Helium is lighter than air, and its molecules bounce around much more quickly. So sound travels through helium almost three times faster. That increases the resonant frequencies of your voice. In essence, the high-frequency tones of your voice end up ultra-amplified, drowning out the lower tones that usually round out the sound.
Here's where the alligators come in: In a study published recently in the Journal of Experimental Biology, scientists wanted to know whether reptiles like crocodiles and alligators used resonance, too. Some animals do -- bird song relies on the same resonance that control of the human voice does, and other animals exaggerate their natural resonance to make it seem like their voice is coming from a larger animal. But frogs, for example, don't seem to use resonances to communicate.
It turns out that helium (or rather heliox, a combination of oxygen and helium that you can breathe for extended periods without suffocating) is the perfect way to test for resonance in an animal. If they were using resonance, their calls would sound different when they inhaled the gas.
Researchers Stephan Reber and Judith Janisch of the University of Vienna found a small Chinese alligator who lived in an isolated tank and was known to get quite vocal when she heard the calls of other alligators. She was the perfect fit: chill enough to live in a special tank that could get pumped with heliox, and responsive enough to make noises on demand.
The alligator's sounds did change. In fact, it actually sounds like they get deeper -- sort of like a burp turning into a more satisfied belch (listen here). But after close examination, the researchers confirmed that her bellows were shifting to higher frequencies under the influence of the heliox, indicating that alligators -- and possibly other similar reptiles -- use vocal resonance.
That could mean that dinosaurs did, too.
"If you see it [resonance] in the last two groups [birds and crocodilians] that share a common ancestor with all extinct dinosaurs, we can infer that dinosaurs probably used formants [resonances], too, for communication," Reber said.
So it's possible that dinosaurs could have used a familiar vocal mechanism to communicate just how big and bad they were. It's too bad we'll never get to make a dinosaur huff a helium balloon.