Geographic Distribution and Habitat
Monk sakis, sometimes called Geoffroy’s monk sakis, are endemic to Brazil and Peru, with some reported sightings in Columbia and Ecuador. They are mainly found in the mature rainforest near the Jurua and Japura-Caquerta rivers in Brazil to the Andes Mountains in Peru. They prefer to use the high, dense forest canopy but travel to the lower levels to search or forage for food.
Taxonomic classification problems with monk sakis have existed since the species was first described in 1812. During the 200 years that followed, confusion surrounding monk saki taxonomy has resulted in what some researchers call the “monachus mess.” The original taxonomy for the genus Pithecia was published in 1987 and described 5 species, including the monk saki. Science followed this classification for almost 40 years before another study, by Laura Marsh in 2014, proposed that there are 16 species in the genus Pithecia. This proposal is still being debated. So far, taxonomic studies have drawn inferences from museum skins and specimens. Preserved museum specimens can lose so much detail, such as fur color, that it makes the job of differentiating species difficult. This makes taxonomy classifications complicated. Currently, most scientists follow Marsh’s classification for the genus in general and the monk saki in particular.
Size, Weight, and Lifespan
Monk sakis are medium-sized monkeys with a body length (head to the base of their tail) between 14.5 and 19 inches (37–48 cm). Their tails are usually just as long at around 16–20 inches (40–50 cm). This sturdy, bushy tail does not grasp objects (it is not prehensile), but is used to balance the sakis as they walk and jump through the canopy.
On average, adult monk sakis can weigh 3.7 pounds (1.7 kg). When they are born they weigh about 4 ounces (121 g).
In the wild, a monk saki’s lifespan may range between 14 and 18 years. A female monk saki lived in a zoo for 20 years and so experts think that monk sakis can live up to 25 years in captivity.
You can identify sakis by their long fur, round faces, bushy tails, and broad noses. Monk sakis have gray-black coarse fur all over their body with the exception of their pale hands, feet, and face. They have a shaggy cap of fur on their head that looks like a poorly fitted wig. When they are frightened or threatened, their fur can stand on end (much like when we get goosebumps). This is called “piloerection” and it makes the monkeys look puffed up and larger than they are, which is meant to intimidate potential predators or other threats.
Males can be larger than females and have slightly different coloration, but you can hardly tell the difference in the wild. Males have a distinct yellowish stripe of fur beneath their eyes, called a malar stripe, and pale fur around their faces. Females have thinner malar stripes and longer hair on their heads, which make their faces look smaller.
One recent study suggests that a rare white-faced color morph of the monk saki exists in Peru.
Monk sakis are herbivores and eat mostly fruits and seeds. They crush seeds with their powerful jaws and molar teeth. They also eat insects or arthropods, like army ants and spiders.
Fruits are high in sugars and offer quick-releasing energy that is great for bursts of activity, like leaping through the canopy. Seeds, on the other hand, are good sources of fat that release higher amounts of energy but require more digestion time. Insects supplement this diet as sources of protein. But catching insects requires energy, and finding insects is less predictable than finding fruits and seeds. The monk saki’s ability to take advantage of these different food sources means they can find sufficient food within a small area and they do not compete with other primates for the same food resources.
Behavior and Lifestyle
There is a considerable knowledge gap for most aspects of monk saki behavior. They are shy and prefer to be high up in the safety of the canopy. Monk sakis tend to quietly slink away at the first sign of perceived threats, such as the presence of curious researchers. It can take months for a troop of sakis to become habituated, or accustomed to, researchers being in their territory. By contrast, other primate species may become habituated within weeks. Once they are habituated to the presence of researchers, the monkeys go on with their lives normally, as if the human intruders are not even there. It is only then that field biologists can observe the sakis’ natural behaviors.
Saki monkeys are diurnal (active during the day) and arboreal (tree-dwelling). Monk sakis rely on daylight hours to find fruits and travel through the canopy. They usually walk along tree branches using all four limbs (quadrupedally). When they need to get to another tree, they use their strong forearms to swing their bodies and land on branches with their feet. In many areas, sakis are called “flying monkeys” because of their ability to soar from one tree to another.
Sakis are called “flying monkeys” because of their amazing leaps across canopies.
These extremely shy monkeys are challenging to study and there is a large gap in our knowledge of their behavior and ecology.
Saki monkey taxonomy was so confusing that it was referred to as “the monachus mess.”
Monk sakis form tight-knit groups of 2–6 family members. In some regions, they tend to stay close to home, traveling less than 1.6 miles (2 km) a day. However, the size of their home range depends on seasonality and the availability of fruit. They will travel further when needed. They may form fission-fusion groups in which multiple troops forage together but then go their separate ways in smaller groups later. Up to 17 individuals have been observed in an area of less than .5 square miles (1 square km). Some field researchers think that monk sakis may not be particularly territorial because the monkeys were not consistently found in the same places even when the areas were surveyed over multiple years.
There is not a lot generally known about wild monk saki populations. One study reported that monk sakis spent 30% of their time in activities related to feeding (either eating or searching for food). They spent almost the same amount of time resting. Given that monk sakis rely on mostly fruit and seeds, which either offer quick bursts of energy or require long digestion times, the large amount of time dedicated to foraging and resting makes sense. They spent slightly less time traveling through the forest (about 25%). As shy animals, they spent 11% of their time being alert and looking out for potential threats.
