Tiny but Mighty: How Monkeys Make Up for Their Size

If you lose your sight, you make up for it with other senses. If one arm is amputated, you learn to do twice as many things with the other arm. The balancing act continues, constantly juggling and shuffling towards some kind of mean. But missing limbs and senses aren’t the only things people feel compelled to make up for; we’re all familiar with the “big truck, small man” phenomenon. And humans aren’t the only species to instinctively overcompensate. Indeed, research has shown that monkeys that scream and howl the loudest often have the smallest packages.

We don’t mean gifts, either. It seems that monkeys who feel compelled to outshout everyone in the jungle are doing so for a compensatory reason: the louder the screamer, the smaller the testes and the lower the sperm counts. However, this screaming index isn’t entirely a behavioral recoupment for a perceived lack of dominance. Monkeys with small testes but loud calls are just as capable of reproduction as their quieter counterparts. Let’s learn more about this fascinating study and what it explains about the relational dynamics between competing male howler monkeys.

Howler Monkey Music

Howler monkeys are one of the few types of monkeys to build nests. As evidenced by the name, they are also some of the loudest animals on earth. Black howler monkeys, for example, are so loud that their calls can reach 100 decibels and be heard up to three miles away. For comparison, the sound of a jet engine can rupture eardrums at 150 decibels. These powerful vocalizations play an integral role in howler monkey social dynamics, but scientists wanted to learn exactly how.

This resulted in the landmark 2015 study “Evolutionary Trade-Off between Vocal Tract and Testes Dimensions in Howler Monkeys,” spearheaded by Dr. Jacob Dunn at Cambridge University and published in the journal Current Biology. The study showed that howler monkeys with the loudest and most impressive shouts produced considerably less sperm than their counterparts. Now, a follow-up study illustrates how the acoustic properties of howler monkey roars, specifically formant spacing, are a reliable indicator of body size.

Hyoid Bone Business

The correlation between howler monkey shout power and testicle size is humorously known as the “calls-and-balls” tradeoff. This compensatory survival strategy, however, hinges on the hyoid bone.

The hyoid bone is found in the neck of animals and supports their tongues. Humans have hyoid bones with a horseshoe shape. Howler monkeys, however, have hyoid bones that have grown much larger through evolution. These large hyoids feature a cup shape that amplifies their howls. Multiple studies, including recent research, show that the size of a howler monkey’s hyoid bone is inversely related to testes size, indicating a developmental tradeoff. Simply put: the bigger the hyoid bones and the louder the call, the smaller the testes. This correlation also works in reverse. The smaller the hyoid bone and quieter the call, the larger the package.

The idea that evolution would favor smaller reproductive organs in exchange for a louder call seems counterintuitive. That is, until you take a closer look at the social dynamics of different howler monkey species. Some animals compete pre-copulation, while others compete post-copulation. An individual howler monkey’s reproductive strategy depends on the size of its hyoid bone.

Reproductive Trade-Off

As shown by the researchers at the University of Utah and the University of Cambridge, howler monkeys take different reproductive approaches depending on their attributes. Monkeys with big mouths and small balls usually live in harems consisting of one or two males and many more females. They use their big, terrifying roars to keep prospective males away. Therefore, competition is minimized. On the other side of the howler monkey spectrum are the quieter males. These males with smaller hyoid bones live in large mixed groups. Since they have to compete with many more males for the same females, evolution puts all their energy into testicular size. The larger the testes, the more sperm have a chance of reaching a female’s egg first.

Effective Exaggeration

The twin studies showed that the hyoid bone of small-packaged howler monkeys contains features not found in humans. For example, these howler monkeys have a tiny ribbon of tissue above their vocal cords. When vibrated, this membrane produces strange, non-linear sounds that are impossible to ignore. As Dunn explained in an article, “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.”

These findings reverberate through primate research. A 2019 study titled “Sexual ornaments but not weapons trade off against testes size in primates,” found that, across primate species, those that invest more in sexual ornaments like the hyoid and larynx tend to have smaller testes. A sufficiently large hyoid bone can make a 17-pound monkey sound like a full-grown tiger.

Surprisingly, it works. The strategy of shouting super loud is so successful for the males with the smallest packages that they usually gather the largest harems. It’s something to keep in mind the next time you see a guy driving down the road in an oversized truck. A little honest exaggeration, as biologists call it, goes a long way.