Penguins use the two-voice system to recognize each other

The sound-producing structure in birds is the syrinx, which is usually a tw o-part organ located at the junction of the bronchi. As each branch of the syrinx produces sound independently, many birds have two acoustic sources. Thirty years ago, we had anatomical, physiological and acoustical evidence of this two-voice phenomenon but no function was known. In songbirds, often these two voices with their respective harmonics are not activated simulta neously but they are obvious in large penguins and generate a beat pattern which varies between individuals. The emperor penguin breeds during the Ant arctic winter, incubating and carrying its egg on its feet. Without the top ographical cue of a nest, birds identify each other only by vocal means whe n switching duties during incubation or chick rearing. To test whether the two-voice system contains the identity code, we played back the modified ca ll of their mate to both adults and also the modified call of their parents to chicks. Both the adults and the chicks replied to controls (two voices) but not to modified signals (one voice being experimentally suppressed). O ur experiments demon strate that the beat generated by the interaction of t hese two fundamental frequencies conveys information about individual ident ity and also propagates well through obstacles, being robust to sound degra dation through the medium of bodies in a penguin colony. The two-voice stru cture is also clear in the call of other birds such as the king penguin, an other non-nesting species, but not in the 14 other nesting penguins. Mie co ncluded that the two-voice phenomenon functions as an individual recognitio n system in species using few if any landmarks to meet. In penguins, this c oding process, increasing the call complexity and resisting sound degradati on, has evolved in parallel with the loss of territoriality.