Purús red howler monkey, Alouatta puruensis
PURÚS RED HOWLER
Alouatta puruensis
Geographic Distribution and Habitat
Purus red howler monkeys are distributed along the Rios Purus and Jurua basins in Brazil and Peru. They are suspected to have a range as far south as the Bolivian border. Their habitat is the Amazon rainforest, which can have parts that are flooded for many months of the year.
The Alouata genus is one of the most widespread Latin American primates adapted to various habitat types. As a result, red howler monkeys have many variations depending on their geographical location. This has made the differentiation of species and taxonomical classification complex. Many scientists have differing opinions on how the Purus red howler monkey should be classified. The Purus red howler monkey was classified as a subspecies of Alouatta seniculus (Colombian red howler monkey). They are currently recognized as an independent species based on their geographical isolation in the Purus River basin of Brazil and Peru.
Size, Weight, and Lifespan
Red howler monkeys are large leaf-eating monkeys. The Purus red howler measures 20 to 26 inches (50 to 67 cm) from the top of their head to the base of their tail. Their tail alone is about the length of the rest of their body, measuring 23-26 inches (59-67 cm).
Males are significantly larger than females. Males weigh between 13.2 and 16.8 pounds (6 to 7.6 kg), and females weigh between 9.9 and 13.9 pounds (4.3 and 6.3 kg).
While we do not know the lifespan of Purus red howler monkeys, other red howler monkey species have lived in captivity for 22 years.
Appearance
Purus red howlers tend to be darker red than other species in the genus. Sexual dichromatism is prominent in the species, with males and females visibly different in coloring. Males have a rich mahogany-red fur color on their back and a lighter maroon on their sides, legs, and arms. Sometimes, their shoulders can have a scattering of gold hairs. Their face is covered in maroon fur, and they have an almost black beard. Females are more yellow-orange throughout their bodies with bright yellow whiskers and tails.
Their fully prehensile tails are muscular and highly flexible. The underside of their tails, of both males and females, are bare from about 2/3 of the way down from their bodies to the tip and are outfitted with a specialized tactile pad that has friction ridges to facilitate a more secure grip, allowing the tail to support their entire body weight so that they can hang hands and feet free from branches. Those same ridges form unique patterns, similar to human fingerprints, which can be used to identify individual monkeys.
Diet
Their primary food source is leaves (they are folivores) and, to a lesser extent, fruits and flowers. They are one of the few monkey species that can subsist on eating mature leaves, though they prefer young leaves when they can get them. Mature leaves are tough to digest because of their thick cellulose exterior. Their teeth and digestive systems are adapted to chewing and digesting the tough exterior of leaves.
The teeth of red howler monkeys have large cusps (the sharp projections on the crown of the tooth) that help tear through the tough exterior of the leaves. Their large intestines have a specialized fermentation chamber called the caecum, where microbes help break down the leaves into more absorbable sugars. The digestion adaptations help red howlers eat a lot of leaves at a time and then take their time to digest them completely.
Behavior and Lifestyle
These monkeys are diurnal (active during the day) and primarily arboreal (tree-dwelling). However, they can travel on the ground. They usually travel on all four limbs (quadrupedally) and need branches or vines to hold on to with their hands, feet, or prehensile tails to cross gaps in the canopy. They also move down trees in unique head-first locomotion and use their tail to balance and control their speed.
Red howler monkeys make and reuse latrines, designated areas where they deposit their feces. In general, this behavior is rare among primates, but red howler monkeys have been observed repeatedly visiting latrines to deposit new feces. The latrines are usually made at the base of familiar trees, and there are multiple theories about their purpose in red howler ecology. The most common reason is that they communicate about the troop’s health and sexual status through hormones released in their feces. Using latrines at the base of trees and defecating away from the food sources in the canopy is a good way to avoid parasites that live in the feces. Red howler monkeys are natural hosts to many pinworm and intestinal parasites that can cross-infect troops through contact with each other’s feces. Latrines are functional in concentrating contaminated waste in one area.
Incidences of aggression between red howler monkeys are rare, but the situation can get violent when they occur. In the wild, many individuals (especially older ones) will have multiple scars from these altercations. Males tend to be more aggressive than females, and given the larger male size, they can do the most bodily harm. In some cases, when a new male takes over a troop, he may kill all the young fathered by the previous male. This killing is called infanticide, and the new male does it to make sure that his efforts in leading and protecting the troop go towards taking care of his offspring and not that of another male.
Female red howler monkeys are typically aggressive against other non-mated females. The females in the troop aggressively call and drive the solitary female away from the area.
