Alouatta seniculus
red howler monkey

Alouatta seniculus have the widest geographical distribution of all the New World primates. Red howler monkeys range throughout the northern half of South America, from Colombia to Bolivia.

Alouatta seniculus are slightly sexually dimorphic. Females have a body length of 46-57 centimeters; males, which are larger, have a body length of 49-72 cm. Both sexes have a long, prehensile tail of approximately 49-75 cm. The coat color of males and females is a deep reddish-brown, although the shade varies slightly with age. Red howlers have a large neck with tremendous lower jaw and hyroid bones, giving them a forbidding expression.

(MacDonald, 1985) (Nowak, 1991).

Due to such an unbalanced sex ratio, fierce sexual competition exists between and within red howler troops. Red howler males, which are expelled from their natal troop upon reaching sexual maturity, are forced to invade an outside group and gain admittance. Once they have accomplished this, they violently kill any infants present in the group. By killing infants in a newly invaded troop, a male can mate quickly and ensure that the new offspring of the group are his own. Mothers do try to protect their offspring against assaulting males. Unfortunately for the female, this is not particulaly successful; less than 25% of offspring survive a male howler invasion.

The mating behavior of red howlers is another interesting aspect of their social interactions. Males and females often form consortships, an unusually close spatial relationship, before any sexual exchange has begun. Once these associations are established, sexual solicitations begin. Although seductive behaviors can be performed by both sexes, the female most often takes on the aggressive role. When attempting to attract a male, the female approaches him and moves her tonque rhythmically. The male may respond the same way, but if he does not, the female may simply try to entice another male.

Alouatta seniculus appears to breed throughout the year. However, in two habitats in Venezuela, the birth frequency is reduced during the early wet season, May through July. The estrous cycle ranges from 16-20 days, with the female being receptive for 2-4 days. Red howler females give birth for the first time around 5 years of age, while males usually do not father an offspring until approximately 7 years. Therefore, a female reaches sexual maturity a couple of years before a male.

(MacDonald, 1985) (Nowak, 1991) (Smuts et. al, 1986)

Newborn A. seniculus are at first quite helpless and are carried around at the mother's belly. Young red howlers begin using their prehensile tails before they are one month old. An infant uses its tail to secure itself to its mother, for in this stage of its life the mother pays little or no attention to her offspring, and fails to give the baby any assistance!

Red howler monkeys live in relatively large social groups, consisting of approximately 10 individuals, with only one or perhaps two of the individuals being males. Alouatta seniculus exhibit many interesting behaviors. They are most famous for their "dawn chorus", a deafening roar that can be heard up to 5 kilometers away! These resonating howls, performed primarily by the males of a group, are answered by all other howler troops within ear shot. This way, one troop can constantly inform another of its precise location, thus avoiding an energetically costly squabble over resources.

During parturition, a newborn usually becomes the focus of attention of several other females. Typically, it is females without infants of their own that are attracted to these infants. In these instances, the females are extremely gentle with the little ones, touching them with their muzzles and hands. They may even occasionally encourage the infant to crawl on them. Males also are known to tolerate the activities of infant howlers in their troop. Similar to the adult females, the male red howlers also allow the infants to climb all over them. This of course takes place long after the male has killed all offspring that are not his own.

(Crockett, 1984) (DeVore, 1965) (Hausfater & Hrdy, 1984)

Alouatta seniculus are primarily folivorous. Leaves are low in nutrients and sugars in comparison with other food choices, and red howler monkeys have two large sections in their hindgut which contain the bacteria needed to digest the cellulose in leaves. This change in anatomy results in a large gut that occupies one-third of their total body volume. In addition, their extremely deep lower jaw bones aid red howlers in chewing. Alouatta seniculus also improve their digestive efficiency by feeding primarily on tender young leaves and on some species of leaves that are unusually nutritious. In addition, they eat sugary fruits and flowers when these are available, but can sustain themselves for weeks at a time on only leaves, providing these are high in quality. Alouatta seniculus spend almost their entire lives near the top canopy of the forest, where such leaves are most abundant (Devore, 1965) (MacDonald, 1985).

Due to their relatively large size, A. seniculus, along with other howler species, are hunted for food and are subject to commercial export (Nowak, 1991).

Although several other Alouatta species are endangered, A. seniculus has no special conservation status. However, red howlers have become rarer in some areas, most likely due to the destruction of their habitat. Fortunately, they are still adundant in Brazil (Nowak, 1991).

Red howlers have an amusing reaction to rainy days during the tropical wet season. In response to heavy rains, they howl, either at the onset, or often at the sound of approaching rain, and sit hunched over until the rain ends (Clutton-Brock, 1977)!

Red howler monkeys have overcome problems that are usually associated with having leaves as a principle food source, including their specialized jaw and stomach structures. Behaviorally, they (along with the other species in their genus) are unique in that they have developed the loudest vocalization of any animal in the New World. These adaptations have aided them in becoming an extremely successful primate--and yet they are still able to sleep for more than 15 hours a day!