A new study reveals how male howler monkeys who possess the loudest, booming voices, apparently reveal an evolutionary strategy suggesting that these loud monkeys have smaller testicles and produce lesser sperm.
Howler monkeys are also among the smallest sized species of monkeys, only weighing up to seven kilograms, where they use their loud voices to attract potential mates. However, this new research is based on Charles Darwin's theory of sexual selection that involves how species that possess strong sexual characteristics to attract mates are not so effective in fertilizing eggs.
According to research author Leslie Knapp from the University of Utah, this Darwinian theory is now seen as evidence for a trade-off between vocal characteristics before mating and sperm competition after mating. This new study is also suggesting that Darwin is probably right regarding how the roars of howler monkeys are crucial for reproduction.
According to Jacob Dunn from the University of Cambridge, there is also strong evidence from howler monkey species that invest in larger vocal organs apparently possess lesser sperm. This is also the first evidence in any animal species that vocal production makes up for low sperm production.
The monkeys utilize their long vocal folds with their hyoid bone to produce very deep and loud sounds to make them appear bigger in size than they actually are. When the hyoid bone is bigger, the more powerful they can shout which obviously has pitfalls.
The international team consolidated data from testicle specimens of 66 howler monkeys from different species where they used 3D laser scans to analyze the size of more than 250 hyoids, where some are 10 times bigger than the other specimens.
Dunn explains that investing in developing a large vocal organ results in lesser energy to develop a bigger testes and alternatively, this large vocal organ can produce effective roaring to compete with rival males that there is actually no need for a larger testes.
He adds that during evolution, all males strive to produce as many offspring as they can, however, there is concrete evidence that in some animals, males invest in physical attributes like larger bodies, brighter plumage or weapons such as large horns or canines since they are unable to invest in reproductive organs.
This new study is published in the journal, Current Biology.