Geographic Distribution and Habitat
Silvery marmosets inhabit lowland forests (under 650 feet or 200 meters above sea level) in the Amazon Basin. The Amazon River bounds their distribution to the north, the Tapajos River to the west, and the Toncantins River to the east. Human infrastructure and agriculture have further fragmented the marmosets’ native range into “a mosaic of forest fragments of different dimensions, interspersed with open pasture and plantations,” though the IUCN Red List does not mark marmoset populations as “severely fragmented.”
Marmosets are known for their ecological flexibility, having been found in primary forests, secondary forests, open forests, and remnant forests in patches of savannah. However, case studies suggest that silvery marmosets prefer edge and secondary-growth forests for their abundance of prey, dense vegetation, and high concentration of fruit species. Because marmosets rely heavily on sap, some primatologists believe the prevalence and distribution of gum trees affect the size and shape of their home range.
Sources variously refer to silvery marmosets as Callithrix argentata or Mico argentatus. The latter designation is considered correct, as it was determined in the late 2000s that Amazonian marmosets warranted their own genus.
Size, Weight, and Lifespan
Silvery marmosets are among the smallest American monkeys. On average, their bodies measure 8 inches (21 cm); their tails are significantly longer, ranging from 11–12 inches (30–32 cm). They weigh between 10–15 ounces (273–435 g). Their lifespan is roughly 16 years.
Like several other marmosets, the faces and ears of silvery marmosets are bare and pinkish-yellow; but unlike any other marmoset species, silvery marmosets have a silver-gray body and dark-brown tail. The tops of their ears curve outward like question marks. Their fingers end in claws rather than nails—as is true of all marmosets—which, alongside their long and sharp incisors, come in handy for gouging gum out of tree bark. The species is sexually monomorphic: that is, males and females do not differ notably in size or appearance.
Gum is the core component of a silvery marmoset’s diet—several physical and behavioral traits help them release gum from certain species of trees. Tree gums are difficult to digest, so silvery marmosets also have digestive systems specialized for exploiting and absorbing hard-to-get nutrients, including an enlarged cecum and an elongated colon. However, gum is less energy-rich and nutritious than other marmoset foods, like insects and fruits, so it’s crucial that silvery marmosets do not rely solely on gum. Consequently, they also eat a wide variety of fruits, plants, invertebrates, and small vertebrates, including flowers, nectar, spiders, frogs, snails, and lizards (among others).
Behavior and Lifestyle
Silvery marmosets spend most (if not all) of their lives in trees, upon which they can use their specialized claws and teeth to extract nutritious tree saps and gums. Some research suggests that they are, nonetheless, more “neophilic”—interested in and willing to explore novel tasks or situations—than primate species with similar degrees of specialization.
Silvery marmosets are diurnal, meaning they are active during the day and sleep through the night. They wake up around sunrise and soon begin foraging in the trees, where they’ll spend most (usually all) of the day. Marmosets avoid predation by keeping to tree cover. In the early wet and late dry seasons, they forage through larger ranges than at other times of the year, and depending on the time of day, they’ll prioritize different parts of the forest. Flooded areas, for instance, are good for resting, social activities, and looking for insects; terra firme forests (empty of flooded rivers) are replete with fruiting trees, especially in the early rainy season. Generally, though, they stick to secondary growth regions—but since these regions emerge only after human agriculture or logging has cleared an old-growth forest, one should hesitate to think of that preference as somehow innate.
At night, silvery marmosets may sleep in one of several sites distributed across their home range. Usually, they’ll pick one closer to major food patches, and if a gum tree is nearby (it often is), they’ll feed on it before sleeping and just after awakening. Case studies suggest that site preferences depend on the pattern of trees, proximity of feeding sites, position inside the home range, and ranging behavior of the day. Vine tangles or tree holes often make for good nests, but primatologists have also seen them use unusual spots like hanging fallen tree trunks and dead palm trees covered by lianas (long-stemmed vines).
In the wild, silvery marmosets live in groups of 4–15 individuals. The core of each group is a dominant male-female pair and their offspring; other socially subordinate adults (generally female) may travel within the group, but limited reproductive opportunities may prompt them to join or breed with neighboring groups. In other marmoset species, dominant females regularly groom subordinate females (the inverse happens too, but less regularly). Some primatologists suggest that grooming thus functions as an incentive from breeding females to their non-breeding counterparts to stay in the group. However, one should note that this pattern (to our knowledge) has not been directly recorded in silvery marmosets.
Marmoset groups often overlap. When they do, they act aggressively toward one another to gain preferential access to feeding sites. They also often overlap with certain other primate species, like tamarins, with whom they rarely interact.
Silvery marmosets use a complex repertoire of sounds for vocal communication. Adults combine discrete vocal elements into dozens of calls, and infants reproduce them in irregular forms known as “geckers,” which some primatologists have compared to babbling in human infants. As baby marmosets achieve independence, their geckers develop into distinct calls; consequently, the development of vocal communication in silvery marmosets is closely linked to their social development. Like most primates, silvery marmosets also use body, facial, and scent-based displays to communicate. For example, aggression signals involve walking with an arched body and bristling hair, as well as anogenital scent-marking, which the marmoset secretes from its hindquarters and rubs against a branch. Facial expressions are especially important for sexual communication—females will often stare backward at males to signal that they’re interested in copulating.
Once or twice per year, the dominant male and female in a silvery marmoset group breed. Subordinate members rarely get this opportunity. Gestation lasts roughly 140 days, after which the female gives birth to (usually) twins. Infant marmosets bear a thin layer of black or gray fur that whitens as they age; their eyes and ears are also much larger relative to their bodies than those of adults.
The mother, father, and other group members all contribute to child-rearing, and one may regularly find infants clinging to individuals other than their mother. At 4–6 weeks, the babies begin ingesting solid food and playing with twins. Around six months, they are weaned, and they reach sexual maturity at two years, after which they may leave to join neighboring groups with better breeding opportunities.
Silvery marmosets serve as seed dispersers in their habitats by consuming fruits and defecating the seeds across their home range. They also may inadvertently assist with pollination by eating nectar and thus transferring pollen between flowers. Additionally, they play the unenviable part of dietary staple for snakes, some predatory mammals, and several birds of prey.
The International Union for Conservation of Nature lists silvery marmosets as Least Concern (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.
Habitat loss is the greatest threat facing silvery marmosets. Logging, cattle ranching, roads, and agriculture are diminishing forests within its range along the Transamazon and Santarém-Cuiabá highways and south of the lower Rio Amazonas. One dangerous consequence (among many) of this trend is habitat fragmentation, which separates marmoset populations and limits opportunities for cross-breeding. Left unchecked, fragmentation can lead to genetic drift (when groups of a single species become genetically distinct) and in-breeding, both of which threaten the long-term survival of silvery marmosets. Local human groups occasionally use silvery marmosets as pets or food, but the IUCN does not consider this a major threat.
Silvery marmosets are 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.
While silvery marmoset populations are declining, the IUCN does not consider their decline especially steep or concerning. Since habitat loss is the greatest threat facing marmosets, further research to examine the scale and effect of agriculture and logging on their native forests is essential. Genetic drift and inbreeding are particularly concerning—primatologists recommend promoting gene flow between populations to stop isolated groups from becoming further differentiated.
Written by Eli Elster, March 2023