Enlarge Image Carolyn M. Crockett

In his 1871 book on evolutionary theory called "The Descent of Man, and Selection in Relation to Sex", Charles Darwin posited that sexual selection was determined in two main ways: through combat, or through display. For those animals who win mates by means of mating displays, those characteristics that are more likely to win a mate come at a cost to the animal. For instance, the peacock's tail feathers hinders its mobility. In other words, there's a trade-off.

For male howler monkeys, that trade-off is an unusual one. The larger their vocal organs, a new study has found, the smaller their testes and the lower their sperm count. The research has been published in the journal Current Biology.

"We have strong evidence that howler monkey species that invest in larger vocal organs produce less sperm," said lead author Jacob Dunn of the University of Cambridge.

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

Enlarge Image Jacob Dunn

Howler monkeys, native to the forests of South and Central America, are some of the loudest terrestrial animals on the planet. Their calls can travel up to 4.8km (3 miles) through dense forest, yet they only weigh 7 to 10 kg (15 to 22 lb). Their prodigious howls are enabled by elongated vocal cords and an enlarged hyoid bone, the hollow bone in the throat that allows the sounds to resonate.

These roars are used to protect their territory from rival tribes, warn other howler monkeys of danger and, for the males, attract mates. Larger hyoids produce sounds of lower frequencies, giving the impression of a larger body behind the sound.

To reach their determinations, the team laser scanned 255 hyoids, 111 female and 144 male, from a number of museums, creating 3D models to calculate volume. This data was then compared with average data on species body weight, skull length, canine length, testes volume, and number of males per group.

The team found a correlation between hyoid size and testes volume. There was also, they found, a correlation between the number of males per group and hyoid size. Males with larger hyoids and smaller testes live in small social groups with few males and a larger number of females, often just one male per group. This indicates that exclusive access to females means the male can "afford" a bigger voice.

Males who live in larger social groups, with several males competing for females, tended to have smaller hyoids and larger testes. The largest hyoid the team examined was 10 times larger than the smallest.

How and why the trade-off exists is still something of a mystery.

"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 said.