New World Monkeys
The six species of howler monkeys in the subfamily Alouattinae possess a throat swelling with a special form of hyoid bone (the bone supporting the tongue muscles) that allows them to produce a deep, thundering roar, which can be heard more than 2 mi (3 km) away. Because of this throat swelling, their lower jaw juts forward more than in most other species of monkey. This jutting is not as noticeable as it might be, however, because the howlers are bearded. The throat is larger in the males than the females, and it is largest in the dominant male in a group of monkeys containing several males.
Howler monkeys are quite large, weighing up to 20 lbs (9 kg) and measuring 33 in (91 cm) long, with an equally long and very strong prehensile tail, which is naked on the end third. Howler monkeys use their prehensile tail to attach themselves firmly to branches, and they may hang from their tail to keep their hands free while feeding on leaves. The mantled howler monkey (A. palliata) has to eat so many leaves to get the nourishment it needs that its intestines make up one-third of the volume of its body. The mantled monkey is found from southern Mexico south to Ecuador and has a black body with a gold mantle, or fringe, down the sides.
Howler monkey species vary primarily in their coloring. The Guatemalan or Mexican black howler monkey (A. villosa) is completely black, from the bare base to the tip of its tail. All the howlers have naked black faces, but the black-and-red howler (A. belzebul) has red hands and feet as well as tail tip. The red howler (A. seniculus) is bright copper. Only the black howler (A. caraya) has different forms of the males and females. The young of black howler monkeys are born golden-brown, and the males turn black as they mature. A howler group contains up to 20 monkeys, with two to four males and five to ten females in the group. A single young howler is born after a gestation of about 180 days. It clings to the mother's abdomen for the first several weeks until its tail matures enough for it to be able to cling to its mother's tail. It then moves around and rides her back, where it remains for a year or more. Howler females reach maturity at about five years, and males at six to eight years.
The mantled, Guatemalan, or brown howler monkey (A. fusca) is an endangered species. Although some howlers live higher in the mountains, this lowland species is losing its habitat to logging and agricultural conversion. They are also hunted, and individual animals are easily caught because their loud voices betray their location.
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