[Sal the Mudokon]

I always figured that it was for native mudokons to know if it was another mudokon that they were talking to and not a slig or some other species that may want entry or a platform that could endanger the native mudokons. I guess it was a whistle because maybe:

1. Only Mudokons can whistle (doubt it).
2. Whistling is the Mudokons' native language, like an African "click-language" (doubt that too, because we never saw whistling amongst the natives of MO).
3. And finally, enslaved Mudokons (scrubs), who had escaped to the forests might have only recently had their lips stiched and whistling was the only thing they could say. I say only recently becauseim guessing that after time, they can become loosened enough to speak and after a mudokon escapes, they often have them removed.

I'm putting my money on #3.