Howler monkeys with deeper roars have smaller testes

The bigger a male howler monkey's vocal organ, and the deeper and more imposing roar they possess, the smaller their testes and the less sperm they can produce, says a study.

Howler monkeys are about the size of a small dog, weighing around seven kilos, 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.

But researchers have now discovered 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.

"This is the first evidence in any species for a trade-off between vocal investment and sperm production," said lead researcher Jacob Dunn from the University of Cambridge.

"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," Dunn explained.

Along with collecting data on the average testes size across howler species, the researchers also used 3-D laser scans to analyse the size of over 250 hyoids.

The team also conducted in-depth acoustic analyses of a number of howler roars.

The researchers also 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, and females mate with all males in the group.

These males do not have exclusive access to females, and the battle for reproduction is geared more towards "sperm competition": quantity and quality of sperm, the researchers said.

The findings appeared in the journal Current Biology.