Crocodiles are noisy creatures. They’re the most vocal reptiles aside from birds (which are part of the reptile family even if we don’t usually lump them together with snakes and lizards). However, we don’t know much about how and why crocodilians (a super fun term that encompasses crocs, alligators, and the like) bellow.
A new study in the Journal of Experimental Biology, led by biologists at the University of Vienna, attempts to get to the bottom of this question in a way you might be familiar with from childhood birthday parties: inhaling helium.
Humans and other animals like monkeys and birds rely on resonance—the vibration of air in the vocal tract—to make the sounds involved in singing and talking. Because sound travels faster through helium than oxygen, talking after inhaling helium involves higher resonant frequencies, amplifying higher vocal sounds. Thus having a bird—or in this case, a crocodile—inhale helium is an easy way to determine whether the animal’s vocals rely on resonance (not all do).
The researchers put a crocodile in a special tank filled with a helium and oxygen mixture, then exposed her to audio recordings of other crocodiles so that she would bellow. Compared to when she bellowed in normal air, her vocalizations shifted to higher frequencies in the helium condition, suggesting that crocodiles do in fact use resonance. (Listen above:The first two sounds are vocalizations in normal air; the second two are the crocodile bellowing in the helium mixture environment.)
This is the first time scientists have observed resonance in reptiles other than birds, and it suggests that crocodiles use their voice to convey certain aspects of themselves to fellow crocs, like their body size.
Since birds and crocodiles are both descended from dinosaurs, and they both seem to use resonance, dinosaurs may have used a similar mechanism to announce themselves to the world.