AMAZON BLACK HOWLER
Geographic Distribution and Habitat
The Amazon black howler monkey (Alouatta nigerrima) is most commonly known as simply the “black howler.” As its name suggests, it is a species of howler monkey, part of the Atelidae family of New World monkeys.
Living deep in Brazil’s south-central Amazon Basin, black howlers are endemic to an area just south of the Amazon River. Locals have reported sightings as far south as Igapó-Açu. The monkeys make their home in the trees of the region’s rainforest, as well as the small groves spread throughout the surrounding savannahs.
Black howler monkeys only became known as a distinct species starting in 2000. However, there is still some debate over whether or not they are indeed a full species. More genetic testing is needed to confirm.
Previously, they had been widely regarded as a sub-species of red-handed howlers (Alouatta belzebul), and that consensus lasted for a period of 50 years. While some earlier primatologists had suggested black howlers may be distinct, it wasn’t until genetic studies were done in the 1980s that modern primatologists had the evidence needed to classify them accordingly.
There is some evidence that Alouatta nigerrima has interbred with another howler monkey species, the Guianan red howler monkey, Alouatta macconnelli. Their offspring are genetic hybrids. The two species occupy much of the same territory, have a similar appearance, and are very close to each other on the howler monkey phylogenetic tree, a diagram representing the evolution and relationship between species.
Size, Weight, and Lifespan
Black howlers are fairly large, with an average body length of 22–36 inches (56–91 cm). This figure does not include their tails, which can be almost the same length. The average adult weighs around 9–18 pounds (4–8 kg).
Howler monkeys, as a whole, tend to have big physical differences between males and females. This is known as sexual dimorphism. In all species, including black howlers, males are at least 25% larger than their female counterparts.
As is typical of members of the Alouatta genus, black howlers have an average lifespan of 15 years. It’s possible for healthy individuals to live for more than 20.
It’s no surprise that black howlers are predominantly black—that includes both their skin and fur. They have prehensile tails, which are flexible and can be used to grab onto branches. Two of the five fingers on their hands are opposable to the other three. Opposability allows these fingers to rotate and bend in a way that can touch the tips to those of the other fingers.
Their faces are relatively long, with protruding muzzles and large mouth openings that tend to take on a frowning expression when at rest. Dense black fur covers their bodies, especially around their cheeks and chins.
Such a gloomy look has not gone unnoticed. Famous conservationist Alexander von Humboldt said that “their eyes, voice, and gait are indicative of melancholy.” When explorer John Lloyd Stephens encountered howler monkeys at the Maya ruins of Copán, he wrote that they looked “grave and solemn as if officiating as the guardians of consecrated ground.”
Black howlers are mainly folivores (leaf-eaters). They are the only Latin American monkeys to regularly eat mature leaves, which tend to be tougher to chew. This gives them a big advantage over other monkeys, who have to limit their range in order to access their more specific food sources.
That being said, black howlers do still prefer younger, softer leaves when they’re available. They also eat leaf stalks, stems, and twigs, as well as ripe fruits (especially figs), buds, flowers, seeds. Black howlers will even eat termites when they can find them.
Behavior and Lifestyle
Black howlers are diurnal, meaning that they are primarily awake during the day. However, much of this time is spent resting, since digesting their leafy diet takes quite a bit of energy.
As arboreal monkeys, they live the majority of their lives in trees. Black howlers are preyed upon by large mammals, such as jaguars, as well as snakes and raptors (predatory birds). This high hunting pressure has forced them to adopt clever defensive strategies. They choose different resting spots depending on whether it’s day or night, in order to avoid predators that are active at those times.
At night, black howlers prefer to sleep in the extended branches of trees high up in the forest canopy, where nocturnal predators can’t easily reach them. These nighttime sleeping spots also tend to lack many connecting vines, which snakes can use to climb. During the day, the monkeys choose to rest in lower trees, close to the trunks, with more cover from vines. This helps them hide from the hawks and eagles that hunt in daylight.
Black howlers are unique from other Latin American monkeys in that both females and males have trichromatic color vision. Like humans, their eyes can detect red, green, and blue light wavelengths, which combine to cover the entire color spectrum visible to our species.
Black howlers typically live in groups ranging from 5 to 19 members of both sexes. Females typically outnumber the males, and unimale harem groups also exist. This means the group only has one male, who has breeding access to all of the females. In some cases, males will live together in bachelor groups, which can be quite stressful due to competition between members.
