It's long been suspected that loudmouths are trying to overcompensate for their inadequacies elsewhere.

Now scientists have found that particularly loud howler monkeys, which are among the noisiest animals on the planet, have smaller testes and produce less sperm than their quieter peers.

But rather than making them less successful mates, the louder members of a group are more likely to have the pick of the females in what the researchers call a 'harem' social model.

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Scientists have found that howler monkeys, which are among the loudest terrestrial animals on the planet (an image of them vocalising is shown), have small testicles and produce little sperm

By comparison, males with larger testes live in larger groups with more males and have to share mates.

Howler monkeys typically weigh about one stone (seven kilos) and are the size of a small dog, but their growl is deeper than a tiger's.

This is to give the impression they are much bigger and stronger than they really are.

Howlers use their roar - which can reach 128 decibels - to attract females, deter love rivals and to scare off predators. But it seems there is a trade off.

Scientists have found that the deeper the howler monkey’s howl, the smaller their testicles are. Those they studied also produced less sperm.

Males with deeper roars but smaller testes live in small social groups with often only one male dominating a number of females, or 'harem'. Males with bigger testes live in large groups with up to five or six males and the females are shared. A computer image of their throat is shown left and a skull, jawbone and hyoid bone, right

Howlers use their roar - which can reach 128 decibels - to attract females, deter love rivals and to scare off predators. But it seems there is a trade off. An image of a male howler monkey roaring is shown above

SIZE MATTERS: MONKEY’S CALLS Howler monkeys have evolved to have a complex and powerful vocal system. Males use their roars to attract females and scare males. The new study has revealed an evolutionary ‘trade-off’ between investments in the size of the male hyoid - the bulbous, hollow throat bone that allows the howlers’ guttural roar to resonate - and in the size of reproductive organs, namely the testes. The bigger a male howler’s vocal organ, and the deeper and more imposing roar they possess, the smaller their testes and the less sperm they can produce. Males with large hyoids and deeper roars, but more diminutive testes, live in small social groups with often only one male dominating a number of females – a ‘harem’ social model. But males with bigger testes and smaller hyoids live in large groups with up to five or six males, and females mate with all males in the group. These males don’t have exclusive access to females, and the battle for reproduction is geared more towards “sperm competition”: quantity and quality of sperm. Advertisement

Dr Jacob Dunn of Cambridge University said: ‘In evolutionary terms, all males strive to have as many offspring as they can, but when it comes to reproduction you can’t have everything.’

‘There is evidence in other animals that when males invest in large bodies, bright colours, or weaponry such as horns or long canines, they are unable to also invest in reproductive traits.

‘However, this is the first evidence in any species for a trade-off between vocal investment and sperm production.’

He added: ‘It may be that investment in developing a large vocal organ and roaring is so costly that there is simply not enough energy left to invest in testes.

‘Alternatively, using a large vocal organ for roaring may be so effective at deterring rival males that there is no need to invest in large testes’.

To test the theory, the researchers used 3D laser scans to analyse the size of the howler monkeys’ vocal organ, called a hyoid, which is a cup shaped chamber in their throat that acts as an echo chamber.

Dr Dunn added: ‘The results of our acoustic analyses show that howler monkeys produce roars at a similar frequency as tigers, which is far lower than we would have predicted from their body size, yet exactly what would be predicted from measuring their giant vocal folds’ which are three times bigger than in a human.'

The researchers found that males with large hyoids and deeper roars but more diminutive testes live in small social groups with often only one male dominating a number of females – a 'harem' social model.

Males with bigger testes and smaller hyoids live in large groups with up to five or six males where the females mate with all males in the group.

The researchers used 3D laser scans to analyse the size of the howler monkeys’ vocal organ, called a hyoid (illustrated in red), which is a cup shaped chamber in their throat that acts as an echo chamber

Dr Dunn said: ‘The results of our acoustic analyses show that howler monkeys produce roars at a similar frequency as tigers, which is far lower than we would have predicted from their body size, yet exactly what would be predicted from measuring their giant vocal folds’ which are three times bigger than in a human'

These males don’t have 'exclusive access' to females and the battle for reproduction is geared more towards 'sperm competition' - quantity and quality of sperm.

Howlers, which live in the forests of Central and South America have long interested humans - from ancient Mayans to modern primatologists - and their cries can travel as far as three miles (5km) through dense rainforest.

Charles Darwin was fascinated by the ‘wonderfully powerful’ vocal organs of the howler monkey, despite describing their chorus as a ‘dreadful concert’ in The Descent of Man.

The new Cambridge research published in Current Biology continues to show just how accurate Darwin was when he wrote in On the Origin of Species: ‘The whole organism is so tied together that when slight variations in one part occur, and are accumulated through natural selection, other parts become modified.’

'This is the first evidence in any species for a trade-off between vocal investment and sperm production,’ Dr Dunn said. a group of howler monkeys calling out from a branch is shown above

Charles Darwin was fascinated by the ‘wonderfully powerful’ vocal organs of the howler monkey, despite describing their chorus as a ‘dreadful concert’ in The Descent of Man. A number of hyoid bones are shown