Description: Alouatta seniculus are less sexually dimorphic than some of the other species of howler monkeys. The Red howler monkey has thick, dark red to brownish red fur, with bright orange or gold underparts. The dark face is naked and surrounded by fur. The nose is somewhat stubby; the wide jaw is covered with a thick beard. The color may vary with age and distribution of the animals. An enlarged hyoid bone at the root of the tongue gives the throat a swollen appearance. Males are larger and have a darker and heavier beard. The prehensile tail can measure up to 31 inches (79 cm) and is without fur on the bottom side of the last third, which allows for better grip when grabbing branches.
Size: One of the larger species of neotropical primates, Red howlers are the largest of the howler monkeys. Females are 18-22 inches (46-56 cm) long; males are 19-28 inches (48-71 cm). Males can weigh up to 20 pounds (9 kg) while females rarely exceed 15 pounds (7 kg).
Behavior: Red howlers live in social groups of not more than ten members, with one or two of the individuals being males. They are rather inactive monkeys, and usually have small territories in which they do little more than sleep and eat. They are most famous for their “dawn chorus” which begins each morning as an accelerating series of low grunts by a single male who is then joined by the howls of other males. Females may also contribute with their higher pitched sounds. This howling continues several minutes, in order to insure that other troops have been thoroughly
informed of the group’s whereabouts, thus reducing potential conflicts. These troop spacing calls, which can be heard from distances of three to four miles (4.8 – 6.4 km), cease with one long resounding roar. Other troops, within hearing distance, answer with howls. These howling sessions are also heard in the late afternoon. They may also howl at the sound of approaching rain or at the onset of rain.
Diet: They eat leaves, flowers and fruit, with leaves making up to 60% of the diet. They are better able to subsist on foliage than many of the other New World monkeys in that they have two large sections in their hindgut where bacteria breaks down the hard-to-digest cellulose. Howlers are selective eaters and avoid leaves that are unsafe or poisonous. They prefer young tender leaves that are easier to digest and contain more protein and sugar.
Senses: As in most primates, the most developed senses are sight and hearing, then smell and touch.
Communication: Red howler monkeys get their name from the loud vocalizations produced by both sexes. Produced by the enlarged hyoid bone and amplified by the throat sac, the “long call” of a male can be heard throughout the forest. Troops, starting with the dominant male, then joined by other males, followed by females, start calling early in the morning. Neighboring troops will respond. These calls allow troops to avoid one another, reducing aggression.
Reproduction: Males reach sexual maturity after five years while females become sexually mature at four years of age. Males are driven from their natal troop at sexual maturity, therefore they must be admitted into new groups. Females usually give birth every 16 months. Male Red howler monkeys will breed with multiple females within the troop. Females will give birth to usually one baby after a gestation period of approximately 191 days. Newborns are quite helpless at birth and are carried on their mother’s stomach. Other females in the group are attentive or at least tolerant of the infants. However, males who take over a group have been observed committing infanticide, shortening the interbirth interval in order to mate quickly.
Habitat/range: Alouatta seniculus have the widest geographical distribution of all the New World monkeys. They live in the middle and upper levels of the forest throughout the northern half of South America, from Colombia to Bolivia.
Status: IUCN Least Concern. CITES – Appendix II.