[3] Despite the name "brown howler", it is notably variable in colour, with some individuals appearing largely reddish-orange or black.

The region spreads through the Brazilian states of Bahia and Espírito Santo through Rio Grande do Sul, and Misiones in Argentina.

Brown howler monkeys that live in higher latitudes tend to eat a lower percentage of leaves in their diet.

A typical brown howler diet will also include mature fruit, wild figs, petioles, buds, flowers, seeds, moss, stems, and twigs[2] The Atlantic forest, where brown howlers tend to live, has an increasing forest fragmentation.

First, when one brown howler becomes aware of a black hawk eagle they will alarm the group and then they will immediately become silent.

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.

When mass amounts of brown howlers are found dead it is a good indication that there may be a yellow fever outbreak occurring.

In order to protect the brown howlers the local communities should limit their killing and become vaccinated to prevent the disease from spreading.

In Argentina, the species that has been shown to carry the yellow fever virus (YFV) is Sabethes albiprivis.

[5] In 2008–2009 there was a yellow fever outbreak among a brown howler study group in the protected Misiones, El Piñalito Provincial Park.