BROWN HOWLER MONKEY
Geographic Distribution and Habitat
The brown howler monkey (Alouatta guariba) is endemic to the lush Atlantic Forest of Brazil and Argentina. They are the only primate species found on the Brazilian island of Ilha do Cardoso and in the Upper Parana Atlantic Forest of Argentina. They occupy a variety of forest types, including lowland and mountain forests, and semideciduous, subtropical, and temperate forests. They are adaptable, able to live even in disturbed habitats, but their adaptability only goes so far. Sadly, their population is in decline due to habitat loss and human interference.
There has been some confusion in the scientific literature over the correct scientific name for brown howler monkeys: A. guariba or A. fusca? While A. guariba is the more commonly used name these days, note that older sources may refer to brown howler monkeys as A. fusca.
Additionally, there are two recognized subspecies of brown howler monkeys: the northern brown howler (A. guariba guariba) and the southern brown howler (A. guariba clamitans). There is still some debate as to whether the subspecies deserve full species status, and this is an ongoing question in scientific literature. Recent research has referred to brown howler monkeys as a “species complex”: a group of organisms so similar that it is difficult to determine what boundaries, if any, exist between them. Until there is more scientific consensus, this profile will consider both the northern and southern populations to be subspecies of brown howler monkeys.
Size, Weight, and Lifespan
Females weigh an average of 9.7 pounds (4.4 kg), while males are larger with an average weight of 14.8 pounds (6.7 kg). Their head and body length ranges from 22 to 36 inches (56 to 92 cm). Their lifespan is about 15 to 20 years in the wild.
Brown howler monkeys are quite large for leaf-eating monkeys. Their bodies are stocky, and covered with hair that ranges from brown to black to dark red. On their bellies, the hair is lighter. They have specially adapted molars that allow them to shear leaves with ease. Their long tails are prehensile, meaning that they can use their tails for grasping, almost like a fifth limb, and the end of their tails has a hairless patch, which helps with gripping. Brown howler monkeys exhibit some sexual dimorphism, or different appearances between the sexes, as the males are larger and have slightly different coloration than the females, such as darker limbs and redder backs and bellies.
Brown howler monkeys have a varied diet. Up to half of their diet is composed of leaves—preferably young leaves, although they eat mature leaves as well. In fact, howler monkeys are the only American primates to regularly eat mature leaves, making it a reliable food source with little competition. The remainder of their diet is made up of flowers, fruit, seeds, mosses, stems, and even termites. Their diet changes significantly depending on the time of year. In the fall and winter, the overall amount of available food is lower, and they must expend more energy to find their meals.
Behavior and Lifestyle
Brown howler monkeys move about quadrupedally—on all fours—and don’t brachiate, or swing by their arms. They are diurnal, meaning that they are awake during the day. They are, at their core, energy conservers, which is another way to say that they are rather lazy. They spend nearly two-thirds of their waking hours resting, especially during the hottest hours in the middle of the day. They spend about 13% of their time moving, almost 20% eating, and about 2% grooming group members. Unfortunately, research has shown that humans are disrupting this carefully balanced energy budget. Because of habitat loss and fragmentation, howler monkeys are increasingly making contact with other groups and competing for food resources more than ever before. Therefore, they need to spend more time and energy finding food each day, which can put significant stress on an animal that has evolved to conserve energy.
Brown howler monkeys carry a number of the same diseases as humans, such as yellow fever and giardia. While disease transfer between our two species has been a problem in the past, work is being done to prevent future cases of transmission. For instance, human vaccination campaigns have been started after noticing signs of disease in wild howler monkey populations. Habitat conservation work will also likely help to reduce disease transmission, as the monkeys have less contact with humans when they have adequate protected habitat.
Brown howler monkeys form a variety of group types. They can be found in groups composed of a single male-female pair, multiple males and multiple females, or a single male with multiple females, though the latter is the most common. A typical group size is between four to five individuals but can be as many as thirteen. Even when multiple adult males live in a single group, one is usually the alpha male who monopolizes the females, and by extension, the breeding opportunities.
Allogrooming—or grooming one other—is an important method of bonding among group members. Males are the most common recipients of grooming, while females are more likely to do the grooming. Brown howler monkeys are territorial and typically have home range sizes of up to 77 acres (31 ha).
Males are the primary defenders of the territory—they use aggressive behaviors like intimidation and fighting to ward off intruders. That said, group ranges overlap quite a bit. They occur in very high population densities when not hunted by humans. Surveys have reported densities of up to 459 individuals per square mile (177 individuals per square km), although this is likely the top end of the density range.
Howler monkeys are so named for their iconic and unmistakable vocalizations. Their bellows are produced in their jaws, which are specially adapted to produce booming howls. Their voicebox and even the bones in their jaw and mouth work in combination to form a resonating chamber from which they can produce vocalizations that can be heard up to 1.2 miles (2 km) away. Their loud calls are used to ward off predators and signal their group’s strength to neighboring groups. They can listen to the calls of other groups to gain an understanding of their size and strength—and determine whether a territory dispute is worth the fight. By signaling group strength through vocalizations, physical confrontations can be avoided.
Brown howler monkeys are also known to use olfactory communication—scent signals. Female genital rubbing on trees and other objects in the environment can signal reproductive status, while male chest rubbing can be used to signal dominance.
Reproduction among brown howler monkeys occurs year round, possibly as a result of their leafy diet, since their energy source is fairly stable throughout the year. Mating is first initiated by the male. If the female is receptive, she moves closer to the male and moves her tongue rhythmically to signal her interest. Females usually only mate with the alpha male, although sometimes they mate with other adult males in the group. Occasionally, they mate with males outside of the group—this is more likely in groups with multiple adult males.
