Howler monkeys with the loudest roars are all mouth and no trousers, scientists have learned.

They pay for their large vocal organs by being somewhat deficient in another part of their anatomy.

Noisy howlers have small testicles and produce less sperm than their quieter rivals, research has shown.

Lead scientist Dr Jacob Dunn, from Cambridge University, said: "We have strong evidence that howler monkey species that invest in larger vocal organs produce less sperm.

"This is the first evidence in any species for a trade-off between vocal investment and sperm production.

"Investment in developing a large vocal organ and roaring may be 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."

Howler monkeys, from tropical Central and South America, are famous for their impressive roars.

Their vocal abilities are linked to long folds in the throat and a "hyoid bone" uniquely designed to resonate sound, making males seem bigger than they really are.

Together with US colleagues, the Cambridge team examined the vocal tract dimensions and calls of male howler monkeys from nine different species.

Those having larger hyoids produced deeper sounds but also had smaller testicles. Conversely, males endowed with bigger testicles had smaller than average-sized hyoids.

The evidence suggests that, for the noisy male howler, having a loud voice that can attract mates is the more important consideration.

"When it comes to reproduction, you can't have everything," said Dr Dunn.

The research is reported in the journal Current Biology.

US co-author Professor Leslie Knapp, from the University of Utah, said: "It is not possible to produce a large hyoid and large testes. This probably arose because individuals within one species produced more offspring if they had large hyoids. And in another species they were more successful if they had large testes.

"The idea has been around since Charles Darwin, but this is the first time that anyone actually has demonstrated a trade-off between vocal characteristics before mating and sperm competition after mating."