Geographic Distribution and Habitat
The red-handed howler monkey has a disjunct distribution ranging from eastern Amazonia to the northeastern Atlantic forests. Their habitats encompass South Amapá, Maranhão, Para, Tocantins, and Sergipe. They have also been spotted in the Amazon estuary, residing on islands such as Marajó, Mexiana, and Caviana. It is possible that their territory extends farther south than these locations; however, red-handed howlers are sometimes mistaken for Spix’s red-handed howler (Alouatta discolor), a howler monkey with remarkable similarities.
The howlers’ habitats within these locations can vary. They live in dry forests, rainforests, wooded savannas, gallery forests, and mangrove forests. They are found living high in the canopy as well as on the ground, although the latter is rare. They seem to be adaptable to changing environments, perhaps more so than other primate species. The red-handed howler can tolerate a reasonable elevation range. They have been recorded at sea level and up to 8,200 feet (2,500 m) above sea level.
The red-handed howler is in the haplorrhini suborder, which includes other monkeys, apes, and, yes, humans! Their family and subfamily are Atelinae, which includes all howler monkeys, spider monkeys, woolly monkeys, and the muriquis.
Size, Weight, and Lifespan
Howler monkeys are the largest Latin American monkeys. Their weight can range from about 10.5–17.5 pounds (4.7–8 kg), and their length can range from 15.75–36 inches (40–92 cm). Red-handed howlers exhibit sexual dimorphism, meaning that males are larger in weight and length when compared to females.
There have been few long-term studies in the wild of red-handed howler monkeys, which presents some issues when estimating their lifespan. For captive individuals, they can live up to 20 years. It has been estimated that wild red-handed howlers have a similar lifespan—perhaps 12–20 years.
Geographically fragmented distribution of animals of the same species at considerable distance.
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The red-handed howler’s appearance can vary slightly depending on their location. For the most part they can be found with three main variations: 1) all black pelage (coats); 2) black pelage with red fingers, toes, and tip of the tail; 3) black pelage with red hands, feet, and ⅓ of the tail.
They are stout in appearance and often exhibit a beard that blends into their medium-length fur. Their fur is thick and coarse; however, it is absent from the underside of the lower tail, which allows for a better grip when high in the canopy. Their tails range from 23–36 inches (58–91 cm) long and often act as a fifth limb when swinging between branches or anchoring them while they snooze in the canopy.
One of their most prominent and unique features is that of their enlarged hyoid bone. The hyoid is a cup-shaped bone that contains an air sac and is located below the jaw. Acting as a resonating chamber, this modified hyoid bone, along with a modified larynx, is responsible for the legendary roars, grunts, and barks the howlers are able to make. Howlers are especially dexterous, as their thumb and index finger are opposable to the remaining three fingers.
Howler monkeys are primarily folivorous (leaf-eaters), yet the red-handed howler has the highest rate of frugivory (fruit consumption) among the species. It has been observed that their preference for fruits or leaves changes, depending on the time of year. Among a colony in eastern Amazonia, it was found that 53.3% of the red-handed howlers’ diet consisted of fruit. During the transition from the wet to dry season, there was a notable shift in diet with 77.9% of their diet then consisting of leaves. During this transition, red-handed howlers tend to eat more mature leaves, as the leaves have secondary compounds needed for healthy digestion.
On some occasions, red-handed howlers practice geophagy, which is the ingestion of dirt or minerals. Geophagy occurs more often when their diet primarily consists of leaves. During this time, they often opt to eat dirt from termite mounds, which are rich in calcium, sodium, and organic carbon. The forest floor does not contain these vital supplements. It is still unclear if they eat this soil because it aids in the digestion of their leaf-heavy diet or, perhaps, as a mineral supplement. Many mature leaves contain tannins, which is a moderately toxic compound. Some researchers believe that eating soil helps in digesting this compound.
Behavior and Lifestyle
The red-handed howler is diurnal, which means their activities occur during the day. Though diurnal, they still spend up to 80% of their days napping in the canopy. They spend about 9 hours per day foraging as a group. They travel through the branches of trees in a quadrupedal fashion, rarely venturing down to the forest floor. Their prehensile tail can support their full weight, although it is not often that they use it in this manner.
As dusk approaches, the howlers congregate at a large tree that they use as their sleep site. They often select a species of Pau-d’arco, which is a larger tree. Many sleep sites are located in trees from the Lecythidaceae, Caesalpiniaceae, and Mimosaceae families. These trees can extend 59–131 feet (18–40 m) high in the canopy. They sit higher above the others and allow the monkeys a better vantage point to avoid predators. The howlers sleep in close proximity to other members of their group, which provides warmth and protection. They do not maintain the same sleep site for more than a few nights in a row. This is most likely to avoid predators and potential parasites.
Howler monkeys are the only Latin American monkeys in which both males and females both have full-color vision! In some Latin American monkey species, only females have this benefit. Most Latin American monkeys have dichromatic vision, seeing primarily blues and greens.
Red-handed howlers live in colonies ranging from 4–14 individuals. Different colonies are separated by a boundary that is maintained by howling matches with their opposing neighbors. Their barks and howls can carry for 2–3 miles (3.2–4.8 km). As well as maintaining their territorial boundary, the call at dawn, dusk, and during rainstorms. Their ranges encompass 32 to 45 acres (13–18 ha); however, there is further variation depending on the habitat they occupy. Within the colony, there is usually a configuration of one male—occasionally two—and two to five females, along with juveniles and infants. When they travel throughout their territory as a group they are generally led by the dominant male or an elderly male.
