Howler monkey don't howl—they roar. To me the sound is that of a cross between a jet plane taking off and a bunch of noisy hogs, a sound you can hear from kilometers away. The anatomical feature that allows the howler monkey to communicate so adeptly is a very large voice box (hyoid bone), which you can see in the attached image. It's the cup-like bone to the right of the skull. Evidently males and females have this large voice box, but it's the male that does most of the yelling. I can't remember how I came by this skull and voice box but I think it was given to me by a rural family in the State of Amazonas.