WESTERN PYGMY MARMOSET
Geographic Distribution and Habitat
Pygmy marmosets are widely distributed throughout the Amazon forest, spanning from Rio Caquetá in Colombia, Rio Madeira in Brazil, to Río Mayo and the Río Huallaga in Peru. They can reliably be found in extensive rainforests along the tributaries of the Amazon River and are capable of thriving in different forest types, from flooded areas and secondary forests to higher-altitude forests, and even some fragmented areas near human settlements. In general, they seem to prefer dense canopy cover as they leap between trees.
Western pygmy marmosets tend to congregate near river edges with up to approximately 274 individuals per 0.4 square miles (1 sq. km) in such habitat while in more interior forest regions researchers have reported up to 51 individuals per 0.4 square miles (1 sq. km).
Pygmy marmosets have recently received a lot of attention in the world of taxonomic classification. In 2018, a few researchers suggested that there may be three groups within the species of pygmy marmosets based on the color of the marmoset’s fur on the chest and belly. But this was soon disputed by zoologists who collected more samples, conducted genetic studies, and showed that fur color was not enough to distinguish pygmy marmoset species from each other. They suggested that there were only two species of pygmy marmosets—the western pygmy marmoset, which is found north of the Napo and Solimões-Amazonas rivers, and the eastern pygmy marmoset (Cebuella niveiventris), located in the south. This classification of pygmy marmosets into these two species was confirmed using genetic studies in 2021.
Size, Weight, and Lifespan
With a body weight of 2–5 ounces (85–140 g) and a height of about 5 inches (13.6 cm), the western pygmy marmoset is one of the smallest New World monkeys. Pygmy marmosets are so small they are called “finger monkeys” because they can comfortably perch on a person’s finger. The skulls are tiny, between 1.3 and 1.5 inches (3.3–3.8 cm)—that’s about the size of an AirPod!
They live to be around 10 years old in the wild and 15 years old in captivity.
The time of pregnancy from conception until birth.
In biology, the niche of an organism is the functional role that it plays within an ecosystem.
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The western pygmy marmoset is the smallest of the New World monkeys. Their body is covered in brown and rusty stripes, which makes it really difficult to spot them in the wild, among the trees of the Amazon rainforest. Their heads are covered in a brindled (streaky) mane, which covers their ears and leaves only their eyes and small muzzle visible. They have a characteristic white nasal stripe and black-colored genitals. There is no sexual dimorphism, so males and females look similar. Their sharp teeth can be seen as they bite into tree barks and overall they can look quite fierce despite their diminutive size.
Western pygmy marmosets have some unique characteristics. They have longer arms than legs, which is unusual for most mammals. Unlike the flat fingernails that most other primates have, western pygmy marmosets have pointed claws to help them clamber up trees. Their tails are longer than their bodies. These anatomical features work together as adaptations for climbing vertically on tree trunks, leaping from trees, and catching insects with their hands.
Western pygmy marmosets’ size and appearance change as they develop into adults. As an infant between 0 and 2 months, they are small but with a large head that is covered with fuzzy hair. At this stage, they look constantly ruffled, they do not have the best motor skills, and an adult usually carries them. As a juvenile, at 6 months to 1 year, they have grown bigger and their head size is more proportionate. The hair around their genitals starts to develop the characteristic dark or black color. As subadults between 1 and 1.5 years they look similar to adults, only a little smaller, and the scrotum in males starts developing the black color that is seen in adults (older than 1.5 years old).
Marmosets eat fruits, flowers, and insects, but they specialize in feeding on the gum or sap from trees, vines, and other plants. Sap is the sugar and mineral-rich fluid that carries food for the tree (similar to blood in a human body) and it is protected by the thick layer of bark. Western pygmy marmosets use specialized tusk-like lower teeth to drill holes through the bark so that they can lick up the sap. A family of western pygmy marmosets can make over 100 feeding holes in a single tree. Each hole is about 0.4–0.6 inches (1–1.5 cm) and is reused by the marmosets to reach the sap. Generally, when marmosets find a suitable tree, they start drilling these holes at the bottom of the tree and make their way up. When marmosets drill these holes, the tree reacts by producing more sap to heal the tree. The tree also forms scar tissue around holes that looks like bumps. These bumps look similar to the marmosets feeding on the tree, which is a great coincidental camouflage for the marmoset.
Behavior and Lifestyle
Western pygmy marmosets are diurnal (active during the day) and at night all the members come together to roost or sleep in one tree. Their small size and camouflage coloring make them difficult to observe in the wild. Western pygmy marmosets are shy animals that would rather hide than fight when confronted by threats from other species, so their behaviors are said to be cryptic or secretive. One reliable way of finding and observing them would be to find their distinct feeding holes in the bark. The marmosets repeatedly visit the holes to feed and so, if you sit and wait near a tree with some recent feeding holes, there is a good chance you will see a troop. To get a reliable estimate of how many western pygmy marmosets there are in a region, it is best to find their roosting tree and watch them go into the tree cavity or nest because it is easier to count them when they gather in one place.
