Howler monkeys are among the largest of the New World primates, weighing 6 to 9 kilograms when adult. They have one of the widest geographical distributions of any New World primates, being found all the way from southern Mexico to northern Argentina. Six species of howler monkey are recognized–one only in Central America, one in Central and South America and four only in South America. The howler species found on Barro Colorado Island is Alouatta palliata, the mantled howler monkey, so-called because of the long reddish ruff of hair along the flanks of adults. This species is found throughout Central America, extending south into Colombia. All howler species appear to occupy the same dietary niche. They are vegetarian, eating new leaves, fruits and flowers for much of the year. But when fruit is in short supply, which on Barro Colorado Island generally occurs at the end of the rainy season and during the transition into the dry season–late October, November, December–howlers are able to live for weeks or months at a time on diets composed entirely or almost entirely of leaves. Howler monkeys have large sections in their gastrointestinal tract where the cellulose and hemicellulose of leaves is broken down by bacterial colonies. This process, known as fermentation, produces energy-rich fatty acids which are used by howler monkeys to help fuel their daily activities.
Visitors to Barro Colorado Island will inevitably hear howler monkey calls, particularly in the early morning and late afternoon but also at other times of day. In fact, the most outstanding characteristic of howler monkeys is their howl– a long, drawn-out sonorous call produced by drawing air into an enlarged hyoid bone in the throat. This vocalization, produced primarily by males, is regarded as the loudest call of any Neotropical animal and can be heard for a distance of two kilometers or more under appropriate conditions. All howler monkey troops typically give this call early in the morning in a type of “dawn chorus” which serves to let other howler monkey troops in their general area know their precise location. Howler troops dislike one another intensely and tend to fight if they come into contact. By howling, troops are able to space themselves efficiently throughout the forest canopy and avoid energetically costly and dangerous fights with other groups. Howler monkeys also tend to howl in the late afternoon to announce their sleeping site as well as prior to heavy rain storms.
On Barro Colorado Island there are an estimated 60 howler monkey troops, averaging 19 monkeys per troop or around 1100-1200 monkeys in total. Most troops are composed of some 3-4 adult males, 7-10 adult females, 2-3 juveniles, and 3 to 5 infants. Female howlers give birth to a new infant every 18 to 24 months. In A. palliata, these babies are pale cream in color and are carried on the belly of the mother until they are a few weeks old. Then their fur begins to darken and they move to the mother’s back. A new howler monkey baby is intensely attractive to other troop members, particularly adult females, and a new mother is constantly harassed and pressured by other members of the troop who want to sniff, look at and touch the new infant. The single infant is carried by the mother until it is around six months old. After the infant completes its first year, it is largely independent of the mother and moves about with the troop on its own.
On BCI, howler monkeys eat an average of 7.7 plant species per day– 5.1 leaf species, 1.7 fruit species and 0.8 flower species; over the course of an annual cycle, foods are taken from more than 125 plant species, largely canopy trees. All howler monkeys show a strong preference for foods from the plant family Moraceae, a family which includes the genus Ficus or “fig”. On BCI, you will often find howlers in huge, tall wild fig trees as they eat both the tender new leaves and fruits of all of the fig species on the island. Because howlers often eat a lot of leaves, which are very low in sugars, they are energy conservers and on BCI troops spend an average of 66% of their daylight hours quietly resting and snoozing. Howler home ranges overlap with one another, each troop using about 32 hectares of the forest over the course of a year.
Howler troops on BCI are persistently infested with larvae of a parasitic fly, Alouattamyia baeri, the howler monkey bot fly, a host-specific species which apparently can only live on howler monkeys. These larvae tend to be localized in the throat region of howlers and can generally be seen with the naked eye. Having only a few larvae does not appear to harm the howler host to any notable degree, but if a monkey is re-infested repeatedly or is in poor physical condition, the cost of feeding multiple larvae may prove too high for the monkey to sustain and it may become ill or die. Howler monkeys die in highest numbers during the mid-to late rainy season on Barro Colorado Island (August through November), the time of year when nutritionally rich plant foods are in very short supply and howler monkeys appear least able to support the cost of bot fly larvae.
© Katharine Milton, Ph.D. 1998