It is hollow but closed at the end, so it was not used to breathe underwater as people thought for a long time. However, it did amplify sounds. The sound travelled from the throat to the nostrils via a system of tubes acting like a resonance box, as with a trombone. The level of the amplified sound depended on the length of the crest. Thus even over long distances the Parasaurolophus could warn of danger, be recognised by its own sort, tackle a rival or attract a female. And the longer the crest, the more impressive it looked and the bigger the impact the trumpet sounds made!