BOLIVIAN RED HOWLER
Geographic Distribution and Habitat
The Bolivian red howler monkey occurs in central and northern Bolivia. Despite its name, this species is also found in southern Peru. In Bolivia, these monkeys are found from the Department of Pando south along the Andean Cordillera and east into central Bolivia. Their range includes the Río Beni basin and extends east as far the Mamoré-Guapore. In Peru, they are found in forests along the Madre de Dios, Tambopata, and Urubamba rivers.
These monkeys inhabits a range of forest types, including lowland tropical forest, seasonally flooded Amazonian forests, and gallery forests, though it seems that they prefer humid and seasonally flooded forests along large rivers. This species is sympatric with black-and-gold howler monkeys (Alouatta caraya), sharing habitat with them over a large part of their range.
Size, Weight, and Lifespan
The Bolivian red howler monkey is a large-bodied monkey and among the largest of the monkeys in Latin America with both sexes weighing between 13.2 and 20 lbs (6–9 kg). They have a combined head and body length of around 21–28 in (54–71 cm). Their long, prehensile tails measure 20.5–23.6 in (52–60 cm).
The average lifespan of this species in the wild in not known, although similar species can live to be over 22 years old.
Bolivian red howler monkeys have a thick red pelage that can vary in tone from a deep orange to maroon and almost brown. Their dorsal pelage (on their upper side or back) tends to be more golden than their ventral pelage (on the underside or abdomen). There are no color differences between males and females.
Dense hair frames their long, dark faces and forms an impressive beard. The unique shape of their jawbone is specialized for both their diet of leaves and their loud calls. Their thick, long tail is prehensile, and they can use it to grasp and swing from branches. They have long arms and legs and large hands that are well-adapted for quadrupedal movement.
Bolivian red howlers are folivorous, with a diet that is primarily made up of leaves. While they prefer young leaves, they will regularly eat mature leaves too. While they will also eat other plant items, including fruits, buds, flowers, and seeds, both their jaws and their digestive systems are particularly well-adapted to eating leaves. They have enlarged caecums, a section of the intestine that contains bacteria that break down the fiber in leaves. Their molar teeth are also particularly adapted for chewing leaves through shearing. It is likely that this ability to primarily eat leaves has allowed howler monkeys to become widespread through a number of different habitats. Wild figs also represent another important food item for this species, though these are less easily obtainable than leaves.
Behavior and Lifestyle
Bolivian red howlers are diurnal monkeys that spend all of their time in the trees of the Bolivian and Peruvian forests. While they are active during daylight hours, these monkeys can spend up to 70% of their day resting in the trees—more so than many other primate species. This is likely because an abundance of leaves means they don’t have to spend more time foraging, as they would if they had to find ripe fruit, for instance. Additionally, they likely spend a lot of time resting because a lot of energy is required to digest the leaves in their folivorous diet.
Red howler monkey groups tend to be very cohesive, sticking close to one another as they go about their daily activities, but their environment can have an effect on their behavior. For example, groups in degraded habitats tend to be less synchronized in their activities than groups in primary habitat, raising questions about the different ways in which habitat degradation may influence primate behavior.
Different groups of red howler monkeys tend to overlap in their relatively small home ranges and their booming calls allow them to communicate between groups, possibly to mark territories and avoid intergroup interactions. If another troop of monkeys does approach, the adults of the group will usually begin roaring to signal their presence.
Howler monkeys have one of the loudest calls of any terrestrial mammal, made possible by their specialized vocal anatomy.
While both sexes usually disperse from their birth groups, females have much more difficulty joining other groups.
Their folivorous diet, which is rich in leaves, allows them to spend a large part of the day resting.
Hunting remains one of the greatest threats to this species.
Social systems of red howler monkeys are fairly flexible, with groups ranging from 2 to 16 individuals. Groups can either contain only one adult male, or multiple adult males, in addition to multiple adult females and their young offspring. In these multi-male groups, there tends to be one dominant male who is physically larger than the other males in the group. Dominance is won through competition between males, which can be violent and result in injuries. When a subordinate challenges a dominant male and wins, the formerly dominant male is forced to leave and try to enter a new group.
On average, there tends to be between two and five adult females per group and all males in the group are dominant over all females. Although both sexes move between groups, females enter established groups at a much lower rate than males. It is likely that the existing adult females in the group prevent new females from entering. Adult females tend to only be accepted into an existing group shortly after the death of an existing adult female, suggesting that there is perhaps a limit of the number of adult females tolerated per group. This may be to reduce within-group competition for resources. As a result, females who leave their birth groups are more likely to establish new groups than enter existing ones.
As their name suggests, howler monkeys are famous for their vocalizations. They have deep jaws that surround a greatly enlarged larynx and hyoid bone. Together, these form a resonating chamber. With this highly specialized voice box, they are able to produce a variety of loud vocalizations, including grunts, roars, and barks. The entire group usually engages in howling sessions, which most often occur in the mornings. These incredible vocalizations can be heard up to 1.2 miles (2 km) away and are likely used to inform other howler monkeys of the groups’ locations. In particular, the Bolivian red howler monkey has one of the largest voice boxes and deepest roars of the howler monkey genus.
In addition to their well-known vocal communication skills, these monkeys also use other signals, such as olfaction and gestures, to communicate with one another.
Both males and females usually leave their birth group at sexual maturity, which tends to be later for males than females. Sometimes, however, individuals of both sexes will stay and breed in their birth group. Both sexes can attempt to enter existing groups, although females are generally less successful at this and usually form new groups. These newly formed groups usually contain just one male. Once a new group is established, several females may breed with the dominant male. Occasionally, females may also breed with the dominant male of another group.
Infants are born around every 17 months after a gestation period of approximately 190 days. They will cling to their mother initially, first to her belly, then to her back, before gradually gaining independence as they grow. Sadly, infanticide can occur in this species when a new male becomes dominant and kills the offspring of the former dominant male. This allows the mother to become sexually receptive again and means that the new dominant male can mate with her sooner and produce his own offspring faster. Thus, changes in male dominance in a group can present a real threat to infant howler monkeys.
With their diet rich in both young and mature leaves, howler monkeys play an important role as gardeners of the forest, pruning the variety of trees they feed from. Additionally, the inclusion of fruits in their diets mean that they also participate in seed dispersal—spreading the seeds of fruits across the forest.
The Bolivian red howler is currently listed as Near Threatened by the International Union for Conservation of Nature (IUCN, 2020), appearing on the IUCN’s Red List of Threatened Species, and their population is decreasing. The main threat to this species is hunting for bushmeat. This is mostly for subsistence, but it is also hunted for use in traditional medicine and for the pet trade.
Habitat loss is also becoming more of a threat to the species, particularly in Peru. Deforestation rates within this species’ range are increasing due to agricultural expansion, logging, and mining. As more habitat is lost, this will undoubtedly lead to a decline in population numbers. It is also thought that climate change will affect this species due to increased temperatures and changes in rainfall patterns, which may particularly affect food availability. It is expected that these threats combined will lead to a population loss of 30% over the next three decades.
There are nine protected areas throughout this species’ range in Bolivia, including the Beni Biosphere Reserve and the Madidi National Park. There are also two protected areas within their range in Peru; Beni Biosphere Reserve and Madidi National Park. However, further action is needed to prevent further population decline, including a reduction of unsustainable hunting. Additionally, more research is needed to better understand this species and how they may respond to these increasing anthropogenic threats.
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Written by Jennifer Botting, PhD, August 2021