Here's an interesting tidbit from Current Biology. Researchers conducted an acoustic analysis of cat purrs and found that hidden within mealtime purring (so-called "solicitation purring") is a high-pitched meow. This hidden meow is reminiscent of a cry which signals urgency and exploits our "sensitivity to acoustic cues relevant in the context of nurturing offspring".
I think Mr. Tumnus uses this purr quite effectively when he wants to be fed. Marble, however, doesn't bother with the solicitation purring and goes straight for the mealtime howl. This is also quite effective, albeit more annoying.