Monkeys with impressive roars possess smaller testes and produce less sperm: study

London: Male howler monkeys with deeper and more imposing roars possess smaller testes and produce less sperm, a new study has found.

Howler monkeys are about the size of a small dog, yet they are among the loudest terrestrial animals on the planet, and can roar at a similar acoustic frequency to tigers. For males, a critical function of the roar is for mating - to attract females and scare off rival males.

The research on howler species has showed 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. Researchers found that the trade-off corresponds to the mating systems of different howler species.

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. 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. The findings are a further example of sexual selection, the researchers said - a theory first proposed by Charles Darwin in 1871 - and in particular the evolutionary trade-off between "pre- and post-copulatory reproductive strategies" - traits that help males compete for access to mates versus those that help males compete to fertilise eggs.

"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," said Jacob Dunn, from the University of Cambridge in UK, who led the study.

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

Along with collecting data on the average testes size across howler species, the researchers also used 3D laser scans to analyse the size of over 250 hyoids - finding a ten-fold variation from the smallest to the largest howler throat bone.

The team also conducted in-depth acoustic analyses of a number of howler roars. "The vocal folds of a howler monkey are three times longer than a human's, yet they are ten times smaller than us, with a hyoid bone uniquely adapted to resonate sound and exaggerate their size," said Dunn.

The study was published in the journal Current Biology.