Alouatta seniculus is found in primary lowland rainforest, dry deciduous forest, Andean cloud forest (including oak forest), gallery forest in the eastern plains of Colombia, mangrove swamps, as well as in várzea forest (Hernández-Camacho and Cooper 1976; Gaulin and Gaulin 1982). This species can be found up to 3,200 m (Hernández-Camacho and Cooper 1976). In Ecuador, it occupies tropical and subtropical evergreen rain forest from 200 m to 2,000 m, but it is more common below 700 m (Tirira 2007). Tirira (2007) recorded that it occurs in many different forest types, from tall primary evergreen terra firma forest to seasonally flooded forests and palm swamp forest.

The howler monkeys are among the largest leaf-eaters of the South American primate communities. Their molar teeth are particularly adapted for chewing leaves through shearing. They spend up to 70% of their day lying and sitting about quietly among the branches, fermenting leaves in their enlarged caecums. Like the spider monkeys, they have prehensile-tails, with a naked patch of skin on the distal ventral side and at the tip. Their most characteristic feature is the deep jaw, which surrounds the enlarged larynx and hyoid apparatus, that works as a resonating chamber. It is with this enlarged and highly specialized voice box that they produce their loud vocalizations (grunts, roars and barks). Howling sessions, usually involve the entire group and can be heard at distances of 1-2 km, particularly in the early morning (Vercauteren Drubbel and Gautier 1993).

Red howler groups are usually small, ranging in size from 2-13 animals and averaging 5.5-9 in Colombia (Di Fiore et al. 2011). Soini (1992) observed group sizes ranging from 3-8 (mean 5.54±1.59, n=74) in the Pacaya Samiria National Reserve, Peru. Adult males will often be solitary. In red howlers, there is usually only one dominant male in the group (occasionally two), others being sub-adults, or juveniles, along with a harem of two to five females. Unlike the spider monkeys, and due to the large proportion of leaves in their diet (up to 52% of the diet, Di Fiore et al. 2011), the howler monkeys generally have quite small and broadly overlapping home ranges (Neville et al. 1988). The ranges of red howlers vary between 3.9 and 182 ha, depending on the type of habitat and perhaps on the species (see Di Fiore et al. 2011). Information for A. seniculus in Colombia shows a range size between 22-182 ha per group (Di Fiore et al. 2011). Two groups studied by Soini (1992) used 6 and 9 ha each in the Pacaya Samiria National Reserve, Peru.

Howlers regularly include mature leaves in their diet, although softer, less fibrous, young leaves are preferred when they are available. Their folivory and ability to eat mature leaves is undoubtedly one of the keys to their wide distribution and the wide variety of vegetation types they inhabit. Mature fruit is the other important food item, especially wild figs (Ficus) in many regions, but they also eat leaf petioles, buds, flowers (sometimes seasonally very important), seeds, moss, stems and twigs, and termitaria. Alouatta seniculus has also been seen to eat and lick clay at so-called “salado” sites in the Colombian Amazon (Izawa 1975). The reason for this and the consumption of soil from termitaria is still not clearly understood, but may involve the need for certain minerals, or may be due to the properties of clay which, by adsorption, can reduce the effects of toxins ingested with leaves.

Infant Alouatta are probably born throughout the year in some areas and tending to seasonality for dry months in others (Di Fiore et al. 2011). Crockett and Rudran (1987a,b) examined seasonal variation in births in the closely related red howlers from northern Venezuela (A. arctoidea), and found that they were less frequent during the early wet season, as weaning would occur at the time of greatest food shortage. There is no precise information on the length of the estrous cycle in A. seniculus, but in A. arctoidea estrous lasts 2-4 days, with intervals between estrous periods of about 17 days. For red howlers, interbirth intervals are generally about 16.6 months, although they may be shortened by the death of an infant to about 10.5 months (Crockett and Sekulic 1984). Mean gestation length is 191 days (range 186-194, n=6) in A. arctoidea (Crockett and Sekulic [1982]).

Size: 
For Alouatta seniculus in Colombia:
Adult male 7.54 kg (n=8, range 5 – 12.5 kg), adult female 6.29 kg (n=9, range 4– 10 kg) (Hernández-Camacho and Defler 1985)

There is no information available that allows for the clear delimitation  of the ranges of the Alouatta seniculus juara and A. s. puruensis of Brazil and Peru from A. seniculus of the Colombian Andes and Venezuela (type locality: Cartagena, Bolivar, near the Río Magdalena, Colombia), so they have been mapped together. A. s. insulanus is found only on the island of Trinidad.

