The deeper the growl of a howler monkey, the smaller its testicles like are, making it more attractive to females at the cost of a reproductive tradeoff.

According to BBC News, authors of a study published in the journal Current Biology were initially seeking to learn more about the "evolution of the animals' incredible roars." In the case of the howler monkey, the loudest males have the smallest testes and vice versa.

For their study, the researchers examined the bone responsible for the howler monkey's growl, the hyoid.

"They are different solutions to the same problem," study senior author Leslie Knapp, professor and chair of anthropology at the University of Utah, said in a press release. "It is not possible to produce a large hyoid and large testes. This probably arose because individuals within one species produced more offspring if they had large hyoids. And in another species they were more successful if they had large testes.

"The idea has been around since Charles Darwin, but this is the first time that anyone actually has demonstrated a trade-off between vocal characteristics before mating and sperm competition after mating," she said. "Our study shows that Darwin was probably right when he suggested that the roars of howler monkeys are important for reproduction."

Jake Dunn, a professor at the University of Cambridge who lead the research on the study, told BBC News there was a significant difference in the sizes of the largest and smallest hyoid bones and testes.

"The largest hyoid is 14 times the size of the smallest," he said. "We became really interested in understanding how and why such amazing variation exists in this trait.

"There's also a dramatic difference in the size of the monkeys' testes.

"The largest are 6.5 times bigger than the smallest."