Howler monkeys are one of the loudest animals on Earth, but new research suggests those with the most impressive are actually overcompensating for something.

A recent study found the male howler monkeys with the loudest calls had smaller testes and produced less sperm than their quieter peers, the University of Cambridge reported. This evolutionary "trade-off" between the size of the male hyoid (the hollow throat bone that allows the guttural call to resonate), and the size of the reproductive organs corresponds with the mating systems of different howler species. The researchers found males with large hyoids and small testes generally lived in small social groups in which one male dominated many females; howlers with small hyoids and large testes lived in groups of five to six males in which the females mated with all of the males in the social circle. Since males in the latter group do not have exclusive access to females, they must rely on "sperm competition," where quality and quantity is key for insemination.

"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 study leader Jacob Dunn, from the University of Cambridge's Division of Biological Anthropology. "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."

To make their findings, the researchers collected data on average testes size among different howler species used 3-D laser scans to analyze the size of over 250 hyoids. They also conducted in-depth acoustic analyses of a number of howler roars.

"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," Dunn said.

This means some howlers can roar at volumes equivalent to animals 10 times larger than their body size. Investing energy into these powerful vocal organs could mean there's not enough energy left to also invest in highly productive testes.

The findings were published in a recent edition of the journal Current Biology.

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