Howler monkeys have short snouts and wide-set, round nostrils.
Their noses are very keen, and they can smell out food (primarily fruit and nuts)  up to 2 km 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 five 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.
This has evolved independently from other New World monkeys due to gene duplication.
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 the Saurischia.
The volume of the hyoid of male howler monkeys is negatively correlated with the dimensions of their testes.
Howler monkeys have short snouts and wide-set, round nostrils. Their noses are very keen, and they can smell out food (primarily fruit and nuts)  up to 2 km 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 five 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. This has evolved independently from other New World monkeys due to gene duplication. 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 the Saurischia. The volume of the hyoid of male howler monkeys is negatively correlated with the dimensions of their testes.
Howler monkey