The brown howler monkey, also known as brown howler, lives in forests in 

south-eastern Brazil and far north-eastern Argentina (Misiones). It lives in groups of two to 11 individuals. Despite the name “brown howler”, it is notably variable in colour, with some individuals appearing largely reddish-orange or black.

Two Subspecies have been recognized

• Northern brown howler, listed as critically endangered with fewer than 250 individuals restricted to the vicinity of the Jequitinhonha River. The northern brown howler is present in several protected areas, including the Mata Escura Biological Reserve, . Unfortunately, this reserve is close to rural settlements and subject to illegal logging, hunting and increased incidence of wildfires. The monkey may also be present in other protected areas, but this is uncertain, because the the other subspecies range limit is unknown -some populations that might be assumed to be Norther brown howlers have been found to be part of the far more numerous Southern brown howler. Of the total population, thought to number less than 250 animals, this is highly skewed towards the young, with perhaps only 50 mature individuals in the wild.
• Southern brown howler are native to southeastern Braziland far northeastern Argentina. Gregorin, 2006, considered the southern brown howler to be a separate species, Alouatta clamitans, but this has not been universally accepted. Its range is eastern Brazil, as well as adjoining parts of Argentina

Brown howler monkeys eat fruit and leaves ( mostly leaves).  Brown howler monkeys that live in higher latitudes tend to eat a lower percentage of leaves in their diet.  Mature leaves are less nutritious and are more fibrous, than the preferred young leaves. A typical brown howler diet will also include mature fruit, wild figs, petioles, buds, flowers, seeds, moss, stems, and twigs. The Atlantic forest, where brown howlers tend to live, has an increasing forest fragmentation. Forest fragmentation means that there would be a decrease in potential food supplies. The brown howler’s feeding ecology, however, does not seem to be affected by the loss in habitat.

Brown howler monkeys are part of the group of monkeys known for their howls/roars. The howlers larynx is enlarged and they have an enlarged open chest that creates a resonating chamber to increase the sound they can make. The howlers also have specialized vocal chords to produce the specific sound. The most frequent reason for the howling is for mate defence. Howling occurs most when there are both female and male howlers present. The males are the dominant group as they begin all cases of howling. Females participate in howling much less than males. Howling can also occur in groups during the dry season suspected to be due to food scarcity. The brown howlers use their sounds to show ownership over certain food or territory.

The black hawk eagle is a predator of the brown howler monkey, and their roar allows the hawk eagle to determine their location. The brown howler’s response has three parts. 

1. When one brown howler becomes aware of a black hawk eagle they will alarm the group and then will immediately become silent. Next they descend in the understory of the trees (making attack from above far harder), and finally they will all disperse in an organized manner. The adults will lead the young away from danger. The young are considered to be the primary target for the black hawk eagle (this is not surprising, as not only will young be weaker, but they are also easier to carry away to eat). There is a more conservative response when adult brown howlers are without the young, and the black hawk eagle is present, thus indicating that the black hawk eagles are targeting the young howlers. When the brown howler monkey is threatened by terrestrial animals they will remain in the tree canopy and remain silent for 5–15 minutes.

Brown howler monkeys rub one another for group communication and domination. The rubbing can be used for various purposes. Males will rub their hyoid bone and sternum against each other to show agonistic and territorial signals. Males will also rub females for sexual intentions. The males are considered to be the dominant over females because they rub much more often than females. Dominate females will rub more often than non-dominate females, but still much less than males.

It is difficult to breed any of the monkeys from genus Alouatta in captivity and therefore the reproduction is relatively unknown (this also makes the zoo population a less effective backup, should the species go extinct in the wild). Brown howlers reproduce year round. There seems to be no correlation to birth rates in rainy or dry season or in times of increased fruit or leaves. It is thought that because the brown howler has a folivorous diet, conception is less dependent on maternal conditions. The average interbirth interval for the brown howler is 19.9 months, which is similar to other howler species. It does not seem that the sex of the infant or the number of females in the population has a significant effect on IBI. The death of an infant will shorten the mother’s IBI and seems to be one of the few factors that affects IBI.

Brown howlers are highly susceptible to the yellow fever virus and have a high mortality rate when infected. When mass amounts of brown howlers are found dead it is a good indication that there may be a yellow fever outbreak occurring. Since the brown howlers have such a high mortality rate they are not considered to maintain the virus in their population. Communities that live near the brown howler populations have previously held the belief that the brown howlers were the cause of the disease, and would kill them to stop the spread of disease, however, given the fact that they die fast, it is highly unlikely that they are the cause of the virus, thought far more likely to be transmitted through mosquito species (interestingly the species that carries the yellow fever can vary from country to country.