Pithecia monachus

Geographic Distribution and Habitat

The monk saki, also known as Geoffroy’s monk saki or Miller’s monk saki, is a New World monkey that inhabits multiple countries in the jungle-strewn and biodiverse regions of South America. The monk saki can be found in the upper elevations of rainforests in Brazil, Colombia, Ecuador, and Peru.

The monk saki is an arboreal, high-canopy dweller—you’d have to tilt your head quite a ways to spot them 32–115 ft (10–35 m) up in the tallest trees of the forest. They can be the only primates inhabiting such a private and elevated niche. This species is difficult to study in the wild due to their shy and wary nature (and the fact that they can hide in trees that are 5–20 times taller than the average human). Hopefully with the advent of camera traps, the enigmatic monk saki will start sharing more of its secrets.

The home range of the monk saki differs by gender—males travel within a 123-acre (50 ha) area, while the females utilize only 82 acres (33.5 ha) of a similar area.

Monk saki geographic range, IUCN 2018

Size, Weight, and Lifespan

Monk sakis measure 12–20 in (30–50 cm) in head and body length. Their bushy, non-prehensile tail adds another 10–22 in (25–55 cm). They weigh around 2 to 4 lbs. (1–2 kg).

Their lifespan is not well documented, but it can be anywhere from 14 to 24 years in captivity.


At all times looking desperate for a haircut, the monk saki is a smaller-sized primate with a dark and woolly pelage. Their partially bald face is circled by a thick fringe hairdo reminiscent of their namesake—a monk. Their body is generally black with lighter gray hands and feet, which makes them appear aged, and they are capable of gripping branches with long and deft fingers. Their short hind legs are designed for leaping quickly over long distances, which is ideal for life high in the trees.

The tail of a monk saki is just as coarse and shaggy as the rest of the body. It is non-prehensile and therefore often sags over the edge of branches like an exceptionally dirty feather duster.


The monk saki is mainly frugivorous (fruit-eaters), but also subsists on a diet of seeds, nuts, and some insects. They are capable of using their large canines to break the hard skin of fruits before ingesting them. They have also been documented preying on some small animals like mice, bats, and birds.

Behavior and Lifestyle

This little primate is very nervous around humans and abnormal disturbances; so, as aforementioned, there is very little data pertaining to the everyday life of a monk saki. However, there are a couple details that are easy to discern.

They are diurnal and quadrupedal leapers—they generally move on all fours but may also teeter bipedally on large branches before leaping to another tree. They are also totally arboreal and will never be seen descending all the way to the forest floor, but occasionally—and perhaps in search of food—they will check out the lower levels of the tree canopy.

Fun Facts

The monk saki weighs as much as a large rabbit. 

A monk saki is one of seven different species of arboreal New World monkeys that has a non-prehensile and furred tail.

Monk sakis do not survive well in captivity, which is one of the reasons why we know so little about them.

Daily Life and Group Dynamics 

During the day, monk sakis move in pairs or small family groups within their home range. Groups are composed of both males and females, though it is not determined if there are any hierarchies.

They feed as they go and engage in allogrooming, mutual grooming that is purely sociable in purpose.


​The monk saki is capable of emitting highly specialized vocalizations. The adults can recognize their mate by a specific vocalization, but otherwise many of the ways monk sakis communicate appear to be designed to express aggression. Squeaks, whistles, and trills are low intensity, like an initial warning call, while barks, grunts, and roars indicate more aggressive intentions.

Reproduction and Family

Adult monogamous pairs of monk sakis mate for life. They will raise a single offspring during each breeding season and retain a family group size of 4–5 individuals on average. The pair will raise their young within their home range, which is typically a defended territory (hence the aggressive vocalizations mentioned above).

Females are in estrus for approximately 18 days and gestation for approximately 170 days.

​Ecological Role

Due to their diet, monk sakis disperse the seeds of the fruits they consume, which in turn benefits the local ecosystem.

Conservation Status and Threats

The monk saki is rated Least Concern by the International Union for Conservation of Nature Red List of Threatened Species (IUCN, 2015). The IUCN describes their population trend as “decreasing,” though there are still believed to be substantial populations of monk sakis in the upper parts of the Amazon basin.

Threats to the monk saki include habitat loss (residential and commercial development and logging) and biological resource use (hunting and trapping). The monk saki is classified as a food source for locals and they are occasionally victims of the pet trade.

Conservation Efforts

Luckily, there are restrictions on the international trade of these primates or any of their parts.

Additionally, the monk saki’s population is not severely fragmented, which enables them to thrive more as a species than other primates that live in habitats drastically altered by deforestation. The fact that the monk saki’s habitat is left intact can likely be attributed to the in-place land protections that exist over their entire range.


Written by Rachel Heim, February 2019