To many first-time visitors to Costa Rica’s varied forests, mantled howler monkeys are the first indication that they are somewhere wild. Often heard before they’re seen, their booming howls can brew a slight apprehension as to how close one should get to the source of the calls.
Should you pluck up the courage to move forward and peer into the branches of the trees, you might be surprised by the size of the roaring beast that you encounter. It’s not a barrel-chested gorilla but a fairly normal sized monkey. The mantled howler monkey (Alouatta palliata).
Of Costa Rica’s four species of monkey, the mantled howler monkey is the second biggest, coming in at about 20 inches in length and 11 pounds. The physical size doesn’t compute with the sounds that you hear tearing out of their little bodies. Their howls can be heard over a kilometer away. Pretty good for an animal about the size of a raccoon.
If you spend some time observing howler monkeys two things become pretty obvious. First, they enjoy eating leaves. And second, they enjoy laying around and napping. It turns out that these two observations are linked.
Mantled howler monkeys are mostly folivorous, meaning they eat a lot of leaves. While flowers and fruit are also eaten regularly (depending on the season), leaves make up the majority of the howler monkey’s diet.
They’ll eat leaves from dozens of different trees. They often prefer new growth because the tree hasn’t had time to put toxins in the leaves, but they will also eat mature leaves of some trees.
If you were to look at the forests of Costa Rica and think, ‘What’s the most abundant food resource?’ one of the most obvious answers might be ‘leaves.’ Trees full of leaves are everywhere, so there should be a ton of mammals chowing down on them.
But there’s a trick to eating leaves, they’re not terribly nutritious. Leaves are full of cellulose, which is difficult to break down, and are often full of toxins (the results of the trees effort to stop things from eating its leaves). Mammalian leafeaters use fermentation to break down the cellulose, which is a time-consuming process.
This leads to our second mantled howler monkey observation, their seeming laziness. They’re often draped across the branches of the treetops doing a whole lot of nothing because their bodies are hard at work breaking down their last meal.
Now let’s bring it all home.
If you were a mantled howler monkey resting in a tree, digesting a huge meal of leaves, would you want to run off and fight with some other group of howler monkeys over territory? Of course not. It’d be a lot easier to just yell ‘Hey! I’m over here! You stay over there!’ without getting up and using a lot of energy.
Maintaining boundaries with other groups without costly physical confrontation is one of the reasons the howlers are doing all of that howling. So, leaf eating → “laziness” → howling.
I see mantled howler monkeys in the field much more frequently than I record them with my camera traps. I have had the good fortune of recording a few individuals in the trees as well as on the ground. Enjoy a few mantled howler monkey clips in the video below.
About the Author
Vincent Losasso, founder of Guanacaste Wildlife Monitoring, is a biologist who works with camera traps throughout Costa Rica. Learn more about his projects on facebook or instagram. You can also email him at: firstname.lastname@example.org