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Howler monkeys are
arboreal and herbivorous: they live in rainforest trees and eat leaves, flowers and fruit. They are large, stocky, and covered in thick black, reddish, or olive brown fur. Some species have "saddles" of light brown or golden fur across their shoulders and backs. Their faces are mostly naked with prominent jaws, short snouts, and small beards.
Adult males weigh 13 to 20 pounds; females weigh 9 to 13 pounds. Bodies are 20 to 36 inches, plus another 20 to 36 inches of prehensile tail! Howler monkeys use their long tails like a fifth hand when they are climbing or feeding.
Those amazing prehensile tails! Howler monkey tails are long, strong, and sensitive. Furry on top, they are partially to mostly hairless on the underside. Monkeys use them like hands to grasp and identify objects. They can also curl them around tree branches and hang happily while they eat or rest.
Among monkeys, the howler's unique characteristic is an enlarged hyoid bone and larynx. The hyoid bone is located at the base of the tongue. In howler monkeys, the hyoid is large and shell-like. It connects with the windpipe and amplifies the monkey's voice. Adult monkeys have an enormous roar that can be heard up to three miles away. The only mammal louder than a howler monkey is the blue whale.
Why Do They Roar?Howler monkeys are highly social and territorial. For most of their lives, they live in family troops of a few males, several females, and their offspring, or in smaller troops of bachelors.
To stake out foraging grounds, troops communicate their location in the forest with loud, throaty howls. At some times of year they conduct daily dawn or dusk "choruses"--hugely noisy shouting matches with other troops. By claiming foraging grounds and broadcasting locations, they avoid unnecessary squabbles over limited food sources. Territories are also marked with monkey dung, which is heaped in large piles or rubbed onto branches.
Occasionally territories overlap. This may make it easier for young monkeys to join new troops, in some species a necessary first step towards mating.
Do Howler Monkeys Communicate in Other Ways?Like most monkeys, howlers are "talkative." Among their own troops, they vocalize in barks, woofs, grunts, and growls, sending messages of anger, warning, or affection. They also spend many hours every day grooming each other. Status within the troop is communicated through social grooming--higher status monkeys groom lower status monkeys. Their resonant voices carry through the trees even when they are not howling. This helps all members of the troop stay in touch when males are high in the tops of the tallest trees or when the troop is spread throughout its territory.
They Live in Trees, So Where Do They Go for Water?Howler monkeys seldom drink water. They get all the moisture they need from vegetation, which means that they must eat a lot every day. In dry seasons, adults may eat up to 15 percent of their body weight in leaves and fruit. In very dry seasons they may descend from the treetops to drink from rivers or lakes or to browse on water-rich marsh plants. They may also take small drinks from the tiny pools that form in the leaf axils of bromeliads (rainforest plants that grows on tree branches).
Fig-eating howler monkeys are primary seed dispersers. They drop fig seeds in their dung onto tree branches, where the seeds sprout and send roots down to the ground. Dung beetles--important secondary seed dispersers--eat monkey dung and carry fig seeds further from the mother tree.
How Do They Digest All That Plant Material?The howler monkey's digestive tract is specially formed to process a diet high in cellulose. They prefer tender leaves, found in the upper canopy where male monkeys spend most of their days. Their molars are large, designed to grind, and they have big salivary glands. Their intestines contain beneficial bacteria that help break down cellulose.
They Must Spend a Lot of Time Eating!In fact, howler monkeys spend only about 20 percent of their day grazing (just under 5 hours). They move slowly through the forest canopy using all four limbs and their tails to grasp branches, and they often suspend from their tails while reaching and feeding.
Adult male monkeys sometimes make great leaps from branch to branch and between trees. Females and younger monkeys are more likely to scramble up and down trunks. All the monkeys in a troop spend a total of an hour or so every day focused on communication, both within their troops and with other troops in their dawn choruses.
What Do They Do the Rest of the Time?They lounge and sleep and DIGEST their meals. Howlers spend the largest portion of their energy digesting all those leaves. So they must lie about and nap a lot. They're diurnal, which means they're awake during the day and asleep at night. But they're often asleep during the day, too, dozing and digesting. Babies and juveniles, whose diets are wholly or partly made up of their mothers' milk, spend lots of time playing.
Don't They Have Anything to Worry About?Howler monkeys are wary of being eaten. Large birds, including harpy eagles, prey on them, and troops must be careful not to let birds carry away baby monkeys. They are also hunted as food for humans, but not very often nowadays.
They compete for leaves with other folivorous (foliage-eating) species and with leaf cutter ants. In some forests, ants and monkeys may become arch rivals for the best leaves. Monkeys solve this problem in part by staying high in the highest tree tops, where ants are less likely to go.
Who Takes Care of the Baby Monkeys?Babies are completely dependent on their mothers for the first few weeks of their lives. They begin eating leaves and using their tails for climbing when they are about one month old and learn to forage soon after. By the time they are three months old, they are moving around and exploring independently. For the first year of their lives, they are cared for by their mother and all the older females in their family troop. After that, they're on their own.
In some species, fathers help take care of babies--but infants are always protected from young males, who will mistreat or even kill them. Sometimes male monkeys entering a new troop will kill all infants.
Monkey's reach maturity when they are three or four years old. Males and females must leave their family groups and live alone in the forest for a year or two. After they are accepted into new families, they must make their way up the social hierarchy before they are allowed to mate.
In their native habitat, howler monkeys may live as long as sixteen years. Old females continue to live with their family troops. Old males are forced out by younger males or leave on their own initiative to live out their last days in peaceful solitude.
Species and Range.Scientists believe that early monkeys arrived in the New World thirty million years ago. They evolved into at least 53 species in two families, including marmosets, tamarins, spider monkeys, capuchin monkeys, howlers and many others--all arboreal and herbivorous. Howler monkeys--the largest New World monkey and the most studied--live in rainforests throughout Central and South America. They are classified in the order Primate, family Cebidae, genus Alouatta. Species include palliata, caraya, seniculus, fusca, and nigra.
In the last decade, scientists have identified ten previously unclassified monkey species in Brazil. On April 22, 2000, Conservation International announced the discovery of two squirrel-sized marmoset species in the Amazon basin.
Are Howler Monkeys Endangered?Some howler species are listed as vulnerable and/or endangered. Their principal threat is loss or fragmentation of habitat. In some areas, reduced populations are recovering by moving into regenerated forests and shade coffee and cacao plantations.
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