The howler monkeys are known for making the deepest and loudest sounds among all land animals. The male howler monkeys use their deep roars for intimidating other males and attracting females for mating. However, during a recent study scientists have found that there’s a curious paradox; they have come to know that males who make calls of lowest frequency are most attractive to their potential mates and are endowed with minimum reproductive potential.

During the study, researchers focused on a cup-shaped bone called hyoid. This bone is located just above the larynx of the howler monkeys. Out of the nine species of howler monkeys they studied, the ones boasting the largest hyoids were found to produce the lowest-frequency and deepest calls. Another feature that was common among howler monkeys with biggest hyoids was their small testes.

Leslie Knapp, an anthropologist representing the University of Utah, said that he and his colleagues spotted the biggest hyoid bones in howler monkeys with smallest testes and vice versa.

In words of Jacob Dunn, an anthropologist from the University of Cambridge, this trend marks an “evolutionary trade-off” between the sizes of the testes and vocal tract of the male howler monkeys. According to Dunn, this indicates that male howler monkeys invest in any one of the two traits; in other words, they are either alluring to their mates or possess great reproductive abilities.

For those who don’t know: the howler monkeys are primarily found in the region between southern Mexico and Argentina. They love spending time on tall trees. They grip the branches of tall trees easily by means of their prehensile tails and keep on munching leaves, flowers and fruits. The howls of these monkeys can be heard from places located 5 km (3 miles) away.

Knapp informed that howls of monkeys sound almost like the roar of lions, but there are some howlers who create more complex and deeper sounds. Deeper howls are markers of big body. So, howler monkeys producing deep howls seem to be larger than they actually are. This makes the ability to make deeper sounds an advantage for male howlers that compete for mating with females.

The entire study has been published in the widely read journal Current Biology.