The guttural calls of howler monkeys reverberate across Central and South American rainforests, and they possess a secret that the males of the species may want to be left unrevealed. Howler monkeys come among the ones who make the loudest, deepest sounds of any land animal. The male monkeys use their roars for attracting the ladies for mating and intimidate other males.

But on Thursday, scientists said that they have found a curious paradox. They have discovered that the males making the lowest-frequency calls are considered to be the most attractive to potential mates. They are endowed with the lowest reproductive potential.

During the study, researchers focused on a cup-shaped bone, the hyoid, present above the larynx that forms a resonating chamber to amplify vocalizations.

They studied nine howler monkey species and found that the ones with the biggest hyoid produced the deepest and lowest-frequency calls, but also had the smallest testes for the production of sperm.

University of Utah anthropologist Leslie Knapp said they have discovered that the largest hyoid bones were found in howler monkey species having the tinniest testes volume, and vice versa.

Biological anthropologist Jacob Dunn from University of Cambridge called the finding an ‘evolutionary trade-off’ for male howler monkeys between vocal tract and the size of testes.

Dunn said, “This means that different species of howler monkeys either invest in one of these traits or the other, but not both. I think the main message is that when it comes to reproduction, you can't have everything”.