The small size and quick movements of the western pygmy marmoset give them the advantage to ambush insects, like beetles and butterflies, when they hunt. They can also leap great distances relative to their body size. The longest recorded vertical jump is 4.5 feet (1.4 m), and they can jump up to almost 50 feet (15 m) from one tree to another.
They are social primates that live in family groups but they can also be territorial, especially around sap trees with established feeding holes. Their home range will usually contain a few trees with sap for feeding and at least one sleeping tree. Troops may live as close as 330 feet (100 m) apart. Sometimes, if a tree is a particularly good source of sap, then the pygmy marmosets will make this tree their “home tree,” where they spend most of their day foraging—and they sometimes sleep there too.
If a tree runs dry of sap, then a family of western pygmy marmosets will move on to another tree and sometimes even displace another troop from their tree!
Western pygmy marmosets are the smallest New World monkey species. They are sometimes called “finger monkeys” because they are so small.
They feed on mostly sap from trees. They have specialized teeth to drill holes in trees and claws to cling vertically on tree trunks.
They live in cooperative breeding troops, where only the main male and female reproduce and the rest of the family helps take care of the babies.
Females often given birth to non-identical twins.
When the troop wakes up, they leave the roost tree in a single file and then disperse in different directions to find feeding holes where they can eat. They tend to be most active during the early, cooler hours of the day and seek shelter during the hottest and most humid times of the day.
Western pygmy marmosets start their day around 6 a.m. when they feed on sap at their favorite tree for about 3 hours. After 9 a.m. most of the family members spend time drilling more feeding holes in the tree, usually in the trunk at a height lower than 32 feet (10 m). Some members may move off and try to catch some insects to eat during this time.
About 32% of a western pygmy marmoset’s day is spent eating sap and about the same amount of time is spent resting. For 16% of their day, they hunt insects, they travel for 11%, and 9% of their day is spent playing, grooming, and on other social activities.
Western pygmy marmosets can be found in three levels of social organizations. They can be in troops, solitary individuals, and incipient associations. In a population of western pygmy marmosets, most of the individuals (83%) are part of a troop.
Most people are probably familiar with primate troops that consist of close family members. In western pygmy marmosets, the troop is formed around the primary adult male and female that mate and produce young. Grown offspring can temporarily be part of the troop and help take care of the next generation of young marmosets. The troop may contain multiple generations of marmosets that can be male or female but only the alpha parents will mate. The adult offspring can break off from their natal troop (where they were born) and form their own troop with another unrelated adult western pygmy marmoset.
When a young adult leaves their natal troop they can travel and forage on their own as solitary individuals. Both males and females can be solitary individuals that are looking for mates. Solitary individuals start out by staying close to their natal troop and slowly increasing their time away from the family. There does not seem to be any form of forced removal by any members of the family.
Multiple solitary individuals can sometimes meet up outside of any troop’s normal home range and spend time together. This type of loose (or beginnings of a) group is called an “incipient association.” These incipient associations are often temporary, but sometimes can lead to mates bonding and forming a stable troop.
Vocal communication is the most common form of communication among western pygmy marmosets. Trill sounds were mostly used to make contact with family members and advertise their locations. The call triggers an automatic response from every western pygmy marmoset in the vicinity. One trill can set off a dozen others. Western pygmy marmosets also trill when they are excited, particularly at the sight of food. They even change the type of call they emit depending on the acoustics of the environment. For example, if they are in denser forests they may opt to use trills that can carry further distances.
Clicks and screech calls are made when they feel threatened and they want to alert or warn members of dangers or desirable food sources.
Western pygmy marmosets develop their calls as they grow up and hear the various calls made by adults in their families. When they are born, baby pygmy marmosets are mostly silent. As they become more mobile and spend more time away from their mothers, they make more trilling calls.
Solitary individuals use long-distance calls when approaching troops to contact potential mates.
Both male and female western pygmy marmosets use scent glands to mark trees especially to demarcate territories and communicate their readiness to mate. When female pygmy marmosets are ready to mate, their hormone levels (especially estrogen) change, which is reflected in their scent gland secretions. Males can detect these hormone changes and investigate scent marks more often as estrogen hormone levels increase in females.
Direct visual communication is only beneficial when the pygmy marmosets can see each other, which mostly happens among family members. Raised eyes, turned lips, or scrunched noses are common facial expressions. The white nasal stripe and light stripes on the corner of the pygmy marmoset’s mouth most likely help define facial expressions and help communicate fear or affection to other pygmy marmosets.
Western pygmy marmosets live in families of 3–7 individuals. Only the primary (or alpha) pair reproduces in the family. They are monogamous (they do not mate with any other pygmy marmoset).
Females typically give birth to fraternal or non-identical twins, but occasionally they can have triplets. Baby pygmy marmosets are tiny, about the size of a ping-pong ball, and weigh about 0.4–0.5 ounces (3–15 grams). But when you compare this to an adult female who usually weighs about 2–4 ounces, baby pygmy marmosets can be relatively large, between 25% and 18% of the mothers weight. To put that in perspective, human babies are usually born at only 5% of their mothers’ weight.
