Illustration of a researcher looking up at monkeys
Illustration: Fernando Cobelo

When I set out to study one of the last remaining populations of the critically endangered northern muriqui monkey in a small forest fragment in southeastern Brazil more than 40 years ago, my goals were to understand these enigmatic primates from within a comparative evolutionary perspective and contribute to conservation efforts on their behalf. It never occurred to me that it would mark the beginning of an international collaborative research and conservation project that is ongoing today.

Those first months in the forest were focused on winning the muriquis’ trust. This meant long days of climbing up and down steep trails, covered in scratches from thorny plants and itchy insect bites, listening for the swooshing sounds of the branches rebounding as the monkeys swung through the canopy. My efforts to locate them were also guided by their vocalizations, which resemble a horse’s whinny, and by the spicy cinnamon scent they left in their wake. It seemed that while I was learning their habits, the muriquis were learning mine. It was only after their wariness gave way to tolerance that I gained my first glimpses into their uniquely peaceful way of life.

Karen Strier
Karen B. Strier is a Vilas Research Professor and Irven DeVore Professor in the Department of Anthropology.

Unlike other primates, northern muriquis live in egalitarian societies in which aggression is rare and inter-individual conflicts are resolved by mutual avoidance or reassuring hugs. They mate in full view of one another, without threats or interference, and females often choose to mate with multiple partners in close succession. Males spend their entire lives in their natal groups, along with their mothers and male relatives, whereas females disperse before the onset of puberty to join new groups where they establish their reproductive careers. Although patrilocality also occurs in primates such as chimpanzees and bonobos, muriquis are the only ones that combine patrilocality with such high levels of tolerance among and between the males and females in their groups.

Comparative studies have since confirmed that the muriquis’ behavior is similar in other populations living under different ecological and demographic conditions. Their egalitarian society has also persisted across generations, even as my study population grew from some 50 individuals in two social groups to more than 350 individuals in five groups, and then declined to its current size of 230 members. Instead of increasing their antagonism toward one another, the muriquis maintain their practice of avoiding direct competition by splitting into smaller subgroups and by expanding their vertical niche to include the ground.

Deciphering which parts of the muriquis’ behavioral repertory are flexible and which, like their pacifism and female dispersal patterns, are resistant to change, has been a slow but incredibly rewarding process. These discoveries have stimulated new questions and new perspectives about muriquis and other primates, and they are directly informing the Brazilian-led conservation management programs now underway. 

Yet despite these advances, the future of the northern muriqui is still far from secure. Fewer than 1,000 individuals are known to live among only a dozen of the last standing tracts of Brazil’s Atlantic Forest, and even populations inhabiting protected areas are increasingly at risk from the impacts of climate change. While the muriquis have captivated my scientific curiosity for more than four decades, they have also revealed what they need to survive. By establishing forest corridors to connect isolated populations, we can provide the muriquis with the protected routes they need to move freely across the landscape as local conditions change. We have the knowledge we need to save them from extinction; now it is just a question of persistence and a race against time.

Read more about Strier’s research here.

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