Howler Monkeys: The Noisiest Animals in the World

In Belize, the black howler monkey (Alouatta pigra) is commonly referred to as a “baboon”.  This is a bit of a misnomer since black howler monkeys are native to Belize, northern Guatemala, and southeastern Mexico while the term baboon is generally reserved for different groups of monkeys that live in Africa.  Alouatta pigra can be howler-monkey-4recognized by the long black hair that covers the entire body of the adults.  Young are born with pale fur that begins to darken around 9 to 10 weeks of age.  Body size averages about 70 cm in length, excluding the tail.  The prehensile tail is around 30 cm in length and acts as a 5th appendage.  Their tails assist in a variety of tasks such as hanging from branches, grabbing food, and holding onto their young.  Adult males weigh around 11.4 kg with adult females weighing around 6.4 kg making black howler monkeys one of the largest primates in the Americas.

Alouatta pigra are typically found in the middle to upper levels of the rainforest canopy. They can be found living in a wide range of forest types ranging from pristine tropical rainforests to riverine forests, deciduous, semi-deciduous, and evergreen forests as well as at low elevations in riparian forests and inland forested areas. However, rainforest areas that have been disturbed by humans often lead to lower survival rates for black howler monkeys and smaller populations.

Even though black howler monkeys are strict herbivores their diet is highly varied.  They will feed upon a wide selection of different parts of various plants that include the leaves, fruits and flowers.  Alouatta are highly frugivorous, meaning they will prioritize fruits over the other available parts of the plant. When fruits are not available they will adjust their diet to consume the leaves or flowers.  In general, fruits are 36%, mature leaves are 30%, new leaves and buds are 25% and flowers are around 5% of their diet.  Members of the genus Alouatta have retained a primitive digestive tract that is very suitable for a frugivorous diet.  The combination of their digestive system along with their dietary habits requires an extended amount of time for digestion.  This helps explain the long periods of inactivity that occupy the majority of their day.

The Sound of the Rain Forest

howler-monkey-5One of the most distinguishing feature of Alouatta pigra is their extremely loud vocalization.  The Guinness Book of World Records has named howler monkeys as the world’s noisiest land animals.  The males have an enlarged bony structure at the top of their windpipe that enables their vocalization to reverberate and amplify.  Their screams can be clearly heard up to 3 miles away. These monkeys will use their calls to announce and defend the groups territory.  The adults in the group will often begin howling when males from neighboring groups invade their territory.  Bouts of howling can last for over an hour at a low but very loud frequency.  Howling is used as a way to defend their territory due to it being less energetically expensive than physical confrontation. The information gained from listening to another groups howling helps the group decide whether or not they should initiate a physical confrontation.  Both adult males and females will engage in group howling.

What Do We Know About Howler Monkey Populations?

Group sizes usually range from 4 to 11 individuals with an almost equal ratio of males vs. females.  Adults generally make up about two thirds of the overall group size.  Most groups are led by an alpha male who is responsible for the primary defense of the group as well as determining the groups daily activities.  Groups also contain several females along with their dependent offspring.  If is a larger group, it may contain several subordinate adult males that will also participate in the groups defense.  These males will sometimes leave their existing group if opportunities arise that enable them to start a group of their own.  They may also make a power play for the alpha position within their own group if they feel they have a chance at overthrowing the existing alpha male.  Newly formed groups tend to be smaller than those that have been established for longer periods of time.

Black howler monkeys reach sexual maturity at four years of age.  Once females become sexually mature they will have an estrus cycle that lasts from 11 to 24 days with a peak conception period of 2 to 4 days.  Mating is not seasonally restricted and can occur at any time of the year.  It is thought that this is due to the fairly stable abundance of their food sources across the entire year.  Pregnancy lasts for about 180 days with a single offspring born.  Once mature, offspring of both sexes will migrate out of their natal group.

Ecology of Howler Monkeys

As black howler monkeys travel throughout their territory they will defecate the seeds of many of the fruits that they have consumed.  This helps increase the success of the plants that they rely on as a source of food.  Ultimately, if these plants succeed in germinating and surviving it will benefit future generations of howler monkeys.  The greatest predatory threat to Alouatta pigra is infanticide by a new adult male who takes over a group containing infants.  Occasionally they are preyed upon by the large predatory cats, birds of prey and arboreal snakes that are found in the tropics.  Humans have been known to illegally catch baby howler monkeys in order to sell them in the black market pet trade and on rare occasions larger individuals are used as a source of food.  Overall, black howler monkeys play an important role in rainforest ecology and are beneficial to humans through increased tourism.

The Status of Howler Monkeys

Currently, the IUCN classifies Alouatta pigra as endangered.  It is estimated that their populations will see a decline of nearly 60% over the course of the next 3 howler monkey generations.  The continued decline is expected to occur due to continued habitat loss and fragmentation as well as hunting of the animals for the pet trade and as a source of bush food.  In some areas their populations have been significantly reduced due to outbreaks of yellow fever.  Fortunately, in Belize as well as various other regions of Central America, conservation efforts have been taken to preserve habitat for black howler monkey populations.  The Blue Hole National Park, Community Baboon Sanctuary, and Cockscomb Basin Wildlife Sanctuary are just a few of the protected areas in Belize that still contain large regions of undisturbed forests where visitors can go to hear the noisiest animals on Earth.

For More Information

Article by Dave Cox. Dave is an Associate Professor of Biology, author, and President of Howler Publications. He leads yearly study abroad trips to Belize, for students and community members, to study tropical ecology, marine biology and Mayan culture. If you would like to join Professor Cox on  a trip to Belize contact him at for more details.

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