RED-HANDED HOWLER

Alouatta belzebul

Geographic Distribution and Habitat
Red-handed howler monkeys are found in Amazonian Brazil and surrounding regions. The northeast states of Brazil consist of Maranhão, south Amapa, Para, Sergipe, and Tocantins and the fragmented Atlantic forest region consists of the states of Pernambuco, Rio Grande do Norte, Piauí, Alagoas, and Paraiba. The range of this species may extend into the lower Amazon as well, but it is poorly understood and often confused with another Brazilian red-handed howler monkey species, Spix’s red-handed howler (Alouatta discolor). Red-handed howler monkeys have also been recorded east of the Xingu and Iriri rivers and in the Rio Pracupy, despite the fact that the Xingu River has been considered the western boundary of their geographic distribution.

In northern Tocantins, red-handed howler monkeys are sympatric with black-and-gold howlers (Alouatta caraya), and there is evidence of a mixed group—a black-and-gold howler (A. caraya) female with a red-handed howler (A. belzebul) male.

Red-handed howler monkeys are found in mixed forest habitats including lowland Amazon rainforest, Marajo varzea forest, and those fragments of the Atlantic forest. This species can be found in both the canopy and the ground, in both undisturbed or modified forests, mangrove forests, wooded savannas, and gallery forests. They are not as susceptible to environmental changes and disturbances as other primate species.

Red-handed howler range, IUCN 2018

Size, Weight, and Lifespan
Howler monkeys in general are the largest New World monkeys; the red-handed howler monkey weighs in at a range of 10.69–17.63 lbs (4.85–8 kg). They are sexually dimorphic in size, with males weighing anywhere from 14.33 to 17.63 lbs (6.5–8 kg) and females weighing between 10.69 and 12.6 lbs (4.85–6.2 kg). They are closer in length, with males ranging from 22.2 to 24.8 in (56.5–63 cm) and females ranging from 15.7 to 25.5 in (40–65 cm). There are, however, some disagreements about the length measurements, with other sources listing the range from 22 to 36 in (55.9–91.5 cm).

There is little information available about the lifespan of red-handed howler monkeys in the wild due to lack of long-term observations in the field, but it can be reasonably inferred that the lifespan is similar to that of other Alouatta species, which have an expected lifespan of 15 to 20 years in the wild.

Appearance
As their name suggests, red-handed howler monkeys have hands (and feet) that are not the same inky-black, reddish, or yellowish pelage that covers the rest of their body. Any guesses what color they might be?

They have a thick, prehensile tail with a naked patch of skin at the tip, shaggy, coarse hair, and a deep jaw that surrounds an enlarged larynx and hyoid. The red-handed howler monkey’s name also gives this next characteristic feature away: the enlarged larynx and hyoid act as a highly specialized voice box that helps them produce their infamous howls, grunts, roars, and barks.

All Alouatta species have a zygodactylous or schizodactylous grip, meaning their first two digits are opposable to the other three. Alouatta species are also the only genus of New World monkey that have fully trichromatic vision, which allows them to perceive the world with the full visible light spectrum. In an evolutionary sense, this advantage gives red-handed howler monkeys the opportunity to snag the best leaves and the ripest fruit. While it was previously believed that having fully trichromatic vision came with a loss of a fully functional vomeronasal organ (VNO), it appears that this may not be the case with red-handed howler monkeys and that they are able to perceive their environment with both color vision and pheromone detection.

Photo credit: Frederico Acaz Sonntag/Creative Commons

Diet
Howler monkeys are the only New World primates that include mature leaves as part of their regular diet, although they do opt for less fibrous, young leaves when they are available. Their folivorous diet is undoubtedly part of the reason why they have such a wide geographic distribution.

Typically, red-handed howler monkeys are dietary generalists (one study observed them feeding on 67 plant species over a 45-day period) and they commonly consume plants from the family Leguminosae (pea, legume, or bean family) and Moraceae (mulberry or fig family). They also feed on fruit during rainy seasons, which makes them the most frugivorous howler monkeys.