Once howler monkeys have matured, they often leave their families to form new groups. For most howler monkeys, this takes place around one year old. Females may stay with the groups they were birthed in.
Like most howler monkeys, black howlers are territorial. They will fight with neighboring groups of primates to defend the area they call home. This can happen frequently since the home ranges of multiple groups often overlap. Fighting among members of the same group is less common, and disputes are usually over quickly.
Communication is so important to black howlers that they’ve evolved to be some of the loudest terrestrial (land-dwelling) animals on earth. As a member of the howler monkey family, black howlers have developed a specialized bone structure in their throat and mouths that allows them to produce very loud, low-pitched howling sounds.
This includes a cup-shaped hyoid bone with a connected air sac that acts as an amplifier. The air sacs inflate when the monkey is vocalizing, causing their throats to puff up. The resulting look is exaggerated by their bushy beards. Males have the potential to develop larger hyoid bones and sacs, especially when they live in all-male groups where competition is high.
Black howlers engage in group howling sessions that typically take place in the early morning. The pesky alarm clocks that no one asked for, these noisy primates can be heard over a mile away (1–2 km). These morning announcements are meant to communicate the group’s presence to other primates in the area.
Competing groups will usually steer clear of other black howlers once they’ve heard these calls. However, when food, mates, or other resources are scarce, they may actually respond to howls by moving closer.
In addition to vocal communication, black howlers also use their feces (poop) to send messages. They will mark their territory with droppings daily.
More research is needed to understand the specific breeding traits of Alouatta nigerrima. However, we can assume that they are probably quite similar to other species in their genus.
This includes promiscuity—howler monkeys aren’t big on commitment. They are polygynandrous, meaning they will mate with various partners over the course of their lives. Males usually reach sexual maturity around 5 years old, while females are ready to breed as young as 3 or 4.
Howler monkeys generally have a gestation period (pregnancy duration) ranging around 152–195 days. Actively breeding females will give birth every 16–23 months. Babies generally weigh around 3 pounds (1.36 kg). Infants will cling to their mother’s front side, and ride on her back as they grow older.
The other members of the group may gather around mothers who are giving birth, and even examine new infants after their arrival. Researchers have observed a group initiating a howling session after a baby is born, with neighboring howler monkeys answering back.
Black howlers reside firmly in the middle of the Amazon’s terrestrial (land-based) food chain. They consume many plants, fruits, and insects, and they are food for many predators. Their presence is critical for encouraging the growth of important plants in their habitat. Howler monkeys in general are known as keystone species: species that play an essential role in ensuring the health and biodiversity of an environment.
By eating so many leaves, black howlers actually stimulate trees to produce more. These new leaves provide food to support larger insect populations, which in turn provide more food for birds and other animals.
When black howlers eat fruit, they carry the seeds in their stomachs and spread them by dropping their feces (poop) farther away from the tree than the tree could manage on its own. This allows fruit trees to take root throughout the rainforest, feeding more animals and fertilizing the soil.
The International Union for Conservation of Nature lists the Amazon black howler monkey, Alouatta nigerrima, as Least Concern (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.
The black howler population has fortunately managed to avoid being split up into separate, isolated groups, a phenomenon conservationists call “habitat fragmentation.” Nevertheless, this is a big risk. Human activity still poses a threat to the forests and savannas where these monkeys live. Crucial habitat areas are being lost to the development of homes and businesses, as well as farms.
Black howlers are heavily hunted by humans for food. This may be an explanation for why the species is particularly rare along the left bank of the middle and upper Tapajós River. A large number of gold miners inhabit the area, and they may be causing intense hunting pressure. It is also possible that the black howler’s scarcity here may have some other natural cause, such as competition with spider monkeys, woolly monkeys, and other primates.
The Amazon black howler monkey is listed in Appendix II of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival.
Since black howler populations aren’t at serious risk, dedicated conservation programs are lacking. But much of their habitat lies in protected areas, such as the Amazonia National Forest, the Tapajós-Arapiuns Extractive Reserve, and the Amanã National Forest.
Responsibly managing the land and water in the black howler’s habitat is the biggest priority for conservationists trying to protect this species. More research is needed to monitor these monkeys and keep track of their population trends. Scientists are interested in finding out how living in degraded habitats may be changing black howler behavior, lifespans, and reproductive success.
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Written by Amanda E. Riley, September 2023