their belonging. He ridicules an older coal shoveler named Paddy because of the man's age:

I belong and he don't. He's dead but I'm livin'. Listen to me! Sure I'm part of de engines! Why de hell not! Dey move, don't dey? Dey're speed, ain't dey? Dey smash trou, don't dey? Twenty-five knots a hour! Dat's goin' some! Dat's new stuff! Dat belongs! But him, he's too old.
(O'Neill, The Hairy Ape, eoneill.com)

Throughout the play, Yank's concepts of "belonging" changes as he realizes the scope of the outside world. His importance in the engine room does not extend to the rest of the world; his rejection by Mildred, who refers to him as a "filthy beast," becomes the incentive for his new vision of himself as a "hairy ape." While he cannot force himself to "belong" to the high-class, civilized city world, he finds solace in the zoo, where he sympathizes with the caged animals; they belong to isolation, and Yank feels that he too has nowhere to belong.