The brown howler (Alouatta guariba), also known as brown howling monkey, is a species of howler monkey, a type of New World monkey that lives in forests in southeastern Brazil and far northeastern 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.
There are two subspecies: (1) Northern brown howler (A. g. guariba), listed as critically endangered and (2) Southern brown howler (A. g. clamitans).
After decades of hunting and habitat loss, only 35-40 Northern brown howler monkeys still survive in Brazil's Atlantic Rainforest. During the last five decades, 93% of this ecosystem has been destroyed by logging and related development. One of its last intact forests, the Serra Bonita Reserve in Bahia, Brazil, is home to five endangered primates, including the Northern brown howler.