Black Howler Monkey

The black howler, also called the black-and-gold howler, is among the largest New World monkeys. It lives in Paraguay, southern Brazil, eastern Bolivia and northern Argentina, in forests of many kinds, and is mostly arboreal with a prehensile tail that has a bare patch on the underside. Adult males turn from golden to entirely black, while females and infants stay golden. Its diet is mainly leaves, supplemented by fruit, figs, bark, seeds and flowers. Howlers rest or sleep for up to 60 to 70 percent of the day, and groups were measured at about 6 to 18 individuals in one Brazilian study.

About the black howler monkey

The black howler, also called the black-and-gold howler, is among the largest New World monkeys and belongs to the genus Alouatta, which includes nine recognized howler species in the family Atelidae. What's most noticeable about it at a glance is its sexual dimorphism: adult males are entirely black, while adult females and infants of both sexes are an overall golden color. That contrast is built into the common name, and it's pronounced enough that a male and female can look like they belong to different species until you know what you're looking at.

A male's coat starts out golden, like a female's, and darkens to full black as he matures. Males are also considerably heavier than females — 47% heavier, weighing 6 to 7.7 kg — and carry a larger hyoid bone, the structure in the throat believed to act as a resonating chamber for the species' signature howl. Like other members of its genus, the black howler has a prehensile tail with a bare patch of skin on the underside that improves grip, making it a confident climber that can move through the forest canopy without needing to touch the ground.

In the wild, black howlers range across Paraguay, southern Brazil, eastern Bolivia, and northern Argentina, with occasional records in Uruguay. They're adaptable by primate standards, turning up in tropical and subtropical evergreen forest, seasonally dry deciduous forest, semideciduous forest, gallery forest along rivers, montane forest, woodland, and flooded forest, and they can persist even in disturbed habitat and forest remnants surrounded by farmland or people.

Behavior and diet

The howl that gives the species its name is one of the loudest vocalizations of any primate, audible up to two kilometers away, and in studies from Brazil it's heard most often around dawn. It functions as a long-distance signal, letting groups advertise territory and keep space between each other without direct contact. Beyond the noise, black howlers are notably sedentary: they're diurnal, mostly arboreal, rarely come down to the ground, and spend 60 to 70% of the day simply resting or sleeping — one of the least active primate groups on record.

Diet is largely folivorous, meaning leaves make up most of what a black howler eats, supplemented by fruit, figs, mosses, bark, seeds, and flowers. Leaves are a demanding food source — low in energy and hard to break down — but the species has a specialized digestive system built to process them quickly, along with strong molar teeth for chewing tough, mature foliage. That diet shifts with the seasons: in winter, when fruit is scarce, black howlers eat mostly leaves and become less active, while in summer, with more fruit available, they're more active across feeding, socializing, moving, and mating. Their gut microbiome even changes seasonally to match.

Groups are typically built around a single dominant male with several adult females and their young, though larger groups with multiple adult males have also been documented — one study along Brazil's upper Paraná River found groups of 6 to 18 individuals, all multi-male, with females outnumbered. Mating is notably promiscuous: females mate with multiple adult males during both fertile and nonfertile periods, actively soliciting partners through grooming and specific postures, and gestation runs about 180 to 194 days. Researchers link this promiscuity to a low rate of infanticide in the species and to genetic diversity within groups.

Range and habitat

The black howler is native to Paraguay, southern Brazil, eastern Bolivia, and northern Argentina, with occasional sightings in Uruguay. Within that range it uses a wide range of forest types — evergreen, seasonally dry deciduous, semideciduous, gallery, montane, woodland, and flooded forest — reflecting a species that can adjust to different tropical and subtropical conditions rather than requiring one specific habitat.

That adaptability extends to landscapes altered by people: black howlers turn up in disturbed ecosystems and forest remnants within agricultural regions, and even in urban settings, where a 2020 study in Pilar, Paraguay, found some local residents valued the monkeys' presence for tourism. That same tolerance of human-altered land, though, brings the species into more frequent contact with power lines and dogs.

Conservation status

The IUCN Red List classifies the black howler as Near Threatened, based on a 2021 assessment, reflecting a recent population decline tied to several human-caused pressures: habitat degradation from residential, commercial, and agricultural development; hunting; the illegal pet trade; and vulnerability to disease outbreaks. Sylvatic yellow fever outbreaks in Brazil, in particular, have caused significant population losses in the species.

In the Paraguayan city of Pilar, a 2020 study found that most residents knew hunting black howlers was illegal, but many did not realize keeping them as pets was also prohibited; about 8% believed the monkeys were hunted for meat. Researchers there also documented direct threats from urban life, including domestic dogs attacking howlers, monkeys being killed by people, and electrocution on uninsulated power lines — three such deaths were recorded during the study period, since howlers often use power lines to travel between trees. Formal conservation efforts in Pilar were described as minimal, and continued urbanization is expected to push howlers onto power lines and the ground more often as tree cover shrinks.