Purus red howler monkeys are found in Brazil and Peru’s Purus river basin rainforests.
They have a large hyoid bone in their neck that makes their throat look swollen. The hyoid helps howlers make their loud howling calls.
They are folivores and one of the few primates that can live on mostly mature leaves (that are tough to digest), though they prefer young leaves.
They make and use latrines where they repeatedly defecate, similar to how people use toilets.
Red howler monkeys are morning primates and usually wake up at first light. They typically start with their dawn chorus calls, letting nearby troops know that they have survived the night and that their area is still actively being defended.
They use their latrines in the morning, typically at the base of a tree near their sleeping site. After this, they spend time looking for food and eating leaves. They travel for about 15% of their day looking for a tree with enough nutritious leaves where they can hang out and consume food for a while. They eat as much as possible because most of their digestion (in their intestine’s fermentation chamber) happens when resting, which takes about 18% of their active time.
They can have two or three bouts of activity where they travel in a day. During the midday resting period, they typically use another latrine site.
Red howler monkeys spend most of their time sleeping (about 40%)! Researchers think their folivore diet requires more rest time to break down tough leaves, and energy is released slowly. Their activity levels change seasonally. In the dry season, leaves are less abundant and nutritious, so they spend less time feeding. In the rainy season, they feed more and rest less, most likely because the cooler temperatures in the Amazon forest make it easier to move around more.
Well before dusk, red howler monkeys return to their sleeping sites, huddling in the high canopy for safety and settling in for the night.
Red howler monkeys live in troops of multiple females led by a single male or multiple males. The size of the troop is variable and depends on food availability. When food resources are inadequate, red howler monkeys form smaller troops that can be separated, giving them a better chance of finding enough food.
Males and females have separate hierarchies, and males are always dominant over females. One male is dominant over everyone in the troop and is typically the only one that mates with the females. The subordinate males can challenge the dominant male and take over the troop. These takeovers are usually violent and result in injury or death. Males are considered the protectors against intruders, especially if they see trespassing troops or other males. Females only take the lead if the intruders are female.
All howler monkeys are known for their loud booming howling calls, and red howlers are some of the loudest. These howls can be heard up to 3 miles (5 km) away. Red howlers are known for an enlarged U-shaped hyoid bone in their neck. The hyoid bone connects to a flexible cartilage structure that expands their vocal muscles. This helps make the signature howling call loud and distinct. Males have a larger hyoid and vocal capacity than females, which explains why males have a louder call than females.
Red howler monkeys start their calls first thing in the morning in a series of long howls called “the dawn chorus”. These specialized calls can last several minutes and are the longest of their communication calls. The males start the dawn chorus, and the females join in afterward. A louder chorus signals a larger and stronger troop, informing other troops that the area is being used and protected by a strong troop of red howler monkeys.
Howling together as a troop has the social benefits of strengthening the bonds. It would be like the familial bond a music band would develop after years of playing together. Similarly, the social bonds are reinforced when males and females call together. Additionally, young red howlers practice their howls and learn to recognize their family’s sound, an important survival tool if the young one were to get separated from the troop.
Red howler monkeys actively defend their feeding and sleeping sites. When a strange troop is seen entering the area, males will gather in the canopy and aggressively roar at the trespassing troop. Females join in on the calls, and soon, a vocal battle between the two troops will continue until the trespassing troop leaves the area or the established troop is driven out of their area. Red howler monkeys erect their fur and appear larger during these interactions, which is a visual signal to others to back down from confrontation. Most of these interactions do not escalate to physical violence. However, if aggression continues, these powerful monkeys can damage and seriously hurt one another, protecting their resources.
Female howls usually trigger howling by males, and researchers think that females do this to excite the males and potentially incite competition among males. The female’s best interest is to have the strongest male as the leader so that he can best defend the entire troop, become the dominant male, and father her children. This strategy gives the female and her offspring the best chance of surviving through adulthood and avoiding infanticide through a takeover by another, more dominant male.
Red howler monkeys have smaller brains than other similarly sized monkeys, and researchers speculate that there is a link between their brain size, leaf-eating diet, and howling. Leaves are low-energy foods with lots of fiber and low-fat content. This does not allow for excess nutrition to develop larger bodies or brains. So, any behavior that does not contribute to getting more food must be energy efficient and not waste these monkeys’ precious energy. Howling is an easy way to make long-distance advertising calls without moving, and their anatomy helps efficiently make these calls without wasting energy. Using latrines is also an energy-efficient method of communication because red howler monkeys can use their feces (a necessary part of their lives) to leave olfactory messages to other red howler monkeys. These long-distance communications reduce the risk of exposing the signaling red howler monkey to predators and rivals.