The howler monkey is the only Latin American monkey in which both males and females have full trichromatic vision, which means they can see reds, greens, and blues—just like humans! Evolutionarily, this helps with choosing the ripest fruit, or perhaps mature leaves. They also have a functional vomeronasal organ (for you cat fanciers, this is the same as the Jacob’s organ), which allows them to detect pheromones. This is key in their perception of their environment. Amongst themselves, red-handed howlers use a variety of calls to communicate with one another, including barking, grunting, and, of course, howling.
Calls can be generalized into two categories: intragroup and extragroup. Intragroup calls occur within the colony and are often associated with warnings and coordination. These calls can be heard in the morning when, as a group, the howlers act as a natural alarm clock for the Amazon. Extragroup calls occur when males are defending their territory or the females within the group. It has been hypothesized that the adaptation of their hyoid and larynx is a way in which the males conserve energy since physical altercations are more energy costing and potentially physically dangerous than howling at an opponent.
Female red-handed howlers reach sexual maturity at age four. Males reach sexual maturity at around age five, though they will not mate until they have achieved status within the group. Females are courted by dominant males through howls and competition with other males. The howling performed by males is deemed acceptable or unacceptable by females and also intimidates other males from neighboring colonies that may be showing interest in the females.
Research biologists have found that, in other howler monkey species, females often choose parasite-free males to mate with. This could indicate an aptitude for parasite avoidance for her young later on.
Red-handed howler monkeys are polygynous, meaning that one male has multiple female partners. Females enter estrus (the period of time during which they are fertile) every 13 to 24 days. They are pregnant for approximately 6 months and give birth to a single infant. On rare occasions, they will have twins.
Females invest heavily in their young through lactation, general care, and carrying them through the treetops. There is some evidence that the differences between maternal care may be cultural and vary from troop to troop, as some females have been observed carrying their infants against their own stomachs, while others carry the infants on their backs. As juveniles begin to reach maturity, both males and females will migrate away from their group to join other troops or to start their own troops in which they may hold more dominant positions.
The red-handed howler monkey’s role as seed dispersers throughout their territories is vital to their environment. They eat a large variety of fruit and help regenerate areas of the forest that have been impacted by logging and human disturbances. It has been found that howlers transport seeds an average of 380 feet (116 m) from their parent trees, and their feces contain, on average, 2.3 different seed species.
Their role in this ecosystem is incredibly important, as they often transport these seeds to pine and ash groves, which aids in the ecological enrichment of the area. Red-handed howler monkey troops will commonly engage in group defecation, which provides a concentrated opportunity for seeds to take root. Many animals contribute to the role of seed dispersers in the Amazon, and red-handed howlers are among the most important in their respective areas.
The International Union for Conservation of Nature lists red-handed howler monkeys as Vulnerable (IUCN, 2019), appearing on the IUCN Red List of Threatened Species. Their populations are decreasing.
In the past three generations, red-handed howler monkeys have lost around 30% of their overall population. The primary threats to these howlers are habitat loss, habitat fragmentation, and hunting. Additionally, the effects of climate change will continue to increase in prevalence.
These howlers inhabit many areas that are not protected by their respective countries, such as the state of Amapa in Brazil. Here, soy plantations have been encroaching on the monkeys’ habitat for years. In the Atlantic Forest, the red-handed howler is vulnerable to the ever-growing sugar cane industry. The issue of protected forest areas is ever-present. One study found that 88% of suitable areas in Brazil with adequate canopy cover are outside the protected zones. In regard to protected zones, it was found that only about 24% of it was considered to be adequate forest area for red-handed howler monkeys; however, 27% of the unprotected areas are considered high priority for furthering conservation efforts. If climate change continues as it is, it is estimated that, ultimately, the red-handed howler monkey may lose up to 94% of its habitats. The silver lining to this is the howlers diet. Because they are adaptable in their food consumption, they will more than likely be able to withstand the change in their environments. However, it will still come at a hefty cost and is estimated to greatly affect their population numbers.
The red-handed howler monkey is not listed in 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.
As mentioned in the previous section, one of the most pressing issues is the need for more protected land in these Amazonian regions. Land, water, and harvest management will be key to stabilizing and, ultimately, reversing the decline of these monkeys. More field research is needed on behalf of red-handed howler monkeys so that we may better understand and protect them. Research in population size, life history, threats, ecology, and distribution trends is vital to create a plan forward.
Fortunately, the red-handed howler is protected in international legislation; however, the farming interests for the area still present a powerful opposition in the protection of their habitats. Local support and education of the primates in the Amazon and Atlantic Forest have gained momentum. Many communities have advocated for decreased logging in certain areas, as well as promoted tourism to howler monkey areas—which can be is a powerful conservation tool!
- Diet of Alouatta belzebul discolor in an Amazonian Rain Forest of Northern Mato Grosso State, Brazil L´ıliam P. Pinto and Eleonore Z. F. Setz
- Instituto Chico Mendes de Conservação da Biodiversidade
Written by Robyn Scott, July 2023