Male howler monkeys (Alouatta spp.) with the most impressive howling roars have paid for those abilities in sperm, according to a study led by University of Cambridge scientists.
This image shows a group of black and gold howler monkeys roaring. This species commonly has several adult males (black fur) and adult females (golden fur) in a group. Image credit: Mariana Raño.
The new findings, published in the journal Current Biology, are a further example of sexual selection – a theory first proposed by Charles Darwin in 1871 – and in particular the evolutionary trade-off between ‘pre- and post-copulatory reproductive strategies:’ traits that help males compete for access to mates versus those that help males compete to fertilize eggs.
“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,” said lead author Dr Jacob Dunn, of the University of Cambridge, UK.
Males of many species are endowed with large bodies, bold colors, or ‘weapons’ – think antlers and horns. In many cases, those sexually selected features come at a cost.
In their study, Dr Dunn and his colleagues wondered if the same could be said of howler monkeys’ howls.
They examined the vocal tract dimensions and the calls of male howler monkeys, gathering evidence to represent a total of nine different species.
The team found that species characterized by single-male groups have larger hyoids and smaller testes, suggesting high levels of vocal competition.
Monkeys with larger hyoids produce sounds with lower frequencies, most likely to give the impression based on acoustics of larger body size.
Across species, testes volume increased along with the number of males per group, indicating the importance of sperm competition. Those males more generously endowed with large testes also had smaller-than-average hyoids.
“It’s hard to say exactly how the trade-off works, however,” Dr Dunn said.
“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.”
The team collected data on the average testes size across howler species. They also used 3D laser scans to analyze the size of over 250 hyoids – finding a ten-fold variation from the smallest to the largest howler throat bone.
The scientists are now carrying out more detailed analyses of howler monkeys’ vocal anatomy to better understand how this determines the acoustics of their calls.
They’ll also be looking to see whether the trade-offs they’ve found between species also exist among individuals within a species.
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Jacob C. Dunn et al. 2015. Evolutionary Trade-Off between Vocal Tract and Testes Dimensions in Howler Monkeys. Current Biology 25, 1-6; doi: 10.1016/j.cub.2015.09.029