When the consumption of mature leaves is unavoidable during dry seasons, red-handed howler monkeys also exhibit geophagy, or the ingestion of soil. The soil they consume is usually taken from arboreal termitaria rather than soil from the forest floor, because the latter has fewer nutrients such as calcium, sodium, and organic carbon. It is not known whether or not this behavior is a response to a lack of nutrients during less plentiful times or whether the soil aids in the digestion of mature leaves.

Behavior and Lifestyle
Howler monkeys are diurnal, though they spend most of their day (up to 80%) sleeping in the branches with their troop members. They are also arboreal and move quadrupedally through the trees in search of food. While their prehensile tails are strong enough to support their entire body weight, they rarely use them in that way and prefer to keep hold with at least two appendages when moving.

As aforementioned, red-handed howler monkeys (like all howler monkeys) are well-known for their behavioral characteristic of vocalizing extremely loudly during howling sessions.

What Does It Mean?

Alarm call:
Specific calls that individuals in a troop make to warn other members of their group of imminent danger—such as predators.

Arboreal:
Physically adapted to living primarily or exclusively in trees.

Deforestation:
The permanent cutting, clearing, and removal of trees to convert forest land for other use, such as pasture, cropland, or plantations.

Diurnal:
Active during daylight hours.

Estrus:
A recurring period of sexual receptivity and fertility in many female mammals.

Folivorous:
Having a diet that consists of leaves.

Frugivorous:
Having a diet that consists of fruits.

Harem:
A group of females guarded by a male, who prevents other males from mating with them; an animal group consisting of one or two males, a number of females, and their offspring. The dominant male drives off other males and maintains the unity of the group. If present, a second male is subservient to the dominant male.

Hyoid:
A U-shaped bone in the neck which supports the tongue.

New World monkey:
Monkeys native to Central and South America.

Polygynous:
A mating system in which one male mates and lives with multiple females.

Prehensile tail: 
The tail of an animal that has adapted to be able to grasp or hold objects.

Quadrupedal:
Using four limbs to move about. This word comes from the Latin meaning “four feet.”

Sexually dimorphic:
When males and females have different characteristics (size, color, etc.) other than their reproductive organs.​

Termitaria:
A nest built by a colony of termites underground, above ground (usually as a mound), or in a tree.

​Tri-chromatic vision:
The ability to see in color thanks to three color receptors (rods and cones) at the back of the eye in the retina. Humans and most Old World monkeys have trichromatic vision—i.e., they distinguish blues, greens, and reds.

Vomeronasal organ (VNO):
Or Jacobson’s organ, is the paired auxiliary olfactory (smell) sense organ located in the soft tissue of the nasal septum, in the nasal cavity just above the roof of the mouth (the hard palate).

Visit the Glossary for more definitions

Daily Life and Group Dynamics 
Red-handed howler monkeys are usually found living in small social groups ranging from 5 to 14 individuals. There is usually only one dominant male in the group (occasionally two) and a harem of 2 to 5 females, with the remainder consisting of juveniles and/or subadults.

Daily life consists of foraging and eating leaves, then sitting around and digesting the leaves that were just eaten. They are able to ferment leaves in their enlarged caecum, a pouch that is located at the beginning of the large intestine.

Communication
Red-handed howler monkeys engage in howling sessions in the wee hours of the morning that are audible at distances of 0.6–1.2 mi (1–2 km) and usually involve the entire group. These howler monkeys have a wider range of frequencies (300–2,000 Hz) and a more sustained call than those of the mantled howler (Alouatta palliata)

Shorter distance calls are associated with group coordination and alerting a social group of danger. Long distance calls are used for mate attraction and resource defense. It is proposed that loud howling is much less costly, energy-wise, than engaging in physical interaction with potential rivals for resources or mates.

Fun Facts

The Alouatta genus is one of several within the family Atelidae, which also includes wooly monkeys, spider monkeys, and woolly spider monkeys.

Red-handed howler monkeys have large salivary glands that help to break down the leaves they eat.

Reproduction and Family
Red-handed howler monkeys are polygynous, year-round breeders (some species of Alouatta have two seasonal birth peaks and thus two breeding peaks). Females only give birth to one offspring at a time—rarely twins—and have a gestation period of 187 days (just over 6 months). Females are sexually mature at 4 years of age, with estrus cycles between 13 and 24 days, and a birth interval of 1 to 2 years.

