BOLIVIAN RED HOWLER MONKEY
Alouatta sara
Geographic Distribution and Habitat
The Bolivian red howler monkey is found in lowland tropical forest including riverine forest and seasonally flooded Amazonian forests, Andean foothill forest, and the gallery forests of the Beni wetland. Populations are also found in igapó forest (seasonally inundated forest) along the Río Iteñez and intermittently and usually confined to riverine forests. However, in the Sub-Andean Tropical Forests of western Bolivia, they are more commonly encountered in lowland tropical forests. Bolivian red howlers are the southernmost of red howler monkey species.
The Bolivian red howler monkey is found in lowland tropical forest including riverine forest and seasonally flooded Amazonian forests, Andean foothill forest, and the gallery forests of the Beni wetland. Populations are also found in igapó forest (seasonally inundated forest) along the Río Iteñez and intermittently and usually confined to riverine forests. However, in the Sub-Andean Tropical Forests of western Bolivia, they are more commonly encountered in lowland tropical forests. Bolivian red howlers are the southernmost of red howler monkey species.
Bolivian red howler range, IUCN 2008
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Size, Weight, and Lifespan
Howler monkeys are the heaviest of the New World primates. Male red howler monkeys are about 21 in (55 cm) tall and weigh about 15 lbs (6.8 kg), while female red howlers are on average 19 in (48 cm) tall and weigh about 12 lbs (5.5 kg). They can live to be over 20 years old. Appearance
Bolivian red howler monkeys are named for their brick-red and orange-red coat, which ranges from golden brown on their back to maroon on their forearms and chest. The hair covers their entire body except for their face and the underside of the end of their tail, which helps them grip on to branches. On average, a red howler’s tail is about equal in length to the rest of their body. Their tail is fully prehensile and can act as a fifth limb when traveling through branches. The howler monkey’s tail is strong enough to hold a fully grown adult by itself, although only babies typically practice this behavior. Howler monkeys have a large lower jaw that allows room for a bigger hyoid bone, which is key to making their signature calls. 
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What Does It Mean?Commensal:
Living in close association in a way that allows one species to benefit without harming the other. Diurnal: Active during daylight hours. Folivore: An animal who primarily eats leaves. Neotropic: Relating to, or constituting the tropical New World biogeographic region that extends south, east, and west from the central plateau of Mexico. New World monkey: New World monkeys are native to Central and South America, whereas Old World monkeys are native to Africa and Asia. Prehensile tail: The tail of an animal that has adapted to be able to grasp or hold objects. |
Diet
Of all the New World monkeys, howler monkeys are the only folivores, meaning they specialize in eating leaves. They prefer young leaves over mature leaves, as young leaves are easier to digest, contain more nutrients, and are less toxic. Because leaves are less nutritious than other food sources, howler monkeys are relatively slow moving when compared to other monkeys in the region.
That said, Bolivian red howler monkeys also eat a variety of fruits, nuts, and flowers. If the rainy season is strong, some troops may even switch to a majority-fruit diet for a few months. They also consume small amounts of soil every few days to combat the toxins from plants.
Of all the New World monkeys, howler monkeys are the only folivores, meaning they specialize in eating leaves. They prefer young leaves over mature leaves, as young leaves are easier to digest, contain more nutrients, and are less toxic. Because leaves are less nutritious than other food sources, howler monkeys are relatively slow moving when compared to other monkeys in the region.
That said, Bolivian red howler monkeys also eat a variety of fruits, nuts, and flowers. If the rainy season is strong, some troops may even switch to a majority-fruit diet for a few months. They also consume small amounts of soil every few days to combat the toxins from plants.
Behavior and Lifestyle
Red howlers are diurnal and spend most of their lives in the trees, although they are strong swimmers when necessary. They spend most of their day sleeping or napping and about a quarter of the day foraging.
Red howlers are diurnal and spend most of their lives in the trees, although they are strong swimmers when necessary. They spend most of their day sleeping or napping and about a quarter of the day foraging.
