A study has indicated that howler monkey with big testicles will always have soft voice. As per the study by an international team of researchers, the primates with louder voice may have small testicles. The big voice of howler monkeys hides their reproductive shortcomings, the study suggested.

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. "There's also a dramatic difference in the size of the monkeys' testes. The largest are 6.5 times bigger than the smallest," said the research paper.

Howler monkeys, native to the forests of South and Central America, are some of the loudest terrestrial animals on the planet. Their calls can travel up to 4.8km (3 miles) through dense forest, yet they only weigh 7 to 10 kg (15 to 22 lb). Their prodigious howls are enabled by elongated vocal cords and an enlarged hyoid bone, the hollow bone in the throat that allows the sounds to resonate.

In the cover story of Current Biology's new issue, the research team provides the first evidence of a trade-off between two traits evolved by different howler monkey species so that males could mate with females and pass their genes to offspring.

To reach their determinations, the team laser scanned 255 hyoids, 111 female and 144 male, from a number of museums, creating 3D models to calculate volume. This data was then compared with average data on species body weight, skull length, canine length, testes volume, and number of males per group.

The discovery was made by researchers when they were studying ‘evolution of animals’ roars’. According to the study published in the journal Current Biology, there are higher chances that such trade-offs are more common than earlier thought.

Males who live in larger social groups, with several males competing for females, tended to have smaller hyoids and larger testes. The largest hyoid the team examined was 10 times larger than the smallest.

Howler monkeys, which are among the largest of the New World monkeys, are well known for their howling roars. They use their impressive roars to threaten their rivals and make an impression on potential mates. The hyoid bone, bone in the primates’ throat, works as a resonator. The bone acts as the body, while the animal’s vocal folds act as the strings.

During the study, the researchers found variation in the size of the hyoid bone of howler monkey species. Dr Jake Dunn, a researcher from the University of Cambridge and lead author of the study, said, “The largest hyoid is 14 times the size of the smallest. We became really interested in understanding how and why such amazing variation exists in this trait”.

A natural question is whether this has any carry over to the monkey's closest relative, humans. A thousand women in a thousand bars might say yes, men who talk a good game are often duds where the rubber meets the bed, but the study offers no answers. The researchers stuck to what they knew. And they really got into the work. They measured testes size across a range of howler monkey species and collected the data to back their findings. They also analyzed the size of the hyoid, a curvy bone in the neck that supports the tongue to power howling, in 250 males.

The study came to one other conclusion, too: The most powerful roarers might live in small groups for a reason.

"In evolutionary terms, all males strive to have as many offspring as they can," Dunn said. But with bodies that generate impressive sound volume but little reproductive volume, "you can't have everything."

The researchers studied nine howler species. They created 3-D scans of 255 hyoid bones in museum collections. In howlers, the hyoid bone creates a resonating chamber for sound and its size is directly related to sound frequency. The bones were saved in museums before the era of genetics as a way to identify species.

The team collaborated with researchers from the University of Utah to perform laser scans to calculate volumes of about 200 howler monkey hyoid bones. These bones were taken from museums in the US and Europe. They also used MRI images of two adult male monkeys.