If you have just arrived in Guanacaste and something woke you at five in the morning sounding like a dinosaur in the garden, you are in the right place. It was almost certainly a howler monkey.
Press play. This is a real mantled howler, the species that lives around Playas del Coco, recorded in Santa Rosa National Park, about an hour north of us.
The short version: that roar comes from an animal that weighs about as much as a house cat. It is the males announcing their troop to the neighbours, mostly at dawn, dusk and before rain, and it is completely harmless.
What you are actually hearing
The sound is deep, rough and rolling. It swells, holds, and fades, and then another troop somewhere across the valley answers it. On a still morning you can hear two or three groups trading calls for ten minutes or more.
Almost everyone guesses wrong the first time. Jaguar, wild boar, a generator, a neighbour in serious distress. Nobody guesses “a small leaf eating monkey,” because nothing about the sound suggests something small.
How something so small is so loud
Two pieces of anatomy do the work.
A hollow bone in the throat. Howlers have an enlarged hyoid bone, the small bone that sits under your jaw. In a howler it is big, cup shaped and hollow, and it works like a resonating chamber that amplifies the voice. It is much bigger in males, which is why the deepest roars are theirs.
Very long vocal folds. Researchers at the University of Cambridge found that a howler’s vocal folds are about three times longer than a human’s, in an animal roughly a tenth of our size. The result is a roar pitched like an animal many times heavier, which is exactly the point. Their research on howler roars is a fun read if you want the full story.
As for size: mantled howlers weighed in Santa Rosa average around six and a half kilos for males and about five for females. A house cat, give or take.
How far it carries
Further than you would believe. The Smithsonian’s National Zoo puts a howler’s call at about 4.8 kilometres (3 miles) under the right conditions, and Cambridge says up to five kilometres through dense forest. That matches life here: you will hear a troop clearly from your bed when the nearest trees are well across town.
It is often called the loudest land animal in the Americas, and once you have heard it at close range you will not argue.
Why they howl, and when
Howling is mostly about spacing. Each troop tells the others where it is, which saves everybody a fight over the same trees. It is not aggression aimed at you.
The timing is reliable enough to set a watch by:
- Dawn. The big chorus, usually a little after five in the morning.
- Dusk. A shorter round as the light goes.
- Before rain. In the rainy season a sudden howl in the middle of the afternoon is often the first sign that a storm is about twenty minutes out.
Where to hear them on this coast
Anywhere there is a stretch of connected trees. The wooded edges of Coco, the hills behind Ocotal and Hermosa, and the dry forest along the roads inland. Look up and look for stillness: a dark shape draped along a branch like a dropped coat, with a tail wrapped around something.
They are the easiest monkey to find here, and the full profile, with photos, is on the mantled howler page in our Flora & Fauna Guide. If you want to know what else is making noise at night, the common house gecko is usually responsible for the chirping in your kitchen.
The etiquette
- Do not feed them. Their gut is built for leaves, and human food makes them sick and bold.
- Do not stand under a troop. For reasons that become obvious quickly.
- Do not try to call back. You will lose, and so will the people trying to sleep next door.
Most people stop noticing the dawn chorus within a few weeks. Some of us never do, and would not want to.