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Daily Life and Group Dynamics
Bolivian red howler monkeys live in troops of 3 to 10 individuals. The troop is ruled by an alpha male who is usually significantly larger than other group members. There may be a few subordinate males, but the alpha male holds exclusive mating rights over the troop’s females. The alpha male must regularly defend his position to keep his status in the troop. Occasionally, a troop forms where all the males are equal in status. When no male is strong enough to establish dominance, all of the males in the group have the opportunity to mate. While they may be weak individually, these males can band together to prevent a single dominant intruder from taking over the troop. The vast majority of red howlers leave the group they are born into once they reach maturity. From there, males tend to form a small group with one or two other males and bully their way into a new group. Females, on the other hand, have a much more difficult time finding new troops, as females in established groups often block any unrelated females from joining. Therefore, they must form their own troops. |
Fun FactBolivian red howler monkeys have been seen using sticks as weapons to ward off predators and other intruders. |
Communication
Howler monkeys are named for the booming roars they frequently make, mostly during dusk and dawn. With a call radius of 3 mi (4.8 km), howler monkeys are considered to be the loudest animals on land (several different sea creatures have them beat for loudest on Earth). The loud calls are amplified by an enlarged hyoid bone, a u-shaped bone that exists at the base of the tongue and is found in most animals. This hyoid is also pneumatized, meaning it has pockets of air within the bone, extending the range of their calls. The bone vibrates during calls and adds to the sound, similar to how a gong works.
When a group chorus sounds off, they are answered by neighboring troops. By informing other troops of their location, howler monkeys can avoid direct confrontation, which could result in violence. Additionally, there is a negative correlation between the range of a male’s calls and the size of their testes. In other words, the louder the calls, the smaller the testes. This fact hints at what researches have long suspected: calls are also used to attract mates. Males that cannot compete with the roars of others due to their smaller hyoid bone have large, often brightly colored testes to attract females.
Bolivian red howlers also vocally communicate through barking and purring. They practice scent-marking by rubbing their throats along tree branches. When threatened or making calls, they will often raise their fur to make themselves look bigger.
Howler monkeys are named for the booming roars they frequently make, mostly during dusk and dawn. With a call radius of 3 mi (4.8 km), howler monkeys are considered to be the loudest animals on land (several different sea creatures have them beat for loudest on Earth). The loud calls are amplified by an enlarged hyoid bone, a u-shaped bone that exists at the base of the tongue and is found in most animals. This hyoid is also pneumatized, meaning it has pockets of air within the bone, extending the range of their calls. The bone vibrates during calls and adds to the sound, similar to how a gong works.
When a group chorus sounds off, they are answered by neighboring troops. By informing other troops of their location, howler monkeys can avoid direct confrontation, which could result in violence. Additionally, there is a negative correlation between the range of a male’s calls and the size of their testes. In other words, the louder the calls, the smaller the testes. This fact hints at what researches have long suspected: calls are also used to attract mates. Males that cannot compete with the roars of others due to their smaller hyoid bone have large, often brightly colored testes to attract females.
Bolivian red howlers also vocally communicate through barking and purring. They practice scent-marking by rubbing their throats along tree branches. When threatened or making calls, they will often raise their fur to make themselves look bigger.
Reproduction and Family
Mother red howler monkeys are pregnant for just over 6 months. Babies hang on to their mother for about a year, gradually gaining independence over time. Females reach maturity at the age of 5, at which point most of them (around 70%) leave their family. Males reach maturity at 7 years old when almost all of them (98%) leave their families. Howler monkeys do not have a set mating season, but they are more likely to give birth during the wet season, between October and April, when the most nutritious food is available.
Fathers take on limited parenting roles, although unrelated males are usually hostile toward the offspring of others. There have also been cases of red howler monkeys adopting abandoned and orphaned young.
Mother red howler monkeys are pregnant for just over 6 months. Babies hang on to their mother for about a year, gradually gaining independence over time. Females reach maturity at the age of 5, at which point most of them (around 70%) leave their family. Males reach maturity at 7 years old when almost all of them (98%) leave their families. Howler monkeys do not have a set mating season, but they are more likely to give birth during the wet season, between October and April, when the most nutritious food is available.
Fathers take on limited parenting roles, although unrelated males are usually hostile toward the offspring of others. There have also been cases of red howler monkeys adopting abandoned and orphaned young.
Ecological Role
Like all animals who eat fruits and seeds, Bolivian red howler monkeys provide the crucial role of seed dispersal throughout the forest. Plants depend on seed dispersal to maintain a healthy and diverse gene pool.
