The voice is a unique characteristic that we use to identify one another – including someone’s sex, age and mood. We speak by using air flow to vibrate our vocal folds, commonly known as vocal cords. The land-living ancestors of the African clawed frog Xenopus also used breath and vocal cords to communicate, but they returned to aquatic life 180 million years ago and had to evolve a different way to create sounds. Today’s Xenopus live in water and use a new mechanism that lets them sing for hours underwater without coming up to breathe. Males from each major group of Xenopus species produce courtship songs with harmonic intervals corresponding to an octave, a perfect fourth or a major or minor third.
Today’s Xenopus species do not have any vocal cords. Instead, they have an elaborate set of vocal components: the muscles of the larynx contract paired, movable rods that end in discs. For the past 40 years, it was thought that these frogs create sounds by collapsing small air bubbles between the discs, similar to snapping shrimp. But such bubbles have never been observed, and exactly how these frogs manage to create sounds underwater has been a mystery.
Here, Kwong-Brown et al. filmed the larynx as it was stimulated to produce sounds and discovered that the rapid separation of the discs excites the larynx and the surrounding tissues to create the harmonic frequencies. Then, to determine how the frog creates its harmonic intervals, Kwong-Brown et al. tried to ‘detune’ the larynx. In a series of experiments, they placed weights on the surface of the larynx, drilled a hole in the cartilage and filled it with helium, or introduced small glass beads. None of these attempts had any effect. However, rupturing the elastic cartilages within the larynx – which separate its internal cavity into three chambers – disrupted the harmonic intervals.
This new way of creating underwater sounds helped to maintain the quality of the frog’s voice and may explain how Xenopus can shape its songs to convey crucial information to others, such as identifying species, sex and social intent.