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Guyanan red howler monkeys on the Brownsberg,
Howler monkeys (genus Alouatta) are among the largest of the New World monkeys. They are famous for their loud howls, which can travel 5 km through dense rain forest. These monkeys are native to South and Central American forests. Threats include human predation, habitat destruction, and capture for pets or zoo animals. Fifteen species are recognized.
Anatomy and physiology
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, excluding their tails, which can be equally long. Like many New World monkeys, they have prehensile tails, which they use while picking fruit and nuts from trees. Unlike other New World monkeys, both male and female howler monkeys have trichromatic colour vision. This has evolved independently of 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 typically 1.5 to 2.0 kg heavier than females.
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, but juveniles do so more frequently.
Social systems
Most howler species live in groups of six to 15 animals, with one to three adult males and multiple females. The number of males in a given group is inversely correlated with the size of their hyoids and is positively correlated with testes size. This results in two distinct groups, wherein one male with a larger hyoid and smaller testes copulates exclusively with a group of females. The other group has more males, which have smaller hyoids, and larger testes, and free copulation occurs among the group. The larger the number of males, the smaller the hyoid, and the larger the testes. Unlike most New World monkeys, in which one sex remains in natal groups, juveniles of both sexes emigrate from their natal groups, 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, but serious injuries can result.
Communication
As their name suggests, vocal communication forms an important part of their social behaviour. They each have an enlarged basihyal or hyoid bone, which helps them make their loud vocalizations. Group males generally call at dawn and dusk, as well as interspersed times throughout the day. Their main vocals consist of loud, deep, guttural growls or "howls". Howler monkeys are widely considered to be the loudest land animals. According to Guinness Book of World Records, their vocalizations can be heard clearly for 4.8 km. The function of howling is thought to relate to intergroup spacing and territory protection, as well as possibly to mate-guarding.
Diet and feeding
These large and slow-moving monkeys are folivores. Howlers eat mainly top canopy leaves, together with fruit, buds, flowers, and nuts. They need to be careful not to eat too many leaves of certain species in one sitting, as some contain toxins that can poison them. Howler monkeys are also known to occasionally raid birds' nests and chicken coops and consume the eggs.
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