NORTHERN BROWN HOWLER
Alouatta guariba guariba
Geographic Distribution and Habitat
Although there has been some posturing and jostling among researchers as to whether this monkey is a distinct species or a subspecies, the International Union for the Conservation of Nature (IUCN) recognizes the northern brown howler monkey (Alouatta guariba guariba) as one of two subspecies of the brown howler monkey (Alouatta guariba); the southern brown howler monkey (Alouatta guariba clamitans) is the second subspecies. There is no disagreement among researchers concerning this primate’s uncertain future, however. The northern brown howler monkey has the dubious distinction of being included in Primates in Peril: The World’s 25 Most Endangered Primates, 2016–2018 and again in the 2018-2020 report. Population may be as few as 250 monkeys with only 50 mature individuals sparsely distributed across diminishing forest fragments.
Native to Brazil, northern brown howler monkeys are restricted to an area north of the Jequitinhonha River in the Jequitinhonha Valley. Small populations follow the flow of the river along the country’s central east Atlantic coast, occupying widely separated submontane, montane, and lowland forests in the states of Bahia, Espirito Santo, and Minas Gerais. (The species had previously been reported in Rio de Janeiro.) Northern brown howler monkeys are separated from their “sister subspecies,” the southern brown howler monkey, by the Jequitinhonha River basin.
One of Brazil’s poorest regions, the hot and humid Jequitinhonha Valley is prone to endemic yellow fever. Both human primates and nonhuman primates, including the northern brown howler monkey, have succumbed to this mosquito-borne viral disease.
Size, Weight, and Lifespan
Howlers are big monkeys, and they are among the largest of New World monkeys. Males are heavier than females, weighing about 14.8 lb (6.7 kg). Females weigh about 9.5 lb (4.3 kg).
Head-to-body length for northern brown howler monkeys ranges from 1.6 to 1.8 ft (49-55.8 cm) in males and from 1.5 to 1.6 ft (45-49 cm) in females.
A ridiculously long prehensile tail adds another 1.86 to 1.97 ft (56.5-60 cm) to males and another 1.67 to 1.88 ft (51-57 cm) to females.
The lifespan for northern brown howler monkeys is not documented. However, the lifespan for brown howler monkeys (the “parent species” of the northern brown howler) is 15 to 20 years in the wild.
Both male and female northern brown howler monkeys are cloaked in coarse auburn-colored fur coats overtaken with swaths of black from their shoulders to the arms. Golden hues accent a thick fur cap and the upper body. If Nature operated a hair salon in Brazil’s Atlantic Forest, you might think that these monkeys had their “tips” highlighted. Their short muzzle is black and hairless, and their jaws are large. Beneath the chin extends a longish beard that you can almost imagine this monkey stroking thoughtfully. Their long tail is auburn with black flecks, except on the underside, which is naked. Their legs are stout compared to other New World monkeys. Their first two fingers of each hand are set apart and are opposable to the other three. Copper-colored eyes assess the world.
Alexander von Humboldt, the famed German naturalist and explorer, offered the following description of his 1812 encounter with northern brown howler monkeys. “[T]heir eyes, voice, and gait are indicative of melancholy,” he said.
The late American explorer, writer, and diplomat John Lloyd Stephens described northern brown howler monkeys “grave and solemn as if officiating as the guardians of consecrated ground.”
Northern brown howler monkeys are “folivore-frugivores.” They eat a lot of leaves (making them “folivores”) along with a lot of fruits (making them “frugivores”). The monkeys prefer to eat younger leaves, but mature leaves are often on their menu. Fruits, particularly mature fruits, increase in their diet according to seasonal availability. Flowers, plant buds, and nuts complement their meal plan.
Behavior and Lifestyle
Behavior and lifestyle habits of the northern brown howler monkey are scantly documented and are therefore largely conjectured, based on its parent species, the brown howler monkey.
Brown howler monkeys are arboreal creatures, meaning they spend virtually all their time in trees. They languidly move through the upper canopy of the forest on all fours (or “quadrupedally,” to use the scientific term), rather than swinging from branch to branch (an activity scientifically known as “brachiation”). Their prehensile tails are strong enough to support their entire body weight while they hang suspended from a branch, although they seldom do so. More typically, they wrap their long tail around a branch for stability while they use their hands to pluck leaves and fruits from trees.
Sensory hairs inside their nostrils enable them to sniff out nourishment up to 3 miles. Unlike other New World monkeys, brown howler monkeys are equipped with trichromatic color vision (they see in color), which helps them discern the best leaves to eat. Their molars are adapted to allow them to devour tougher leaves.
Howler monkeys often travel to find seasonably available fruit. More time is spent foraging during fall and winter, when the quality and quantity of available food decreases.
No one would accuse brown howler monkeys of keeping a frenetic pace. They are all about conserving energy. Diurnal creatures (active during daylight hours), these monkeys spend about 64 percent of each day resting. Peak rest time occurs during the middle of the day when temperatures soar. The remainder of the day consists of travel (13 percent), feeding (18.5 percent), and allogrooming (2 percent). Allogrooming is the practice of social grooming between members of the same species. It serves to develop and reinforce social structures, to strengthen family links, and to build companionships. With brown howler monkeys, males are the main recipients of allogrooming by the females.
Brown howler monkeys form multi-male, multi-female groups; single-male, multi-female groups; and single-male, single-female groups. The most common group composition is a single-male, multi-female group of up to 10 individuals. A group’s alpha male gets to breed with all the reproductive females.
Group size for northern brown howler monkeys has been documented as 3 to 4 individuals.