Gestation lasts around 140 days, and the young are born with a full coat of fur and open eyes. The young become independent after around 6 months and there is usually a 5–7 month time period between births in a troop. Males reach sexual maturity earlier than females at around 9 months. Females reach sexual maturity later, at around 15 months.
Young western pygmy marmosets rely on both parents for protection and sustenance. Fathers may carry baby marmosets as soon as the first day after birth, which is rare in most mammals where the mothers are the only ones to maintain close contact with their newborns. While mothers will initially do all the feeding, fathers will take on the role of carrying the baby and help feed them when they can eat solid food. Other family members also help take care of the young. There are four distinct stages of dependence:
- Stage A is complete dependence, which lasts for around two weeks and is characterized by nonstop parental care.
- Stage B is parental-initiated independence, which involves parents pulling babies off their backs, nipping at the baby’s head or feet, or rubbing the baby up against something to encourage some degree of separation. Parents at this stage are still protective of the baby and will become fierce at the hint of danger. This stage lasts until about the fourth week.
- Stage C is young-initiated independence and, at this stage, the baby voluntarily disengages from the adult’s back and exhibits more coordinated movements as well as social engagement with other members of the family group.
- Stage D is complete independence, which occurs at around 4 to 5 months of age. Young at this stage are capable of moving independently and do not instantly flee to their parents for safety. The arrival of a fresh set of twins at five months old seemed to correspond with this independence.
In many primate species, troop sizes are large because there is strength in numbers, and many family members mean better protection against predators. In these species, the father rarely takes on parental roles and mothers are the primary caretakers of their infants. This can make young primates vulnerable to threats from rivals or predators. For this reason, large troop sizes are often associated with higher infant and juvenile survival in primates. However, western pygmy marmoset infants are successful without protection from a large troop because all members, including the father, take care of the infant. This close-knit group shares the responsibility of constantly carrying the infant on their backs and even feeding them. This type of co-parenting behavior is called alloparenting or cooperative breeding. Therefore, infant western pygmy marmosets are less exposed to predators than other primate species where infants are more mobile. The only time we see western pygmy marmosets following a trend like other primate species, where larger troop sizes are associated with higher offspring survival, is when pygmy marmosets grow into their juvenile phase and move more independently. Juvenile pygmy marmosets are no longer physically carried by adults and they are more exposed to predators, which decreases their survival rate.
The scientific consensus is that a primate’s diet is an important evolutionary driver of their body size, so smaller food sources (like insects or sap) cannot sustain large primate bodies. This is why the western pygmy marmoset is so small and has a lower metabolic rate than other marmosets. The high sugar content in saps gives western pygmy marmosets quick bouts of energy. So they can do short bursts of jumping, ideal for catching insects, but they cannot do these quick movements for long.
Sap is also a food source that is not easily accessible to a lot animals because it is protected by thick bark. Once the bark is broken through, it can take many hours for the sap to trickle through. In addition, the amount of sap produced is also small. This is why so many feeding holes are made on one tree. Therefore, the western pygmy marmoset occupies a niche that does not have a lot of competition with other species. This specialized diet has determined many anatomical and behavioral aspects of pygmy marmosets, such as their sharp teeth and claws and their ability to move vertically (up and down) on tree trunks.
Western pygmy marmosets are sources of food for tree-dwelling snakes, and raptors like hawks and eagles. The marmosets also feed on the abundant insect populations that live in the Amazon forest. They also cohabit with other arboreal species, like squirrels and tamarins, that sometimes compete with pygmy marmosets for space and food. In this way, western pygmy marmosets are a part of the complex ecosystem whose health depends on the interaction of different parts (like predators and prey).
The International Union for Conservation of Nature lists the western pygmy marmoset as Vulnerable (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.
Even though the western pygmy marmoset can live in disturbed secondary forests, there has been a decline in their population in fragmented habitats near human populated areas in Ecuador, which tells us that they cannot make these disturbed areas their home for too long.
While habitat destruction in the Amazon forests is a threat to almost all wildlife species in the region, western pygmy marmosets are mostly threatened by the illegal pet trade. Their small size and cute appearance has boosted their appeal in the exotic pet market industry. Unfortunately, that has lead to illegal trappers capturing and trafficking these amazing primates out of their natural homes. In many cases, the marmosets do not make it out alive and the wild population of western pygmy marmosets in some regions are damaged beyond their capacity to recover.
Fragmented habitats near human populations also increases the visibility of western pygmy marmosets, which makes them easier targets for hunters and trappers.
Western pygmy marmoset is listed in Appendix I 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. This designation is given to species that are the most endangered by the threat of illegal trade.
The western pygmy marmoset habitat is protected in conservation areas within their home range such as the Amacayacu Natural National Park (Colombia), Yasuni National Park (Ecuador), and Manu National Park (Peru).
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Written by Acima Cherian, January 2023