Maranhão Red-handed Howler Monkey

The Maranhão Red-handed Howler Monkey is a medium-to-large New World primate endemic to the transitional forest zones of northeastern Brazil. Like all howler monkeys, it possesses an enormously enlarged hyoid bone that acts as a resonating chamber, enabling its powerful, far-carrying vocalizations. It inhabits a fragmented landscape at the ecotone between Amazonian forests, Cerrado, and Caatinga biomes, making it ecologically specialized and particularly vulnerable to habitat loss. It is considered one of the least studied and most threatened howler monkey species in Brazil.

Northeastern Brazil, primarily in the states of Maranhão and Piauí, with possible occurrence in adjacent areas of western Ceará and eastern Pará.

Endemic to northeastern Brazil; the core range spans the state of Maranhão and extends into adjacent Piauí, with some records suggesting presence in western Ceará and eastern margins of Pará. The species is bounded by the Parnaíba River to the east and the transition into denser Amazonian vegetation to the west.

Altitude and depth

Generally found at low to moderate altitudes, typically between 0 and 600 meters above sea level, consistent with the lowland and foothill forests of northeastern Brazil.

Seasonal movements

No major seasonal migration is documented. Dietary composition shifts seasonally—more fruit is consumed during the wet season when availability peaks, while leaf consumption increases during the dry season. Group home range use may shift accordingly.

Occupies semi-deciduous transitional forests, gallery forests along river corridors, and babaçu palm (Attalea speciosa) dominated woodlands. It can persist in degraded secondary forests and forest fragments, though population viability in such patches is limited.

Adaptations to its environment

The greatly enlarged hyoid bone and associated vocal sac allow extreme amplification of vocalizations for long-distance communication across fragmented landscapes. A specialized digestive system with an enlarged cecum allows efficient fermentation of tough, tannin-rich leaves. The prehensile tail provides a reliable fifth limb for secure arboreal locomotion and resting, reducing energy expenditure.

Endangered (EN) on the IUCN Red List. The species qualifies under criterion A due to a significant ongoing and projected population decline—estimated at over 50% over three generations—driven primarily by rapid deforestation and habitat fragmentation throughout its restricted range.

Estimated population

No precise global population estimate is available; the population is considered small, severely fragmented, and declining. Subpopulations are isolated in forest patches and many are likely too small to be viable in the long term.

The primary threat is large-scale deforestation for cattle ranching, soybean cultivation, and charcoal production in the MATOPIBA agricultural frontier. Hunting for bushmeat and the illegal pet trade exert additional pressure. Epizootics of yellow fever (Flavivirus) pose a catastrophic periodic threat, as seen across the Alouatta genus in Brazil. Road construction and infrastructure projects further fragment the remaining habitat.

Diseases and parasites

Yellow fever (sylvatic cycle) is the most significant documented disease threat, causing mass mortality events. Gastrointestinal helminths (nematodes, cestodes), protozoans (e.g., Trypanosoma, Plasmodium), and ectoparasites (ticks, botfly larvae) are commonly recorded in the genus. Captive and wild individuals are also susceptible to herpesvirus infections.

The species is listed under Brazilian federal law (MMA Ordinance No. 148/2022) as Endangered. Some populations occur within or near state protected areas in Maranhão, such as the Mirador State Park. The ICMBio (Chico Mendes Institute for Biodiversity Conservation) monitors threatened primates nationally. It is also covered under CITES Appendix II restrictions on international trade.

Recommended actions

Priority actions include establishing biological corridors connecting forest fragments, expanding the network of strictly protected areas within the species' range, enforcing anti-deforestation legislation in Maranhão and Piauí, conducting population surveys to define viable subpopulations, and developing monitoring protocols for yellow fever outbreaks. Community-based conservation programs with local landowners are also essential.

The body is predominantly black with contrasting reddish-orange to golden-red coloration on the hands and feet, which gives the species its common name. The face is bare and darkly pigmented. The tail is long, robust, and prehensile with a bare patch on the underside near the tip for gripping.

Size

Head-body length approximately 48–65 cm; prehensile tail adds an additional 55–75 cm. Males are notably larger than females.

Weight

Males typically weigh 6–9 kg; females weigh approximately 4.5–6 kg, reflecting significant sexual size dimorphism.

Distinctive features

Distinguished from the closely related Alouatta belzebul by subtle differences in pelage coloration, geographic range (more westerly and inland), and molecular genetic markers. The extent and intensity of the reddish-red coloration on extremities may differ slightly. Its restricted distribution in the Maranhão–Piauí ecotone is a key diagnostic criterion.

Differences between males and females

Pronounced sexual dimorphism exists: males are significantly larger and heavier than females. In some Alouatta species males are darker or differ in throat coloration; for A. ululata specifically, both sexes share the general black-and-red coloration pattern, but males possess a notably larger and more prominent hyoid apparatus.

Lives in cohesive social groups typically composed of 5–15 individuals, usually including one to three adult males, several adult females, and their offspring. Groups are philopatric and defend home ranges through vocal advertisement rather than overt physical aggression. Dominance hierarchies exist among both males and females.

