Howler Monkey

Howler monkeys (genus Alouatta monotypic in subfamily Alouattinae) are among the largest of the New World monkeys. Fifteen species are currently recognised. Previously classified in the family Cebidae, they are now placed in the family Atelidae. These monkeys are native to South and Central American forests. Threats to howler monkeys include human predation, habitat destruction and being captured for captivity as pets or zoo animals. These monkeys are famous for their loud howls, which can travel three miles through dense forest.

Classification
A. palliata group
Coiba Island howler, Alouatta coibensis
Alouatta coibensis coibensis
Azuero howler, Alouatta coibensis trabeata
Mantled howler, Alouatta palliata
Ecuadorian mantled howler, Alouatta palliata aequatorialis
Golden-mantled howler, Alouatta palliata palliata
Mexican howler monkey, Alouatta palliata mexicana
Guatemalan black howler, Alouatta pigra
A. seniculus group
Ursine howler, Alouatta arctoidea
Red-handed howler, Alouatta belzebul
Spix's red-handed howler, Alouatta discolor
Brown howler, Alouatta guariba
Northern brown howler, Alouatta guariba guariba
Southern brown howler, Alouatta guariba clamitans
Juruá red howler, Alouatta juara
Guyanan red howler, Alouatta macconnelli
Amazon black howler, Alouatta nigerrima
Purus red howler, Alouatta puruensis
Bolivian red howler, Alouatta sara
Venezuelan red howler, Alouatta seniculus
Maranhão red-handed howler, Alouatta ululata
A. caraya group
Black howler, Alouatta caraya
Anatomy and physiology

Male mantled howler
Howler monkeys have short snouts and wide-set, round nostrils. Their noses are very keen, and the Howler monkey can smell out its food (primarily fruit and nuts) up to 2km away. Their noses are usually roundish snout-type, and the nostrils have many sensory hairs growing from the interior. They range in size from 56 to 92 cm (22 to 36 in), excluding their tails, which can be equally as long, in fact in some cases the tail has been found to be almost 5 times the body length. This is a prime characteristic. Like many New World monkeys, they have prehensile tails, which they use for picking fruit and nuts from trees. Unlike other New World monkeys, both male and female howler monkeys have trichromatic color vision.[2] This has evolved independently from other New World monkeys due to gene duplication.[3] They have lifespans of 15 to 20 years. Howler species are dimorphic and can also be dichromatic (i.e. Alouatta caraya). Males are, on average, 1.5 to 2.0 kg heavier than females.

The hyoid of Alouatta is pneumatized, one of the few cases of postcranial pneumaticity outside Saurischia.

Locomotion
Howler monkeys generally move quadrupedally on the tops of branches, usually grasping a branch with at least two hands or one hand and the tail at all times. Their strong prehensile tails are able to support their entire body weight. Fully grown adult howler monkeys do not often rely on their tails for full body support, whereas juveniles do so more frequently.

Behavior

A Bolivian red howler (Alouatta sara).
Social systems
Most howler species live in groups of six to 15 animals, with one to three adult males and multiple females. Mantled howler monkeys are an exception, commonly living in groups of 15 to 20 individuals with more than three adult males. Unlike most New World monkeys, in which one sex remains in natal groups, juveniles of both sexes emigrate from their natal groups,[4] such that howler monkeys could spend the majority of their adult lives in association with unrelated monkeys.

Physical fighting among group members is infrequent and generally of short duration. However, serious injuries can result. Both males and females rarely fight with each other, but physical aggression is even more rare between sexes.[4][5] Group size varies by species and by location, with an approximate male to female ratio of one male to four females.[4]