Unfortunately there is very little information available about reproduction that is specific to red-handed howler monkeys, like birth weight, the age at which weaning occurs, and ages of sexual maturity in males and females. A similar monkey, the mantled howler monkey (Alouatta palliata), weighs 9.7–14.1 oz (275–400 g) at birth and infants are weaned at 10 months. Males are sexually mature within 5 years, but they do not reproduce until they have achieved a dominant role in the troop. Males will compete for a mate, but they often choose to do so with a howl rather than a fist, which allows them to assess their opponent without engaging in a physical fight.

The parental investment of red-handed howler monkeys is also largely unknown, but in general, howler monkey males and females leave their parents’ group after they gain independence to form new troops and establish themselves—hopefully at a higher rank—in a new social hierarchy. Females are strongly invested in the gestation, lactation, and care of their young, whereas infanticide by males has been observed in other Alouatta species when a dominant male takes over a new group. One study suggests that birthing females learn from other members of the group as they age.

Photo Credit: Ric/Flickr/ShareAlike License

Ecological Role
Howler monkeys are important seed dispersers and Alouatta species specifically will commonly disperse large seeds throughout the forest and in restricted areas like canyons through the practice of group defecation, where undigested or partially undigested seeds may become buried in the soil or secondarily dispersed by dung beetles. This behavior has aided in forest regeneration after fragmentation or destruction by human activity.

Conservation Status and Threats
The red-handed howler monkey is listed as Vulnerable (VU) by the International Union for Conservation of Nature (IUCN, 2019). The population trend for this species is decreasing with a grim outlook—there has been a 30% population decline in the past 36 years mostly due to hunting, habitat destruction, and logging. Additionally, more than 95% of red-handed howler monkey populations live in Brazilian Amazonia (with the remainder residing in the Atlantic Forest region). Amazonian forests are projected to be reduced by as much as 40% by the year 2050 due to illegal deforestation, hydroelectric projects, road-building, and cattle-ranching. Populations are also reduced due to hunting activity. The small populations that reside in the Atlantic Forest region are also dealing with habitat loss and fragmentation due to the expansion of sugar cane plantations.

Red-handed howler monkeys are hunted and captured for human export and used as test subjects for extensive genetic and medical research projects. According to one study, red-handed howler monkeys are significant to the diet, religion, and social structure of the Guaja people. The red-handed howler monkeys are primarily hunted and eaten during the wet season, but the Guaja also believe all monkeys are kin, and they always take in infants of mothers that were killed for food and treat them as their own children. The religion of the Guaja people portrays this symbolic cannibalism as a religious way of life.

They are also a prey species for eagles, hawks, tayras, and jaguars.

Conservation Efforts
Red-handed howler monkeys are dietary generalists and opportunistic foragers, so they can adapt more easily to habitat changes, but their small home ranges are growing more affected by habitat fragmentation. It is still very important to provide a suitable habitat for this species and many local Brazilian communities have taken steps to do just that—they are managing logging more effectively and focusing on ecotourism.

Red-handed howler monkeys are also one of five species included in the Brazilian government conservation action planning system: Plano de Acao Nacional para a Conservacao dos Primates do Nordeste. It is also listed on Appendix II of the Convention on International Trade in Endangered Species (CITES) and listed as Endangered on the Brazilian Red List. 

This species also resides in a variety of protected areas, like Caxiuana National Forest, Tapirape Biological Reserve, Carajas National Forest, Tapirape-Aquiri National Forest, Igarape Gelado Protected Area, Gurupi Biological Reserve, e Quilombo do Frexal Extractive Reserve, and Araguaia National Park.

The IUCN also states that more research is needed for population size, distribution, and trends, life history and ecology, and threats. They recommend the continued monitoring of population trends.

References:

  • https://animaldiversity.org/accounts/Alouatta_belzebul/
  • https://www.iucnredlist.org/species/39957/17925370#habitat-ecology
  • https://www.britannica.com/animal/howler-monkey#ref1120946
  • https://link.springer.com/article/10.1023/A:1019928529879
  • http://www.theprimata.com/alouatta_belzebul/
  • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3036160/

Written by Rachel Heim, February 2020