Ursine Howler Monkey, Alouatta arctoidea
URSINE HOWLER MONKEY
Alouatta arctoidea
Geographic Distribution and Habitat
The ursine howler monkey (Alouatta arctoidea), also referred to as the ursine red howler, is one of 15 species of howler monkeys that occur throughout various parts of Central and South America. According to geographic data, the home range of the ursine red howler monkey includes Venezuela, and possibly parts of Colombia.
In Venezuela, the ursine howler monkey is an endemic species; endemism is a biological term that describes when a species is native to a specific geographical region. In the case of this species, their Venezuelan geographic range runs along the northern Venezuelan coast, particularly La Cordillera Central. The range further extends to the eastern border of Lake Maracaibo, before stretching south to Venezuela’s largest sedimentary basin, Los Llanos.
Throughout their range, ursine howler monkeys live in a variety of habitats, many of which include savannas, deciduous forests, and gallery forests. A gallery forest is a narrow strip of trees that grows along a river or wetland in an area that is otherwise mostly treeless.
Elevation limits can vary between 3.3 feet (10 meters) to 3806 feet (1,160 meters) and the average temperature is between 55 to 75 degrees Fahrenheit (12.78 to 23.89 degrees Celsius).
Beginning in 1962, scientists and primatologists worked closely together to classify nine subspecies of the red howler monkey, including Alouatta arctoidea, the species that is the topic of this profile. In 1995, researchers concluded that the number of chromosomal differences between two of the species (A. seniculus sara and A. seniculus arctoidea) was significant enough to elevate both to full-species status.
Taxonomic research for this species remains ongoing.
Size, Weight, and Lifespan
On average, male ursine howler monkeys weigh around 14 pounds (6.3 kilograms) and have a body length that is between 19.3 to 28.25 inches (49 to 72 centimeters). Females weigh an average of 10.3 pounds (4.67 kilograms) and have a body length that is between 18 to 22.44 inches (46 to 57 centimeters). Males are typically longer and heavier than their female counterparts. This is an example of sexual dimorphism.
In the wild, the average lifespan of ursine howler monkeys is 15 to 20 years.
Appearance
Male and female ursine howler monkeys have deep reddish-brown fur, often compared to the color of brick, that extends along their bodies and onto their faces. Although both males and females may have fur-outlined faces and have many of the same features, one difference between the sexes is that males typically have shorter beards that are darker and heavier than their female counterparts. This is another example of sexual dimorphism, that is, a physical characteristic or characteristics that differ between the genders other than those that define the sexes.
Like all other howler monkey species, ursine howler monkeys have prehensile tails that are used for grasping, hanging, balance, and suspension. Often considered a howler monkey’s fifth limb, the tail is roughly the same length as the monkey’s body, ranging between 19.3 to 29.53 inches (49 to 75 cm). Thumbs of howler monkeys are not fully opposable, so having a prehensile tail is a great evolutionary advantage! As their prehensile tails have tactile pads with few hairs on the lower third portion, they can sense objects similarly to how humans do with their hands.
In addition to having deep jaws and an enlarged larynx, which functions as a voice box for producing sound, one of the most unique evolutionary adaptations for howler monkeys is their large, cup-shaped hyoid bone. The hyoid bone serves as an attachment structure for the tongue and muscles of the floor of the oral cavity. It functions to deepen the pitch and resonance of sound, which is how the vocalizations of howler monkey calls are amplified. Interestingly, of all howler monkey species, the hyoid bone in red howlers is the largest and can vary by gender with males having larger hyoid bones than females, thereby producing louder and deeper roars. Talk about good vibrations!
Diet
Ursine howler monkeys are folivores, herbivorous animals that specialize in eating leaves. Although they consume leaves above all else, they may also eat a variety of other food products, such as fruits, strangler figs (Ficus aurea), flowers, buds, stems, moss, twigs, etc. They even have a great sense of smell and are capable of detecting some foods at a distance.
At times when fruit and flowers are in short supply, ursine howler monkeys spend up to 85% of their daily feeding time eating leaves. To help their digestion, when consuming leaf products, they prefer younger leaves because they are softer and more digestible. That being said, because of resource availability, much of their regular diet includes tougher, more mature leaves. To accommodate this leafy lifestyle, howler monkeys have digestive systems that are well-adapted to their daily routines. All howler monkeys have a simple stomach, in addition to two enlarged ceca. In the case of howler monkeys, ceca are pouches that form the first part of their large intestine. Ceca, for many herbivorous species, contains bacteria that break down fiber contained within leaves, making the overall digestive process easier.
To support food digestion, on average, howler monkeys spend up to 70% of their days lying around and sitting amongst the trees. Howler monkeys may obtain as much as 31% of their required daily energy from fermentation, so rest plays an important role in their ability to carry out daily routines with high energy and vigor.
Because their diet is heavily influenced by leaf consumption, these monkeys have dental formations specifically adapted to meet their digestive needs; their dental formula, which consists of their upper and lower jaw, is very distinctive. Howler monkeys have two incisors, one canine, three premolars, and three molars. Incisors and canines are incredibly important for an animal’s ability to eat foods that are especially hard to tear, such as tough plant products. Since howlers are herbivores, having a greater ratio of premolars and molars to incisors and canines is essential. Molars and premolars are particularly adapted for leaf chewing, particularly for tearing and shearing.