White-tailed deer have been observed following groups of howler monkeys and eating the food they drop from the canopy. This is called a commensal relationship, with the deer benefiting from the monkeys while the monkeys are not affected one way or the other.
The main predator of the red howler monkey is the harpy eagle. Jaguars, caimans, and boa constrictors may also prey on these monkeys, but such instances have never been directly observed.
Like all animals who eat fruits and seeds, Bolivian red howler monkeys provide the crucial role of seed dispersal throughout the forest. Plants depend on seed dispersal to maintain a healthy and diverse gene pool.
White-tailed deer have been observed following groups of howler monkeys and eating the food they drop from the canopy. This is called a commensal relationship, with the deer benefiting from the monkeys while the monkeys are not affected one way or the other.
The main predator of the red howler monkey is the harpy eagle. Jaguars, caimans, and boa constrictors may also prey on these monkeys, but such instances have never been directly observed.
Conservation Status and Threats
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Bolivian red howler monkeys are classified as Near Threatened by the International Union for Conservation of Nature (IUCN, 2020), appearing on the IUCN's Red List of Threatened Species. Although the species has a relatively large range, hunting pressure and habitat loss are suspected to lead to an ongoing population reduction approaching 30% over three generations (33 years).
The main threat to this species is subsistence hunting, which is common and has led to local extirpations and lower densities and biomass in some hunted areas. Hunting for this species in Bolivia has been recorded for food, as pets, skins, and for use in traditional medicine. Habitat loss is becoming more of a threat to the species, particularly in Peru. The main causes of habitat loss are expansion of the agricultural frontier, logging, and mining, both legal and illegal. In the future, climate change will affect the species due to increased temperatures and changes in rainfall patterns. As their habitat decreases, howler monkey troops are forced away from the safety of the inner forest into the edges of the forest. Not only are they more vulnerable to human hunters when at the forest edge, but they can also be easily spotted by their main predator, the harpy eagle, as well as other birds of prey. Fortunately, howler monkeys are surprisingly resilient and can adapt to new situations quicker than most other primates. In many habitats where other primates have disappeared, howler monkeys still persist at relatively healthy population levels. |
Conservation Efforts
The Bolivian red howler is listed on Appendix II of CITES. Although the Bolivian red howler monkey is not a main priority for many conservation groups, the species benefits from living alongside a number of rare and threatened plants and animals which do require protection.
Because of this, the Bolivian red howler monkey appears in several protected areas according to the IUCN: Isiboro Sécure National Park (1,200,000 ha), Noel Kempff Mercado National Park (1,500,000 ha), Manuripi Heath National Reserve (1,884,000 ha), Ríos Blanco y Negro National Reserve (1,423,900 ha), Amboró National Park (180,000 ha), Pilon Lajas National Park (400,000 ha), Carrasco National Park (622,600 ha), and Madidi National Park (1,571,500 ha).
The Bolivian red howler is listed on Appendix II of CITES. Although the Bolivian red howler monkey is not a main priority for many conservation groups, the species benefits from living alongside a number of rare and threatened plants and animals which do require protection.
Because of this, the Bolivian red howler monkey appears in several protected areas according to the IUCN: Isiboro Sécure National Park (1,200,000 ha), Noel Kempff Mercado National Park (1,500,000 ha), Manuripi Heath National Reserve (1,884,000 ha), Ríos Blanco y Negro National Reserve (1,423,900 ha), Amboró National Park (180,000 ha), Pilon Lajas National Park (400,000 ha), Carrasco National Park (622,600 ha), and Madidi National Park (1,571,500 ha).
References:
- http://www.iucnredlist.org/details/41546/0
- http://pin.primate.wisc.edu/factsheets/entry/red_howler/taxon
- http://www.wildinfo.net/facts/Redhowler.asp
- https://alltheworldsprimates.org/Members/Home/MasterPrimate.aspx?tid=296
- https://www.scientificamerican.com/podcast/episode/howler-monkeys-trade-testicles-for-decibels/
- https://animaldiversity.org/accounts/Alouatta_seniculus/
- Monkey Planet (2014) http://www.bbc.co.uk/programmes/p01r52gr
Written by Eric Starr, May 2018, Conservation Status updated May 2021
