It’s often said that men that make the loudest displays of their masculinity have the least to show for it in the bedroom. Now a study of male howler monkeys seems to confirm that relationship.
Howler monkeys are known to bellow in their jungle habitats at unusually loud decibels, louder than the bellows of an elephant or tiger. Their main goal is to catch the attention of prospective female mates, apparently.
But why do some bellow so loudly compared to others?
Researchers at Cambridge University hypothesized that the loudest monkeys were also the least able to spread their seed — their aggressive cat-calls were meant to disguise their inability to mate as effectively as quieter males.
To test their hypothesis, they performed a simple test. They captured dozens of howler monkeys and compared the size of their testes with the number of folds in the hyoid bones located in their voice boxes.
And what did they find? Monkeys that howled at a higher decibel level displayed a much less robust testicular endowment.
In other words, “the louder the calls, the smaller the balls.”
Researchers also decided to compare the mating patens and social structures of the two male groups. It turns out that the louder monkeys had fewer female mates and also lived with them in comfortable isolation from other monkeys — in a harem, in effect.
By contrast, the quieter, big-balled monkeys lived among a much larger community of monkeys, including potential male rivals. Apparently, they had the confidence to score and spread their seed no matter where they were.
“In evolutionary terms, all males strive to have as many offspring as they can – but when it comes to reproduction, you can’t have everything,” says Dr. Jacob Dunn, from the University of Cambridge’s Division of Biological Anthropology.
“There is evidence in other animals that when males invest in large bodies, bright colors, or weaponry such as horns or long canines, they are unable to also invest in reproductive traits. However, this is the first evidence in any species for a trade-off between vocal investment and sperm production,” he says.
There could be another reason for the loud bellows in the less endowed males. Rather than attempting to attract mates, they’re trying to keep other mate-seeking monkeys from encroaching on their own.
There’s also the issue of why some female howlers decide to choose one male over another. Do the female monkeys have their own varied distribution of sexual or reproductive capabilities?
Dunn has no answer for that.
Whatever the size of their scrotum, all howler monkeys emit sounds at an impressively high decibel level. Mated males and females also howl in unison, which magnifies the effect.
The father of evolutionary biology, Charles Darwin, in the Origin of Species, noted the “wonderfully powerful” vocal organs of howler monkeys but also described their sound like a “dreadful chorus.”
Anyone who has visited the jungles of Mexico or Guatemala can attest to this experience. Howler monkeys can keep you up all night. And in the end, you might not have any nookie to show for it.
Are we humans really just like our simian relatives?
Many women would likely say so. After being schmoozed in a bar by an avowed alpha male, they often find that his performance in bed doesn’t measure up to his boasts.
Then again, we’re no longer comparing the size of testicles here.