Just out Hootin' and Hollerin'
The Howler Monkeys are a genus of New World Monkeys native to Central and South America. They are THE loudest terrestrial animals. At 140dB, their territorial calls are as loud as a jet engine starting up. When you think about it, a monkey has a lot of the same hardware as you. What makes them the loudest land animal on Earth and you just kinda loud? Well it starts with a fragile little bone in your throat.
This is the hyoid bone! You might be able to find yours if you tilt your head back, gently place a finger on each side of your throat at the base of the jaw, and swallow. It should jump! The hyoid is the connection point for some of the muscles in our tongue and the muscles used to swallow. It helps us talk, but I wouldn't say it really makes us loud. You'll notice the male hyoid is larger and more dramatically shaped. This is due to testosterone. Let's see what testosterone does to a howler hyoid!
The howler hyoid is shaped like a cup! If you were to grab a cup right now, you could yell into it until you find a note the cup likes very much. A resonant frequency resonates a lot more and leaves the cup amplified! Just as you suspect, a higher amount of testosterone makes a larger hyoid and a more terriorial male. But also, as in humans, high testosterone shrinks the testes :/
Enough talking about it, let's listen!
(this is very loud and unpleasant)
Yes, that could cause permanent hearing damage really quickly. That is their first order of business in the morning, howling and listening to assess where other howler troops may be and maybe pick up tips on food locations. Howler monkeys are Folivores, which means they eat foliage or leaves. Like a sloth or a hoatzin, they don't get a ton of energy from their leaf-based diet. They spend most of the day eating leaves or lazing about as they digest the leaves. Fruit, however, has tons of energy in it! Usually less than 30% of their diet, access to fruit is what these howlers are hollering over. A big loud call in the morning tells everyone that you're prepared for a (short) fight over highly valuable resources.
In the black howler monkeys, as shown above, the males are black while the females and young are this goldish color. The females have quieter voices and smaller hyoids, but they help generate noise to show the strength of the troop as a whole. They live in groups of 15-20 monkeys with either on loud male or a few endowed males, depending on testosterone levels.
They have a muscular prehensile tail with a furless grippy pad on the bottom. It's capable of supporting their full body weight and they use it like a fifth limb or a safety rope to navigate the thin branches of the canopy.
Here's a male howler skull, you can see the hyoid in place below it. But also notice how much muscle you could attach to that lower jaw! That's one half of the picture, the other half is that high cranium! That is where the other side of the muscle attaches. Howler monkeys use these large muscles to grind up leaves. It also gives them a really cute, sloping, pouty face.
What cuties! They can stay far away from me! You could be chilling with them, a picnic with fruit, and they get excited and deafeningly loud. I like music way too much to spend my ears on monkey dates. Aside from their volume, they're mostly sweet, lazy monkeys that make a living by chilling hard.