Red howler monkeys breed throughout the year. The dominant male in the troop is the only one who mates with the troop’s females. He generally keeps other males away from the females, especially when she is in estrus (when she is hormonally ready to mate).
The gestation period (the time she is pregnant) is about 6 months, and the newborn red howler monkey is pale and utterly dependent on their mother. The dominant male (the newborn’s father) spends more time with the mother and other females than the other males. But he will rarely play or help take of young red howler monkeys.
For the first month, the infant is carried across the mother’s chest, where she can secure her baby while she travels through the canopy. As the young get stronger, they migrate to riding on their mother’s back. The baby usually wraps their tail around their mother’s tail to hold on securely. When the young red howler monkeys are four months old, they use quadrupedal locomotion and can move around more independently. Young monkeys start eating leaves when they are about 7 months old but may nurse until 18 months old.
Females are sexually mature at around 2 to 4 years old, while males develop later, maturing at around 4 to 6. Both males and females disperse or leave their natal troop (the troop they were born into) to form new troops. This prevents closely related individuals from mating with each other.
Most troops are started with one male and a couple of females. Dispersed males can join an established troop, but solitary females are often prevented from joining by the other females. So dispersed females usually travel much further than their brothers in search of some bachelor males.
Once a troop is established, only one male will have the dominant designation and breeding access with the females of his troop. The dominant male is visibly larger than the other males. Subordinate males can challenge the dominant male for the alpha position, but this usually has violent outcomes for both parties. If a subordinate male overpowers the dominant male, he must leave his troop for the new dominant male.
As frugivores and folivores, red howler monkeys have an intertwined relationship with trees and the forest ecology. They help seed dispersal as they feed on fruits and travel long distances, dropping seeds along the way. These seeds are usually dropped away from the parent tree, where they have less competition for light and soil. This way, red howler monkeys play a role in forest regeneration.
When these primates feed on leaves, they thin out the dense canopy and create light gaps where sunlight can penetrate the forest floor. Smaller plants depend on these light gaps for photosynthesis (make food from sunlight) and successfully grow amongst tall trees. Red howler monkeys contribute to the diversity of plant life in the Amazon forest.
Red howler monkeys make latrines, which increase the organic material and micronutrients in the soil around a latrine, a limited resource in neotropical forests like the Amazon. Red howlers also defecate seeds at latrines. As a result, latrines become a hotspot of plant diversity as seeds successfully take root in the nutrient-dense soils at and around the latrine.
Purus red howler monkeys live in potential competition with other frugivore and folivore mammals such as squirrels and other primates. They generally avoid competition by feeding at different heights in the forest. However, with the increasing pressures from deforestation and hunting, more competitors are forced to tolerate each other. Purus red howler monkeys have been observed feeding next to and peacefully interacting with natural food competitors such as black-faced spider monkeys (Ateles chamek) and common woolly monkeys (Lagothrix lagotricha). Mixed feeding groups like these offer protection in numbers against hunters and natural predators.
Raptors, like the Harpy eagle, are the primary predators of red howler monkeys. For the first time, a boa constrictor was observed hunting and consuming a female Purus red howler. These rare interactions may occur more frequently as these primates lose dense forests’ safety and space routes through habitat loss.
The International Union for Conservation of Nature lists Purus red howler monkeys as Vulnerable (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.
Habitat loss and hunting are the two biggest threats to Purus red howler monkeys. As the population and needs increase, most of the Amazonas have been deforested for timber, mining, and human development. Howler monkeys are highly arboreal and require continuous forests of tall trees to meet their food and cover requirements. As forests become more fragmented, these monkeys are forced to live in secondary forests with lower-quality food and habitat conditions.
Red howler monkeys are striking because of their vibrant coloring and interesting vocalizations. This makes them appealing to illegal poachers, traders, and the pet industry. They are also hunted for meat throughout their range, and in some local cultures, their hyoid bone is harvested for traditional medicine.
Purus red howler monkey is listed in Appendix II of 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.
Many parts of the Amazon forest in Brazil are protected in national parks, where development and hunting are restricted. However, enforcing these protections is complicated, and illegal deforestation and hunting are widespread in the region.
There is a pressing need for enhanced conservation efforts to protect the region and the red howler monkeys, ensuring sufficient quality habitat for their ongoing survival
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Written by Acima Cherian, January 2025