The captain asks the first mate how he knows that there are four stowaways. “Why they’re singing Sweet Adeline,” the first mate answers.
Viewers who take this to mean that all four Marx Brothers are singing, even Harpo, are missing the joke. The humor of that line is that the first mate doesn’t specifically say that he heard four voices singing, he only infers it from the fact that the song they’re singing is Sweet Adeline.
And when we enter the ship’s cargo area in the next scene and hear the voices for ourselves, we hear them singing in the style of barbershop quartet. But listen, there are only three voices. The voice singing in the lower range is obviously Chico. The tenor voice matches Groucho’s straight singing voice. That leaves one more voice, a comic falsetto, which would fall to Zeppo rather than Harpo.
Monkey Business is the shortest Marx Brothers film, but it seems to gives Zeppo more screen time than the other four that he appeared in. Very little setup or backstory is given, so the roles of each of The Brothers (and other archetypical characters from the Cocoanuts formula) is less defined than in the other films. Each of The Brothers is less constricted by an assigned role. He sinks or shines based exclusively on his performance. Zeppo is not Groucho’s paige, he is simply on the loose. He does okay.
Zeppo has an interesting scene that I want to point out: He pauses, while being chased by the crew, to decide which direction to run in and is distracted by his first glimpse of The Love Interest (played by Ruth Hall). He turns to follow her. When crew members come into view, he jogs up alongside her, takes her arm and says something bold and charming. She rebuffs him.
The crew passes out of sight quickly. So Zeppo can forget about the pursuit for a moment and can focus on her. He catches up to her again and drops a handkerchief conspicuously. He picks it up.
“Pardon me. Is this yours?” he asks her. He is turning the old dropped handkerchief trope on it’s side.
She responds, “No,” coldly.
He stands, dejected, and watches her walking away without breaking her stride. Then a few steps on, she purposely drops her own handkerchief. Zeppo’s face brightens, he scampers over to retrieve it. But before he can reach it, she picks it up. “Is this yours?” she asks him. “Why, yes,” he answers. They lock arms and walk away together.
The handkerchief gag follows a very Marxist (to coin a term) logic. Zeppo’s costar goes with the Marx logic in a way that few outsiders do without getting confused or going mad. She accepts the logic of the scene and even gets the better of Zeppo. Her passions turn in a moment for no apparent reason, very Marx. This may be her only comic bit in the movie, but it’s a good one. Also, notice how Zeppo pockets her handkerchief before they pass out of camera range?