The Senegambian languages are a branch of Niger–Congo languages centered on Senegal (and Senegambia), with most languages spoken there and in neighboring southern Mauritania, Guinea-Bissau, and Guinea. The transhumant Fula, howewever, have spread with their languages from Senegal across the western and central Sahel. The most populous unitary language is Wolof, the national language of Senegal, with four million native speakers and millions more second-language users. There are perhaps 13 million speakers of the various varieties of Fula, and over a million speakers of Serer. A special feature of the Senegambian languages not found outside the group is its non-tonality.