Babies are born after a 6-month gestation period. They nurse from their mother for about a year after birth. The mothers are highly invested in caring for their young, while fathers have minimal involvement. Unlike many primate species, there is little allomaternal care—that is, adult females are unlikely to help care for young that are not theirs. Young females reach sexual maturity at about three and a half years of age, while males reach maturity at about five years of age. Males disperse from their natal groups—the ones they were born into—and join new groups, after competing with the alpha male.
Brown howler monkeys are preyed upon by animals such as birds of prey, cats like ocelots, and even feral dogs. They attempt to deter predators through their loud calls, but if that doesn’t work, they try to hide by staying still and silent. If the predator is still in pursuit, the dominant male distracts the predator, allowing other group members to lead the young away to safety. Brown howler monkeys are sympatric—meaning they live in the same places as—black-and-gold howler monkeys (A. caraya) in parts of their range. Hybridization between the two species has occurred in captivity and it is suspected to occur to some degree in the wild where their ranges overlap.
The International Union for Conservation of Nature lists the brown howler monkey as Vulnerable (IUCN, 2020) appearing on the IUCN’s Red List of Threatened Species. Brown howler monkeys are in significant population decline and are predicted to lose 30% of their population over three generations by 2044. The northern brown howler subspecies is listed as Critically Endangered. It is believed to have lost 25% of its population over just one generation, and there are fewer than 50 adults left of the subspecies. The threats that are impacting northern brown howler monkeys can be summarized quite simply: human interference. This has led to a number of issues, notably habitat loss and fragmentation, but also the spread of disease, vehicle and power line accidents, hunting pressure, and even predation by domestic dogs.
Habitat loss and fragmentation is a major concern for all brown howler monkeys. Five hundred years ago, the Brazilian Atlantic Forest covered an area twice the size of Texas—about 330 million acres (1.3 million square km). Over 85% of the forest has since been cleared, and what remains is extremely fragmented. Habitat fragmentation is the process of breaking up a wide swath of habitat into disconnected “islands” of habitat, by introducing roads, power lines, and other types of human development into an ecosystem. While brown howler monkeys are not quite as impacted by habitat fragmentation as some other primate species due to their relatively small home ranges, they are certainly not immune to its impacts. In particular, conservationists are concerned that habitat fragmentation is causing gene flow issues within the species. In other words, when populations are small and isolated, there is not enough genetic diversity and inbreeding problems can arise. Only 12% of the original forest cover is left in the brown howler monkey range, and most of the forest exists in only small pockets less than 120 acres (50 ha) in area.
To add to that, brown howler monkeys can be impacted by a number of the same diseases as humans. In 2007 to 2009, an outbreak of yellow fever among brown howler monkeys led to a significant population loss. Human retaliation against the monkeys was reported after this incident, as locals feared the disease transmitting to the human population. In 2008, a large number of brown howler monkeys in Argentina were found to have died from giardia, a parasitic disease, which prompted a human vaccination campaign. Additionally, increased human encroachment in their habitat has led to deaths caused by vehicle collisions and power line accidents, hunting (in addition to hunting for its meat, the monkeys’ hyoid bone—the oversized throat bone that helps generate their loud calls—is used in local rituals and folk medicine), as well as the emergence of feral domesticated dogs as a major predator.
Climate change is also expected to impact the brown howler monkey population in the coming decades. Extreme climate changes have been modeled in the Atlantic Forest in the coming years. In fact, climate scientists predict that it will change more in the next 50 years than it has since the last ice age. It is difficult—impossible, really—to predict all of the ecological impacts that these climate changes will have, but ecologists are sure that it will be an uphill battle for the many species that rely on the forest.
Brown howler monkeys are 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.
Brown howler monkeys are protected by a very large number of protected areas across their range, which helps to alleviate—but not halt—concerns about habitat loss and fragmentation. A number of conservation programs have focused on the species. For example, the Projecto Bugio (Howler Project) conducts research, education, and conservation activities in the northern brown howler’s range. The Programa Macacos Urbanos (Urban Monkeys Program) works to help alleviate the effects of human encroachment in Brazil, such as road kills, dog attacks, and power line accidents. One project they’ve implemented is the installation of rope bridges in the city of Porto Alegre to allow monkeys (as well as porcupines, opossums, and other arboreal mammals) to safely cross roads. The municipality of São Paulo works to educate local communities about the monkeys and reintroduces individuals into the wild. The Projeto Barbado-Vermelho (Red Howler Project) works to release howler monkeys that were illegally kept in captivity and manages almost 5,000 acres (2,000 ha) of primate reserves.
- Monticelli, C., Comassetto Maciel, P., & de Oliveira Garcia, F. 2022. Rope bridges provide safe connectivity for the southern brown howler monkey (Alouatta guariba clamitans Cabrera, 1940) in an urban Atlantic Forest remnant. Folia Primatologica, 93(3-6), 519-532.
- Povill, C., de Oliveira, M.B., de Abreu, F.V.S. et al. 2023. Genetic Diversity and Insights into the Distribution of Brown Howler Monkeys (Alouatta guariba Group) (Atelidae, Alouattinae). Int J Primatol 44, 517–539.
- Sobral, G., Fuzessy, L.F. & de Oliveira, C.A. 2023. The Challenge of Coexistence: Changes in Activity Budget and Ranging Behaviour of Brown Howler Monkeys in Response to the Presence of Conspecifics and Heterospecifics. Int J Primatol 44, 558–580.
Written by K. Clare Quinlan, October 2023