The Howler is the largest of Costa Rica’s monkeys, and despite its ferocious growls, bark and roars, the least aggressive. With a standing height of about 20 inches and weighing up to 11 pounds, the Howler or Congo as it is named in Spanish, spends most of its life high in the tree canopy.

A Diet of Leaves
Howlers are folivorous eating mostly leaves, but their diet also includes flowers and fruit. Leaves offer little nutrition and are often toxic: the monkeys seek out young leaves which have more proteins, less cellulose and fewer toxins. Leaves take a long time to digest and Howlers, lacking any enzyme which breaks down cellulose, ferment digesting leaves in their gut (sloths have a similar practice).

A Sedentary Life
Digestion is a slow process and Howlers spend up to 80% of their day and all night resting. They like to spread themselves out in the forks of branches, limbs dangling, waiting for the early morning sun to warm their bodies and speed up their internal fermentation. They sleep high in the tree branches, often spread through neighboring trees. During the day they conserve energy and can spend all day in one small area or even one large canopy tree. Their range is comparatively small (only up to 190 acres). This may explain why they continue to be successful in Costa Rica despite shrinking forests and increased development.

A Social Group
As many as 40 can live together, though it is more commonly around 15 – 20 individuals. Hierarchies exist within each group, but Howlers are unusual amongst monkeys in that it is not the oldest pair which dominate. Rather both dominant male and female are the youngest adults in the community. This means that each Howler will have the opportunity to be alpha – for the more plentiful females, their period of dominancy lasts roughly a year. Due to the fewer number of males in each troop, a male will be dominant for roughly 4 years of his life. The oldest monkeys are at the bottom of the hierarchy. Dominant pairs have access to the best food and sleeping positions.

Offspring
One or sometimes two offspring are born after a gestation period of 6 months. The young are born throughout the year and spend the first three months of their lives clinging to their mother. They wean at around one year. Other females help raise the babies. Female offspring remain with the group but males leave the troop sometime after their first year and spend 2 to 4 years alone or with other young males. When a young male attaches himself to a troop he will spend time on the periphery slowly socializing with lower members before fighting the alpha male for dominancy. Fights are mostly verbal though sometimes end in a very brief physical skirmish. If the new male is successful he will kill all unweaned babies and mate with all the females, thus ensuring his offspring will have a favorable troop position (unless another male comes along).

Making Noise Together
Otherwise Howlers are fairly peaceful monkeys who would rather co-exist than spend energy fending off intruders. Their incredibly loud vocalizations allow them to communicate amongst each other and with other troops without moving. The vocalizations are possible because of the Howler’s hyoid bone – a hollow bone set alongside the vocal chords which acts like a drum. Howling is an inaccurate way to describe the noise: males will make a gurgling, pulsing roar and bark; females a softer, higher pitched bark. They have a pre-dawn chorus and can be heard in the late afternoon. They also respond throughout the day to other noises – another troop, barking dogs, engines, even the eruption of volcanoes. Howlers will make noise at the beginning of heavy rain showers. The sound of a male monkey can travel up to 3 miles through even dense rainforest.