Behavior and Lifestyle
Ursine howler monkeys are arboreal monkeys, meaning they spend most of their time living in trees. On the occasion that they venture to the ground, they get around via quadrupedal walking and running. Quadrupedalism is a method of movement in which animals who have four limbs use them to bear weight and move around. Additionally, howler monkeys may perform vertical descents, a locomotive form of movement in which the monkeys will face the ground and walk quadrupedally down a tree, supported by their tails for control and speed. This technique also allows monkeys to cross horizontal and vertical tree gaps.
Gap-crossing typically occurs in the upper canopy of many forest types. In the context of habitat fragmentation, because these animals are arboreal, gap and road-crossing behaviors are becoming more common. Because howlers have long tails capable of gripping branches, it is easy for them to stretch both their bodies and tails across two disconnected trees to create a bridge. For example, for a mother to get her infant across this gap, she may wrap her tail around one branch, hold another by a foot, and extend her arm to the tree she is attempting to travel to. In this position, the mother’s body can support the safe journey for her offspring. Because they have a strong ability to disperse, this technique can be especially useful when also escaping from intruders or potential predators.
The natural predators of ursine howler monkeys may attack from either land or the sky. From the skies, harpy eagles (Harpia harpyja) are the most threatening predator. From the land, jaguars (Panthera once) and large snakes, specifically the red-tailed boa (Boa constrictor), are major threats.
Howler monkeys are notably one of the loudest land mammals on Earth, especially since their calls can be heard for over 3 miles (5 kilometers). Whether they communicate early in the morning, or late in the day, these animals are sure to keep entertained. In fact, they’re bona fide experts!
Troops of ursine howler monkeys consist of 2 to 16 individuals, although the average troop size is 4 to 10. These troops usually consist of one adult male (sometimes 2) and between 2 to 5 females. The remaining troop members are juveniles and infants.
Female ursine howler monkeys reach sexual maturity at the age of 4, while male ursine howler monkeys reach sexual maturity at 5 years old. Once matured, both males and females leave the troops they were born into to join a new one. Because of this, the majority of their adult lives are spent living among monkeys they are not biologically related to.
As a diurnal species, these animals are most active in the morning when searching for feeding areas, and when traversing their home ranges. Depending on the forest type, home ranges can be small, and broadly overlap. A typical size of an ursine howler monkey’s home range is between 12.35 to 111 acres (5 to 45 hectares).
As mentioned, because of their enlarged hyoid bone and large throat sacs, ursine howler monkeys are known for their loud, distinctive calls. Famous for their howling morning chorus, the roars, whoops, and barks of a howler monkey are mainly performed by males for territorial defense, danger warning, and seeking mates. Often, two competing troops may perform call-and-response howls over one another, sometimes for up to an hour, or until one troop gets tired.
Another form of communication used by ursine howlers is olfactory (scent) and used to mark territories. Adults may transfer their sweat onto tree branches by rubbing their beards on specific territory markers, as well as rub their anal areas on tree branches after defecating together as a group. This behavior acts as a warning for other troops to stay away, while also providing predators the perfect opportunity to track and stalk their prey.
Ursine howler monkeys are polygynous, meaning one male mates with multiple females. When a female is pregnant, her average gestation length is 191 days, after which one, golden-haired infant is born. Upon birth, the average weight of this offspring can vary anywhere between less than 1 pound to up to 2 pounds (0.45 to 0.91 kilograms).
Infant ursine howler monkeys stay with their mothers for 18 to 24 months, or until fully weaned. Weaning, by definition, is the process of gradually transitioning young animals away from consuming their mother’s milk, to a solid food diet. Although howler mothers are individually responsible for caring for their young, other female troop members may also help. Within a troop’s natal group, all adult females may carry, groom, and protect any infant or juvenile, regardless of parental lineage.
Once an infant becomes more independent, it will stay within its troop until it reaches sexual maturity. Once matured, juveniles leave in search of their own troop to become a member.
Ursine howler monkeys are folivores and frugivores, meaning they have a diet that primarily consists of leaves and fruits. In both cases, when these animals consume food products and defecate, they are likely to disperse seeds. Seed dispersal is very important for the survival of plant species, especially within ecological areas that are environmentally threatened. If seed dispersal occurs across a wide area, the land’s genetic diversity will increase. Although wind, water, and gravity also play a role in seed dispersal, as one of the largest leaf-eaters of the South American primate community, the ecological role of these animals is inherently linked to the overall success of the environment.
The International Union for Conservation of Nature lists ursine howler monkeys as Least Concern (IUCN, January 2015), appearing on the IUCN Red List of Threatened Species. According to the IUCN Red List, this species is listed as Least Concern because of its relatively large distribution across Venezuela, and parts of Colombia, and lack of major threats to drive a significant range-wide decline.
The main threat to this species is hunting, particularly for food, and trapping. Other threats include habitat loss for livestock farming and ranching and agro-industry grazing.
The ursine 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.
Luckily, to prevent this species’ extinction, conservation sites and protected areas occur throughout their endemic home, including El Avila National Park, Sierra Nevada National Park, Guatopo National Park, Henri Pittier National Park, Aguaro-Guariquito National Park, and more. Also in Venezuela, ursine howler monkeys are included in international legislation and subject to both international management and trade controls.
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Written by Brooklynne Mitchell-Arno, October 2024