Whistled language

From Yahoo! News: Oddly Enough:

No one knows how long the shepherds on the island of La Gomera have used the rare whistled language called the Silbo Gomero, but American and Spanish researchers said on Wednesday that the brain processes it like a spoken language.

I’d like to know what other activities cause those regions of the brain to be used. Obviously, the whistling transmits semantic information, but I don’t think this study proves that it is a true language. How much variation is possible in a whistled language? Enough for a robust lexicon? What kinds of inflection exist in Siblo Gomero?