A group of researchers from Cambridge University, University of Roehampton, University of Vienna, and the University of Utah have published a study on Thursday in the journal Current Biology suggesting that bass tone is seductive in certain species of monkeys, among them the howler monkey, but these monkeys pay the price with smaller testicles.
Social scientists have established over time that women naturally find men with deep voices attractive, it seems the same is true for howler monkeys. But while men with deep, guttural voices do not necessarily have smaller testicles, the same cannot be said for species of howler monkeys.
One of the leaders of the research, Jacob Dunn of Cambridge University, blamed evolution for the choice of either a deeper voice for howler monkeys or a smaller testicle – both they can’t have it both.
W. Tecumseh Fitch of University of Vienna and Leslie A. Knapp of University of Utah with other researchers analyzed the size of hyoid bones – bone in the vocal apparatus, from museum collections, because these are related to depth of deep calls, to see how they affect size of testicles in nine species of howler monkeys.
The scientists compared the size of hyoid bones in the museum with those in zoos and related it to testicular measurements and found a great variation.
For howler monkeys in Central and South America, deep calls attract males and scare off rivals; but the animals pay little attention to size of testes that influence sperm production. In other species of howler monkeys in other places, the females have several male partners and the male with the strongest sperm fertilize her eggs. This can also be seen in gorilla and chimpanzees.
The researchers tried to theorize why the species of howler monkeys do not have deep calls together with bigger testicles. They found that larger testicles interfere with immune functions. They also found that bigger testicles are vulnerable to being caught on a branch or scratched by another monkey. And another thing: evolution causes a species to either have deeper voices with smaller testicles, or the exact opposite.