A recent study has shown that a whistling language developed by shepherds in the Canary Islands is “processed by the brain in the same way as ordinary speech”:http://news.independent.co.uk/europe/story.jsp?story=598140. It implies that the brain can process other non-verbal communication as language, too.
If you’ve read David Brin’s “Startide Rising”:http://www.scifi.com/sfw/issue154/classic.html, this should ring a bell. (If you haven’t, consider this a recommendation.) The book has neo-dolphins and humans communicating in a common language, developed to allow either species to make the appropriate sounds.
[Deana points out an "earlier post about Silbo":http://www.globalspin.com/mt/archives/000184.html, the language in question. The new article is more about how the language is processed in the brain. ~c]