In Colombia, A. seniculus is absent from the Pacific coast (western slope of the western Andes) and the desert of the Guajira Peninsula (Hernández-Camacho and Cooper 1976; Defler 2004). Otherwise, it is present throughout the country, except in non-forested areas and mountainous regions above the cloud forest belt, with altitudinal limit at 3,200 m in the Central Andes (Hernández-Camacho and Cooper 1976; Defler 2004). In Venezuela A. seniculus occurs west of the Sierra de Merida and around Lake Maracaibo.  It is not clear whether the distribution of A. seniculus in Venezuela extends further east, as Bodini and Pérez-Hernández (1987) indicate that the howler monkey north of the Rio Orinoco and west through Apure basin north of the Rio Meta is a distinct, as yet undescribed, form (although referred to as A. arctoidea by Linares 1998). A. seniculus occurs in eastern Ecuador and Peru in the upper Rios Marañon, Napo and Putumayo and into Amazonian Brazil, North of the Rio Solimoes and south to the Rio Negro (Mittermeier et al. 2014; Boubli et al. 2015).

Alouatta s. juara is believed to occur in Western Amazonian Brazil south of the Rio Solimoes and into the Peruvian Amazonia, but the limits of its distribution range are uncertain (Mittermeier et al. 2014; Cortés-Ortiz et al. 2015).

Alouatta s. insulanus is found only on the island of Trinidad. Red Howler monkeys on Trinidad range from the western peninsula of Chaguramas to the Diego Martin Valley in the east, as well as the Mathura Forest Reserve in the east and south to Charuma Forests. They are also found in the Central Range Forest Reserves including Bush Bush Sanctuary, and Iros Forest and Trinity Hills in the south.

This species occurs, or may occur, in numerous protected areas:

Colombia

• Amacayacu Natural National Park (293,000 ha) (Defler 1994) 
• Cahuinarí Natural National Park (575,500 ha) (Defler 1994) 
• Serrania de Chiribiquete Natural National Park (1,280,000 ha) (Defler 1994) 
• Cordillera de los Picachos Natural National Park (286,600 ha) (Defler 1994) 
• Cueva de los Guacharos Natural National Park (9,000 ha) (Defler 1994) 
• El Tuparro Natural National Park (548,000 ha) (Defler 1994) 
• Tayrona Natural National Park (15,000 ha [12,000 ha on land]) (Defler 1994) 
• Isla de Salamanca Natural National Park (21,000 ha) (Defler 1994) 
• Paramillo Natural National Park (460,000 ha) (Defler 1994) 
• El Cocuy Natural National Park (158,125 ha) (Defler 1994) 
• Sierra Nevada de Santa Marta Natural National Park (383,000 ha) (Defler 1994) 
• Tamá Natural National Park (48,000 ha) (Defler 1994) 
• Pisba Natural National Park (45,000 ha) (Defler 1994) 
• Los Nevados Natural National Park (58,300 ha) (Defler 1994) 
• Chingaza Natural National Park (50,374 ha) (Defler 1994) 
• Sumapaz Natural National Park (154,000 ha) (Defler 1994) 
• Las Hermosas Natural National Park (150,000 ha) (Defler 1994) 
• Los Farallones Natural National Park (150,000 ha) (Defler 1994) 
• Serrania de la Macarena Natural National Park (630,000 ha) (Defler 1994) 
• Puracé Natural National Park (83,000 ha) (Defler 1994) 
• La Paya Natural National Park (442,000 ha) (Defler 1994) 
• Tinigua Natural National Park (201,875 ha) (Defler 1994) 
• Nukak Natural National Reserve (855,000 ha) (Defler 1994) 
• Catatumbo-Bari Natural National Reserve (158,125 ha) (Defler 1994) 

Ecuador 

• Yasuní National Park (982,300 ha) (Tirira 2007)
• Sangay National Park (517,765 ha) (Tirira 2007)
• Sumaco-Napo Galeras National Park (205,249 ha) (Tirira 2007)
• Podocarpus National Park (146,280 ha) (Tirira 2007)
• Cayambe-Coca Ecological Reserve (403,103 ha) (Tirira 2007)
• Cofán-Bermejo Ecological Reserve (55,451 ha) (Tirira 2007)
• Cuyabeno Faunal Production Reserve (Tirira 2007)

Peru 

• Pacaya Samiria National Reserve (Soini 1992)
• Zona reservada Pucacuro (Aquino et al. 2016)
• Zona reservada Santiago-Comaina (Aquino et al. 2016)

Venezuela

• Perija National Park (295,288 ha) (Venezuela, INPARQUES, 1982).

Trinidad

• Found in several forest reserves including the Central Range Forest Reserves (Bush Bush Sanctuary), and Iros Forest and Trinity Hills in the south.

It is listed on Appendix II of CITES.