You Don’t Need to Shout!
Primates’ hearing is pretty much as good as man’s. So you have to wonder why, and how, the Howler Monkey of South America, developed the bellow that can travel up to eight miles.
Males in a group will give long calls, often at sunrise, in order to communicate with neighboring groups, or an individual in that group. The normal call is amplified by the hyoid bone, which creates a resonance as they "howl". The calling allows them to establish the location, composition, and distance away of another group of howlers.
This is relatively important in the pecking order of things, because each group is dominated by a single male. Other male offspring of his mates are thrown out of the clan at sexual maturity, which for males is generally around seven years of age. The outcast males must then fight their way into a new group. Generally when this happens, and a male defeats the reigning monkey king, he will kill all the infants in the group, so subsequent offspring are all his.
This natural re-arrangement of the social structure in the primate world, is more responsible for the infant mortality rate of the Howler Monkey than environmental factors. The species is not currently on the endangered list, although numbers appear to be dropping with the encroachment of man on their rainforest habitats.