When it is active

Strictly diurnal. Activity begins at dawn with characteristic group howling bouts. The species is notably sedentary, spending up to 70–80% of its daily time budget resting to conserve energy from its fibrous, low-calorie diet. Active foraging and movement occur primarily in the morning and late afternoon.

Diet

Primarily folivorous (leaf-eating), consuming young and mature leaves as dietary staples. Supplements diet with ripe fruits, flowers, and seeds depending on seasonal availability. The reliance on leaves, which are abundant even in degraded habitats, confers some resilience but limits energy intake.

Communication

The howl is the defining communication: deep, resonant roars produced primarily at dawn and dusk that can carry up to 3–5 km through forest. These vocalizations advertise territory, coordinate group movement, and signal group location to neighboring troops. Scent glands on the throat and chest are used for marking substrates. Body postures and facial expressions mediate close-range social interactions.

Interactions with other species

Natural predators include large raptors such as the Harpy Eagle (Harpia harpyja) and other large hawks, as well as felids such as ocelots and pumas. Competes for food resources with other sympatric frugivorous and folivorous primates. As a significant seed disperser for various forest trees, it has important mutualistic relationships with plant species. Botfly (Cuterebra sp.) larvae are common ectoparasites.

Role in its ecosystem

Functions as an important seed disperser for numerous tree species, particularly those producing large fruits. As a mid-to-high canopy browser, it also influences forest structure and leaf litter composition. Its position as a prey species contributes to supporting apex predator populations.

No strongly defined breeding season; births can occur year-round, with some evidence of a peak during the wet season. Gestation lasts approximately 186–194 days (roughly 6 months). A single infant is born per pregnancy; twins are extremely rare. Interbirth interval is approximately 16–24 months.

Raising the young

The mother is the primary caregiver, carrying the infant ventrally for the first weeks of life and then dorsally as it grows. Adult and sub-adult females within the group may show allomaternal interest. The infant begins sampling solid food at approximately 3–4 months and is fully weaned by 12–18 months. Males do not contribute direct parental care but provide group-level protection through vigilance and territorial defense.

Maturity and lifespan

Sexual maturity is reached at approximately 3–4 years in females and 4–5 years in males. Wild lifespan is estimated at 15–20 years; individuals in captivity may live up to 20–25 years under optimal conditions.

Classification

Mammalia > Primates > Atelidae > Alouatta > Alouatta ululata

Scientific synonyms

Alouatta belzebul ululata Elliot, 1912. The species was long treated as a subspecies of the Red-handed Howler Monkey (Alouatta belzebul) before being elevated to full species status.

Recent studies

Recent phylogeographic analyses of the Alouatta belzebul complex have reinforced the distinctiveness of A. ululata based on genetic divergence and geographic isolation. Studies of habitat suitability modeling in northeastern Brazil indicate that projected deforestation under current land-use scenarios will drastically reduce available habitat for this species within the next two decades. Yellow fever monitoring programs following the 2016–2019 Brazilian epizootic have included surveys of this species.

Genome and DNA

Chromosome number for the closely related Alouatta belzebul (from which A. ululata was separated) is reported as 2n = 52. Species-specific karyological data for A. ululata remain poorly documented. Molecular phylogenetic studies using mitochondrial and nuclear markers have supported its recognition as a distinct species.

Compared with similar species

Most closely related to Alouatta belzebul (Red-handed Howler of Pará and Maranhão), from which it was historically lumped. Both share the distinctive reddish extremities contrasting with a black body. A. ululata is geographically more westerly and associated with drier transitional forests, whereas A. belzebul is more closely tied to humid Amazonian forest margins. Both differ from the fully red A. seniculus and the sexually dichromatic A. caraya (males black, females buff).

The howler monkey's morning chorus is considered the loudest call produced by any land mammal—audible up to 5 km away. Despite its impressive vocal power, A. ululata is one of the laziest primates on Earth, spending roughly 80% of its waking hours motionless to conserve the limited calories extracted from its leaf-heavy diet. The enlarged hyoid bone in males is so large that it is visible externally as a pronounced bulge in the throat, essentially functioning as a built-in megaphone.

Where and how to see it

Best observed in the early morning hours (just after sunrise) when groups conduct their characteristic howling bouts from high in the forest canopy. The Mirador State Park in Maranhão and forested gallery zones along tributaries of the Parnaíba River offer potential observation sites. A local naturalist or research team guide is strongly recommended given the fragmented and difficult-to-access terrain.

Risks to humans

No notable direct risk to humans. Like all primates, individuals may bite if handled or cornered. Wild populations could theoretically serve as sylvatic reservoir hosts for yellow fever, posing an indirect public health consideration in outbreak zones.

Human uses

Historically subject to subsistence hunting by rural and indigenous communities in Maranhão and Piauí for bushmeat. Occasionally captured for the illegal pet trade, though this appears less prevalent than for other primate species. No documented use in traditional medicine specific to this species. It is legally protected under Brazilian federal legislation and cannot be legally traded.

Myths and legends

Howler monkeys in Amazonian and northeastern Brazilian indigenous traditions are sometimes associated with forest spirits or regarded as omens due to their eerie, far-reaching calls at dawn. Local communities in Maranhão occasionally refer to their howls as the 'voice of the forest.' Specific documented mythology for A. ululata is sparse due to limited ethnographic study of this species.