Muriquis are total herbivores, eating mostly fruits and leaves (the southern species eats more fruits than leaves, while the northern species eats more leaves than fruits). A small amount of flowers, tree bark, buds, and seeds supplement their diet.

Southern muriquis consume food from 145 plant species. Their menu varies according to the season, linked to availability of their preferred food.

While the northern species practices geophagy, or “soil consumption”—they eat dirt—this behavior has not been documented in the southern species. 

Southern muriquis, like northern muriquis, are diurnal (active during daylight hours) and arboreal (spending most of their time in trees). Daily life for these monkeys consists of foraging, feeding, socializing, playing, and resting (about half their day is spent resting). Overnight, they sleep close together in the dense canopies of large trees.

Brachiation—swinging by their arms, hand over hand, from one tree limb to the next—is the main mode of travel for both species as they advance through the forest canopy. Southern muriquis typically move through the forest single-file while foraging, a strategy that reduces the risk of serious falls.

Muriquis are fast and agile. Both species are assisted by their long, prehensile tail. A bare patch of skin on the underside at the tip of the tail functions as a gripping pad, providing stability as the monkeys hang solely by their tail while feeding.

NOT HIPPY MONKEYS
Unlike their sister species to the north, whose modus operandi of peace, love, and group hugs has earned the northern muriqui the nickname “hippie monkey,” southern muriquis are much less touchy-feely. And while civility reigns in the north, hostile encounters—even acts of violence against one another—are not uncommon in the south.

But southern muriquis may suffer from an unjustly besmirched reputation. Blame it on the males.

In the north, where the species feeds foremost on leaves, an abundance of leaves typically keeps members of a group together in a big, loving tangle while they eat. Aggressive behavior is rare. But in the south, where the species feeds foremost on fruits, fruit trees are widely scattered, necessitating that members forage farther from the main group. When too many females disperse, the males who remain behind have fewer mating opportunities. They become sexually frustrated—and aggressive.

In the course of a long-term study of southern muriquis, researchers documented a deadly attack on an older adult male by a group of younger adult males. An adult female and two unidentified individuals also took part. Scientific speculation suggests that an imbalance in the monkeys’ environment, related to food sources, led to the dispersal of female members; which, in turn, led to behavioral imbalances within the group; which led to tensions relating to mating competition; which, ultimately, triggered the vicious ambush. (This attack shares commonalities with attacks by chimpanzees on other chimpanzees, where a lone chimp is murdered by members of his group.)

Researchers hypothesize that imbalances of power, like those observed in southern muriquis, lead to intraspecific competition—and killing—not only with muriqui populations but also with other primates, including chimpanzees and humans.