We've all been at a party where someone has inhaled the helium from a balloon to make their voice go squeaky. Rarely do we consider this to be a useful scientific tool for determining how animals produce their characteristic sounds, but that's what Peter Madsen, of Aarhus University, Denmark, and his team have done. In a new paper published online in Biology Letters, Madsen and co. use heliox breathing to determine the mechanism by which dolphins produce their ‘whistles’. Dolphins use tonal whistles for communication but it is unclear how they achieve consistency in a challenging underwater environment where they encounter a great range of hydrostatic pressures, which could dramatically alter the sounds that they produce just by virtue of their depth. Are their cries true whistles, produced by air flows in their complex nasal system, or are they the result of a vibrating structure?
When humans speak, vibrations from the vocal chords cause the air in the throat to vibrate. When we breathe helium, our vocal chords still vibrate at the same frequency but, because sound travels faster in helium than in air, there is a shift in timbre. Timbre describes the quality of a sound and enables us to distinguish between different sounds; for example, if you play the same note at the same volume on a piano and a clarinet, you can hear the difference between the instruments because the timbre is different. Our voice sounds squeaky after inhaling helium because of the resulting change in the resonance frequency of the vocal tract. This enhances the higher frequencies while attenuating the lower frequencies so that we sound a bit like Donald Duck. Dolphins could have a similar problem, sounding squeaky at the surface and more sonorous at depth if their calls are produced by whistling. This is because the air volumes of the dolphin nose are reduced when they dive and so the resonance frequency of them increases.
Madsen and his team made use of this phenomenon to test whether the dolphin's whistle is actually a true whistle or a misnomer by analysing the sounds produced by a dolphin in heliox and normal air. If the ‘whistles’ are produced by an airflow (true whistle), the fundamental frequency would change during heliox breathing; if they are produced by a vibrating structure (not a true whistle), there would be no change in frequency. Having access to recordings made by Sam Ridgway and Donald Carder in the 1970s of sounds produced by a bottlenose dolphin (Tursiops truncates) inhaling first a heliox mixture (80% helium and 20% oxygen) and then normal air, the team decided to analyse the calls to find out whether dolphins whistle.
Analysing the power distribution in each call, Madsen and his colleagues showed that although there was less energy in the fundamental frequency (the lowest frequency) in the heliox calls, there was no significant difference in other frequency variables between the two conditions. This suggests that, although there is an indication of some air effect on timbre, the fundamental frequency is consistent and therefore produced by tissue vibrations. If the sound was produced by whistling, the team would have found a change in frequency. This means that the term ‘whistle’ is not technically correct as the calls are not produced by resonating air volumes but by vibrating structures that are the nasal equivalent of the vocal chords of humans and other mammals.
Madsen and colleagues' results showing that these sounds are produced by vibrating structures, rather than vibrating air columns, enable a better understanding of how dolphins communicate information and signal identity regardless of depth.