Large Testes vs Loud Howls

This week in Current Biology a team of scientists have found the first evidence for an evolutionary trade-off between vocal investment and testes size.

The research was carried out on howler monkeys, one of the only nest building monkeys and the largest of the new world monkeys. As their name suggests, their howls are among the loudest of all terrestrial vocalisation, with the ability to travel for three miles through dense forest. Female howler monkeys find deeper howls more attractive.

The howler monkeys have a highly modified larynx with a greatly enlarged cup shaped hyoid bone that contains an air sac. This acts as a resonating chamber for their calls.

Using laser scanners and bones from museum collections the scientists created 255 virtual 3D models of hyoid bones for 9 of the 10 classically recognised species. Using phylogenetic analysis and species data on various other features, such as testes size, the team tried to establish whether differences in male hyoid volume were related to male competition among species.

They found that species with more males per group tended to have smaller hyoid bones and larger testes plus a direct relationship between large hyoids and small testes. Additionally they found that howls increase the impression of body size.

The scientists suggest that this trend between hyoid and testes size may be an evolutionary trade-off. Monkeys with limited resources may face a choice of investing energy in roaring or more sperm. Another idea is that males with loud voices are better able to fend off other males reducing competition for their sperm and so do not need large testes. Dunn one of the lead researchers suspects ‘that there may be an element of both’. Stefan Lupold from the University of Zurich also pointed out that vocalising can be very energetically costly which may be a contributing factor to the smaller testes.

This is the first clear example, Dr Dunn says, of a trade-off “vocal investment and sperm production” in any species. However a study in 2012 suggested that deeper voiced men had lower sperm counts. So beware that deep voice may not mean what you think …