A new study has found that primates either develop big voices, or big testes, but not both. This suggests that there could be some biological drive that leads a howler monkey to become louder and more shriller if it is capable of producing lower quantities of sperm.
Howler monkeys (Alouatta) are found in South and Central American forests. The monkeys produce loud howls, when they are not sleeping for 15 hours per day. These howls can travel three miles through dense forest. These deep guttural growls are generally considered to be the loudest of any land animal.
The howl is a means of communication. One such role is to allow males to signal their presence (and intentions) to females. Research suggests that the volume of the howl is a biological trade-off in relation to the size of a monkey’s testes. Howls vary considerably with the loudest in a troop typically being 14 times louder than the quietest monkey.
Interviewed by BBC Science, lead researcher Dr. Jake Dunn from the University of Cambridge, noted: “Females find a deeper howl more attractive.”
Dr. Dunn found “There’s also a dramatic difference in the size of the monkeys’ testes” with “the largest are 6.5 times bigger than the smallest.” However, the largest correlates with quieter howls and the smallest with the loudest howls.
The research has been published in the journal Current Biology, in a research paper titled “Evolutionary Trade-Off between Vocal Tract and Testes Dimensions in Howler Monkeys.”
Interestingly, a study about people published three years ago, and featured in National Geographic, argued that deeper voiced men have a lower sperm count that men with higher pitches.