Indicative of its name, howler monkeys HOWL—loudly! They are one of Nature’s loudest animals. Their impressive calls emit from deep jaws, surrounded by an enlarged larynx and a specialized hyoid bone (also known as the lingual bone or tongue bone)—unique to howler species—set within the thyroid cartilage. The effect is a resonating chamber that transmits howler monkeys’ calls up to 3 miles (4.8 km).
Their call repertoire includes a range of howls, grunts, whoops, barks, and roars. Howler monkeys use high-amplitude calls to alert one another of predator danger, to claim resource rights, and for mating purposes. Loud calls are sounded to signal group strength; by listening to the calls of rival groups, the howlers can assess the strength of their potential opponents to reduce occurrences of direct physical confrontation.
Brown howler monkeys rub themselves to convey various messages. For example, in what is considered a combative display, an adult male rubs his breastbone to assert his dominance and to claim territory.
An adult female rubs her anus and genital area—spreading strong olfactory signals through her feces, urine, and vaginal secretions—to announce her reproductive status and to get male attention.
Female brown howler monkeys reach sexual (reproductive) maturity at about 3-1/2 years old; males lag a bit behind, reaching sexual (reproductive) maturity at about 5 years old.
It’s the female who solicits the male. Besides rubbing her anus and genital area, she performs rhythmic tongue movements while advancing toward the male object of her sexual desire. The male is quick to oblige, and the act of copulation lasts all of one minute. Genital inspection might occur.
Sometimes, female brown howler monkeys initiate extra-group copulations: orgies. They are more likely to engage in this behavior in multi-male groups.
After a gestation period of 6 months, a female gives birth to a single infant. She nurses her baby for 12 months, after which time the infant is considered weaned.
Researchers speculate that the monkeys’ folivorous diet may account for their year-round breeding: mature leaves that are available throughout the year provide a relatively stable source of energy.
Northern brown howler monkeys are habitat ambassadors. Thanks to their frugivorous diet, they disperse seeds of the many fruits they eat, through their feces, to help regenerate numerous plant species.
The northern brown howler monkey is listed as Critically Endangered by the International Union for Conservation of Nature (IUCN, 2016), appearing on the IUCN Red List of Threatened Species.
As of 2015, the remaining population size is unknown, but it is suspected to number not more than 250 individuals, with no more than 50 mature individuals, sparsely distributed in diminished forest fragments.
Locals have long-hunted these docile and slow-moving monkeys for their flesh, resulting in a devastating toll on populations. The monkeys are thought to be nearly wiped out from the cacao-growing region of southern Bahia.
Habitat loss has further fragmented northern brown howler monkey populations. Lush forest has been lost to logging, agriculture, and cattle grazing . . . nudging the species toward extinction.
Yellow fever poses a serious threat to all howler populations residing in Brazil’s Atlantic forests. Because of their already dramatically reduced populations, northern brown howler monkeys are especially vulnerable to this potentially deadly disease. After a yellow fever outbreak in February 2017, researchers discovered five dead northern brown howler monkeys in the Brazilian state of Minas Gerais, not far from the Mata Escura Biological Reserve, the only strictly protected federal area where northern brown howler monkeys are known to reside.
Whether oversight, slight, or deliberate omission due to its taxonomic ranking as a subspecies, the Critically Endangered northern brown howler monkey is not currently listed as protected by 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. However, its parent species, the brown howler monkey, classified as “Least Concern” by the IUCN, is listed in Appendix II of the CITES document.
Despite its omission from CITES, the northern brown howler monkey is included in a conservation action plan, established in 2010, as one of Brazil’s 27 threatened mammals of the country’s Atlantic Forest. Conservation groups participating in this action plan include the Instituto de Estudos Socioambientais do Sul da Bahia (IESB), Instituto Uiraçu, the State University of Santa Cruz (UESC), and the National Center for Research and Conservation of Brazilian Primates (ICMBio/CPB), with the support of Conservation International (through the Margot Marsh Biodiversity Foundation’s Primate Action Fund), the Rainforest Trust, and the Mohamed Bin Zayed Species Conservation Fund. Since 2012, researchers from the aforementioned groups have conducted field surveys to locate and count surviving populations, better understand the threats against the species, and clarify the monkeys’ geographic distribution.
Since 1980, several protected areas have been created within the northern brown howler’s range in Bahia and northeastern Minas Gerais. However, the only strictly protected area under federal governance where the species has been confirmed is Mata Escura Biological Reserve, just north of the middle Jequitinhonha River. The northern brown howler monkey coexists here with the Critically Endangered northern muriqui (Brachyteles hypoxanthus), a woolly spider monkey; and the Endangered golden-bellied capuchin (Sapajus xanthosternos). But nearby rural settlements of “quilombolas” (descendants of Afro-Brazilian refugee slaves) continue to encroach upon the reserve. Logging, hunting, and other practices detrimental to the northern brown howler population occur just outside the boundaries of the reserve.
The 2017 IUCN report cites an occurrence of two confiscated pets (a male and a female) who, in February 2014, were successfully released into the Serra Bonita Private Reserve, Camacan, Bahia, a location where the species has not been seen nor heard for more than 50 years.
A conservation initiative in southernmost Bahia in Pau-Brasil National Park and in surrounding private reserves holds some promise for the species. Led by a team of researchers from Federal University of São Paulo, Diadema, along with the park’s administrator and local stakeholders of the corporate sector, a major goal of the initiative is to establish the northern brown howler monkey as a flagship species for the region. Cultivating public interest for this Critically Endangered primate and creating a donor base to raise funds for the species’s preservation are key facets to the initiative’s success.
The advent and deadly impact of yellow fever must be factored into conservation plans. Some researchers have called for a “metapopulation management plan” that includes relocating individuals and smaller groups to larger groups—in the hope of reinforcing and strengthening remnant populations.\
Written by Kathleen Downey, June 2018, Conservation status updated July 2020.