Monk sakis are not aggressive and appear to spend little time in social interactions. Adults seem happy to be with each other without a great deal of grooming or otherwise apparent social activities. However, it is important to note that since there are few long-term studies of wild monk saki behaviors and activities, we need to exercise caution regarding the degree to which we base all monk saki monkey behaviors and activities on a few observations of these complex primates.
Monk sakis communicate using low grunts, chirps, and whistles. Wild males have been observed emitting loud alarm calls to warn family members of danger. However, in captivity, they seem to be quiet most of the time. As it turns out, the manner in which saki monkeys behave in captivity can be quite different than how they behave in their natural habitats.
Male and female monk sakis have scent glands on their necks that they rub onto tree branches. Researchers think that when other monk sakis smell the gland secretions they can tell whether they belong to a male or female and if they are an adult or maybe even if they are ready to mate.
The family unit of monk sakis is still a bit of a mystery. Much of what we know comes from captive individuals or from studies of the closely related white-faced saki (Pithecia pithecia). Some studies suggest that monk saki adults form monogamous bonds (in which an adult male and female mate with each other for life). This notion was primarily informed by captive saki monkey behaviors. But, as we said, it’s been learned that captive sakis behave differently than wild sakis. After further wild monk saki studies, scientists now think their initial understanding of saki society is probably underdeveloped. They have reported monk sakis in multi-adult groups with as many as nine individuals in a troop. It is possible that the main family unit is formed by a male and female breeding pair with their young and multiple generations of juveniles. Alternatively, family groups could contain multiple unrelated adults.
Monk sakis may breed seasonally, meaning that there are specific months when they mate and give birth. In many primate species, births are timed with the fruiting season when food is plentiful. Monk sakis tend to give birth to one infant at a time. Babies are carried across the mother’s belly as she travels through the high canopy.
As noted, wild monk sakis are shy and live high in the forest canopy. It is, therefore, difficult to observe them for long periods of time. Plus, it is almost impossible to distinguish adult males from females. This makes them challenging observation subjects. Field researchers need to rely on innovative techniques, like remote cameras, to study monk sakis. As of now, more research needs to be done before we can make definitive claims about how monk sakis interact in their natural environment.
Monk sakis peacefully interact with other primates, like red howler monkeys (Alouatta seniculus) and Bolivian squirrel monkeys (Saimiri boliviensis). The monk saki diet trait of consuming fruits, seeds, and insects means they can share their habitat with other primates and herbivores without competing for food resources. This phenomenon is what ecologists call “resource sharing” or “niche partitioning.” This is one reason we find so many primate species coexisting in the Amazon rainforest.
Primates are often credited with helping trees reproduce by either pollinating flowers or spreading seeds. We do not know enough about monk saki behavior to say if or how they have a role in maintaining forest tree species in their habitat. Sakis take a long time to digest their food. The seeds they eat are almost completely digested in their stomach. This means they may have a limited role in dispersing these seeds and helping trees regenerate in the forest. However, for those specific tree species where monk sakis eat only the fruit and throw away the seeds, it is possible they have some role in seed dispersal.
Monk sakis are part of the Amazon food web and play ecological roles as predator and prey. As seed predators, monk sakis help control the overpopulation of certain tree species and allow other plant species to establish and grow. Monk sakis are also a food source for predators such as leopards. This may explain why they spend so much time in the high canopies of the forest.
The International Union for Conservation of Nature lists the monk saki as Least Concern (IUCN, 2015), appearing on the IUCN Red List of Threatened Species. While there is little known about their populations, most of their habitat is still intact. Therefore, monk sakis are not considered to be at risk at this time. We do know, however, that their populations are decreasing.
The dominant threat for monk sakis is habitat destruction. Like other species in the Amazon forest, deforestation by logging companies and human development threaten food and space availability.
Monk sakis are commonly hunted by locals for bushmeat. They are also kept as pets. In traditional Amazon communities, sakis’ bushy tails are used as dusters and their skins are used to make hats.
Studying monk sakis and other saki species is a challenge. In order to get reliable scientific data, field researchers need to collect data over long periods of time on multiple individuals. These primates live in dense forests and high in the canopy, making them difficult to reach. Monk sakis are also shy and hard to locate. Once spotted, they quickly run away. As a result, researchers have a hard time observing monk sakis for extended periods of time. These challenges mean that the conservation status of monk sakis is uncertain, and more long-term studies on wild populations are needed throughout their distributed regions.
The monk saki is listed in Appendix II of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival. Appendix II means that monk sakis are not currently threatened with extinction but trade in the species may lead to extinction.
Though the monk saki population is decreasing across their distribution, in parts of Peru, national parks have established some protection against habitat destruction. In these parks, monk saki populations are relatively stable. Some expeditions into restored and protected habitats have reported seeing monk saki families thrive—shouting at the human visitors and fearlessly gliding through the forest. This is a hopeful sign for species and ecosystem recovery.
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Written by Acima Cherian, March 2023