Howler Monkey Guide: The Loudest Land Animal in the Americas

Howler monkeys produce the loudest call of any land animal in the Americas — a guttural roar that carries up to five kilometres through dense rainforest. They are also among the most sedentary monkeys alive, a direct consequence of a diet built on leaves.

Howler monkey at a glance

Scientific name	Alouatta spp. (about 15 species)
Family	Atelidae (New World monkey)
Weight	4-11 kg; males up to 50% heavier than females
Tail	Fully prehensile, with a bare gripping pad
Lifespan	15-20 years wild; 25+ in captivity
Diet	Folivore-frugivore: 40-70% leaves
Troop size	6-19 individuals
Range	Southern Mexico to northern Argentina

Key takeaways

• A hugely enlarged, hollow hyoid bone in the throat acts as a resonating chamber.
• Calls at dawn and dusk advertise troop position so neighbouring groups avoid costly fights.
• Leaves are low in energy, so howlers sleep or rest up to 80 percent of the day.
• They are the only New World monkeys with reliable full trichromatic colour vision in both sexes.
• Yellow fever outbreaks can wipe out entire local populations.

Why howlers are so loud

The male howler’s hyoid bone is expanded into a large hollow shell that amplifies sound the way a guitar body amplifies a string. Combined with an enlarged larynx and a deep jaw, this lets a 7 kg animal produce a call audible at three miles. There is a trade-off: species with the biggest hyoids have smaller testes, suggesting evolution traded sperm competition for vocal competition.

The chorus is territorial signalling, not aggression. By roaring at first light, troops broadcast where they are and how many males they have, letting neighbours steer clear of contested fruit trees. More on primate vocal signals in How Monkeys Communicate.

A low-energy lifestyle

Mature leaves are abundant but nutritionally poor and full of tannins and toxins. Howlers cope with an enlarged caecum and colon where gut bacteria ferment cellulose, and with behaviour: they move short distances, choose young leaves and ripe fruit when available, and rest for most of the daylight hours. Their home ranges are small compared with spider monkeys of similar size. See Monkey Diet by Species and What Do Monkeys Eat?

Species and where to find them

• Mantled howler — Mexico to Ecuador; the one most tourists hear in Costa Rica.
• Black howler — Argentina, Paraguay, Bolivia, southern Brazil; males black, females blonde.
• Red howler — Amazon and Orinoco basins; the most widespread.
• Guatemalan black howler — Belize, Guatemala, Mexico; endangered.
• Brown howler — Brazil’s Atlantic forest; hit hard by yellow fever.

Regional context in South American Monkeys and Rainforest Monkeys.

Social structure

Troops usually contain one to three adult males, several females and their offspring. Males compete to take over troops, and takeovers can be violent. Grooming is less frequent than in capuchins or macaques — another energy economy of a leaf-eater. Background reading: Monkey Social Hierarchy and Why Monkeys Groom Each Other.

Threats

Habitat fragmentation is the biggest issue, followed by electrocution on power lines in agricultural landscapes, road deaths, and yellow fever epidemics that can eliminate whole populations within months. Canopy bridges over roads have proved genuinely effective in Costa Rica and Brazil. See Threats to Monkey Habitats and Conservation Success Stories.

Frequently asked questions

How loud is a howler monkey?

Around 128 decibels at close range, audible up to five kilometres through forest.

Why do howler monkeys howl?

To advertise troop location and strength so neighbouring groups avoid direct conflict over feeding trees.

What do howler monkeys eat?

Mainly leaves, supplemented with fruit, flowers and buds. They favour young leaves, which are easier to digest.

Are howler monkeys dangerous?

No. They are shy and slow-moving, and they flee rather than confront. The noise is far bigger than the animal.

Do howler monkeys have prehensile tails?

Yes — fully prehensile, with a bare textured pad for grip.

Related: Spider Monkey Facts, Squirrel Monkey Guide, Ultimate Guide to Monkeys.