Some howler monkeys have huge balls, others have loud calls.

Why Do Some Howler Monkeys Have Such Humongous Testicles?

Why Do Some Howler Monkeys Have Such Humongous Testicles?

Wild Things
Slate’s animal blog.
Oct. 26 2015 3:11 PM

Hung Like a Howler Monkey

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Scientists are calling it the "calls and balls" paper.

Courtesy of Flickr photographer Scott Robinson.

Just hours after stepping off the plane in Costa Rica, I was confronted with my first pair of monkey testicles.

My wife and I were on our honeymoon, our destination a spa and hot springs at the foot of the Arenal Volcano. I was anxious to get away from the city we flew into, San Jose, as quickly as possible, mostly because I wanted to see some wildlife. And then, less than an hour’s ride from the airport, our bus driver pulled over so that we could snap a few pictures of a monkey. A mantled howler monkey, to be precise.

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I remember being disappointed upon looking out my window. The monkey was straddling a power line stretched across the mountain road. Not exactly the candid wilderness shot I was hoping for, but I pulled out our fancy new camera all the same. And that’s when I saw them.

Huge, white testicles—slung over the side of the power line in a way that recalled a few choice original verses of the children’s song “Do Your Ears Hang Low?

Forget about the beaches and the all-inclusive dining and the couples massage—I knew right then, staring down the barrel of two of the biggest, baddest gonads I’d ever laid eyes on, that we had come to the right place.

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Courtesy of Jason Bittel

Most people probably don’t think about testes when they see the words howler monkey. The animals are known for having one of the loudest calls in nature, a grunting, vibrating cacophony that sounds like an enormous, angry bullfrog trapped in a trash can.  

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But now, in what very well may be the most fun scientific paper of the year—the veritable chocolate-in-my-peanut-butter moment of 2015—scientists have managed to bring these two aspects of howler anatomy together in a study that investigates the relationship between vocal tract size and testes dimensions.

Naturally, they’re calling it the “calls and balls” paper.

“Of course, you can only say that so loud at the university,” says Leslie Knapp, a primatologist and chair of the anthropology department at the University of Utah.

Indeed.

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At the base of the calls and balls paper is a bunch of data that doesn’t seem to be related to anything. Knapp and her co-authors worked with zoos to conduct testes volume measurements, analyzed recordings of howler monkey calls, and traveled to museum collections around the world to measure a particular bone found in howler monkey throats. Called the hyoid, this large, hollow structure acts like a resonating chamber, which amplifies the primates’ primal screams.

Anyway, it’s when Knapp and company put all of this information together that some mighty interesting correlations started to emerge.

For starters, howler monkey species with larger hyoid bones were able to create vocalizations with a deeper pitch. This part isn’t so surprising. Howler monkeys produce a bass pitch comparable to animals of much larger body sizes, such as red deer and elephants, and in general, larger vocal chambers can produce deeper sounds.

Much of the howler monkey’s characteristic croak is thanks to the hyoid bone. Humans and many other animals have hyoids, by the way, but none are so big and hollow as the howler’s. Magnetic resonance images of the howler’s throat also reveal extremely long vocal folds for an animal of its size. Some species’ folds are more than two-and-a-half times the length of a human’s.

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It’s thought that male howler monkeys use this apparatus and the hoots it produces to guard territory from neighboring troops. Deeper bellows tell rivals that they’re dealing with a big monkey who doesn’t take any guff. Presumably, says Knapp, this also helps the male ingratiate himself with the ladies.

Now, this brings us to the second part of Knapp’s study. You might think that the howler monkey species with the largest hyoids—the biggest badasses, as it were—would have the biggest balls. But you would be mistaken.

“To make a long story short, what we find is that when males have a large hyoid bone, they also have smaller testes,” says Knapp.

That strapping howler I saw on my honeymoon? Turns out, this species (Alouatta palliata) had the smallest hyoid bones of any howler species surveyed but the largest testes. (To that, I can teste-fy.) (Sorry.) And we’re not just talking about a little bit bigger. The mantled howler’s testes are close to six-and-a-half times as big as the red howler featured in the video above.

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To understand what all of this might mean, Knapp says you need to look at yet a third correlation found in the paper—the fact that howler monkey species with larger hyoid bones tend to have fewer males per troop.

This is important because group size and composition can tell you a lot about an animal’s anatomy. Humans, for instance, have relatively small testicles for our body size when compared to other closely related primates, such as chimpanzees. Knapp says this is likely because chimps live in multi-male/multi-female groups where everybody’s competing with everyone for reproductive rights. Thus, chimps have evolved big, honking testicles that produce tons of sperm in the hopes that this will give them an advantage over their rivals. Male humans, on the other hand, don’t have to invest as many resources into competitive sperm thanks to the benefits of monogamy.

Getting back to the howlers, Knapp says what the calls and balls paper seems to show is an evolutionary tradeoff. Each howler monkey species displays a different strategy of resource allocation. Some, like the mantled howler monkey, whose picture has been up on my computer all day and which my wife keeps laughing at, invested in enormous testicles to help them pass on their genes. Others blew their evolutionary wad on raucous hyoid bones, the better to impress females and keep other males at bay.

But wait, you may be wondering, why wouldn’t some monkey invest in both big balls and deep calls? After all, if deeper, Barry White–like vocalizations help you get laid and larger testes increase the chances of fertilization, wouldn’t it behoove the howlers to go big on both?

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“If it was possible for males to have large testes and also large hyoids, we would have observed that in the study,” says Knapp. “It really seems to be one or the other.”

Knapp says this is probably because either route requires a vast amount of limited resources, and with all the things a howler must deal with in the wild—finding food, avoiding predators, fighting off rival males—there simply aren’t enough resources for the howler to have it all.

Of course, the calls and balls study is far from the final word on the teleology of howler testes. For starters, Knapp says they’d like to do genetic tests within species to determine whether deeper calls result in more offspring. (Hyoid size and call pitch vary within species, too, mind you.)

Furthermore, these are only a few of many potential variables involved in sexual selection. And all you have to do is look at the pictures I took of howlers on my honeymoon to spot another. I mean seriously, why would a monkey covered in thick, dark fur have a scrotum that looks like it’s been waxed and bleached?

“Maybe if the females aren’t impressed with the male’s vocalization, they can look at the testes size,” says Knapp, humoring me. “Or maybe it looks scary to another competing male.” (Or it may be that bald, pale testicles contribute to temperature control—there are a lot of theories about testicle evolution.)

Holsters for sperm, indicators of fitness, and maybe even dangling devices of intraspecies intimidation—I’ll bet you never look at testicles the same way again.