Geographic Distribution and Habitat
Common marmosets, also known as white-tufted-ear marmosets, are New World monkeys native to Brazil. Once found only in Atlantic coastal forests in the northeastern region of the country, years of habitat destruction have forced them to seek new places to live.
Habitat loss, coupled with the release of captive marmosets (former pets) outside of their original range, has led to the species’ geographic distribution throughout Brazil, as far west as the country’s Rio Grande (left bank) and as far south as Argentina.
A highly adaptable species, common marmosets are able to thrive in environments that some other primate species might find too hostile, including dry secondary forests with shorter tree canopies and more seasonal fluctuations in temperature and rainfall, riverine forests, savanna forests, and semi-deciduous forests inland. The monkeys typically reside at the edges of these habitats rather than deep within the forests.
Common marmosets can even be found within the city zip codes of Rio de Janeiro, Brazil, and Buenos Aires, Argentina. Unfortunately, the human populace who lives there regards these “city slickers” as an invasive species.
Size, Weight, and Lifespan
Common marmosets are tiny monkeys with long tails. Males are slightly larger than females.
Average body length for males is 7.4 in (19 cm) and they weigh about 9 oz (256 g). Average body length for females is 7.3 in (18.5 cm) and they weigh about 8.3 oz (236 g).
Tail length adds another 11.6 to 13.8 in (29.5 to 35 cm) to the common marmoset’s body.
The average lifespan of a wild common marmoset is 12 years. Average lifespan of a captive common marmoset is 16 years.
Mother Nature was having a good time for herself when she created this unusual-looking little monkey. Showing a sense of humor, and also tenderness, she got busy with her color palette and her imagination.
The common marmoset’s pelage is anything but common. Brush strokes of brown, gray, and splashes of yellow-orange grace this primate’s tiny body in a distinct, mottled pattern with muted vertical stripes along the back. Its long tail is encircled by long, light-colored stripes.
But it’s the head sitting atop that fanciful, furry body that captivates. Wild, white tufts of fur shoot outward from the temples, evoking the look of a mad scientist. A dark fur cap, dark fur trim that generously outlines the face, and a patch of white fur emblazoned upon the forehead add further distinction. Pale facial skin surrounding the eyes and lips darkens when exposed to the sun, and the muzzle is dark. Wide brown eyes complete the common marmoset’s beguiling appearance.
Infants are born with a brown and yellow coat and develop their white ear tufts and forehead blaze as they grow older.
Common marmosets are known as exudativore-insectivores, which simply means that they like to eat plant exudates—gum, sap, and tree resin—and they eat a bunch of insects, too.
Exudates can comprise up to 70 percent of their diet. Fortunately, the compact bodies of these monkeys are uniquely adapted to obtaining this reliable food source. While clinging to the side of a tree with their claw-like nails, they simultaneously gnaw a hole in the tree with their large, chisel-shaped incisor teeth, stimulating the flow of edible plant exudates. They then use their tongue to leisurely lick the desired exudates or use their teeth to scoop them into their mouth.
Second to exudates, insects are the important dietary staple for common marmosets; if they aren’t foraging for exudates, they’re usually foraging for insects, with a penchant for grasshoppers and crickets. Their small body size makes full use of the protein derived from their insect meals, a biological feat not possible for larger-bodied primates. A specialized gut, specifically the cecum (a pouch at the beginning of the large intestine), is uniquely adapted to their exudate-insect diet and aids in digestion.
Fruits, flowers, nectar, fungi, spiders, and the occasional small lizard, tree frog, or birds’ eggs complete the common marmoset’s diet.
Behavior and Lifestyle
Common marmosets are diurnal (active during daylight hours) and arboreal (tree-dwelling) monkeys. Their claw-like feet allow them to hang onto branches, while vertically suspended, and leap between or run across branches on all fours (quadrupedally) with the agility of a squirrel. In the understory and middle layers of the forest, they silently stalk and pounce on their insect prey. They stay within their home range, which is based on density of gum trees (a preferred dietary exudate) and encompasses only a small area, from .003 to .04 mi (.005 to .065 km). In addition to gouging their own tree holes to get at the gum, common marmosets opportunistically visit the holes created by other animals; they often revisit the same holes.
Leaving their sleeping nest 30 minutes after sunrise, common marmosets feed intensively for about an hour and then spend the remainder of the day alternating between feeding, socializing, and resting (or in human-speak parlance, “chilling”). They are “chill masters” when it comes to rest. They assume a full-body sprawl and can remain in this position of complete repose for more than 30 minutes. About one hour before sunset, the monkeys return to their sleeping sites in the dense, vine-covered vegetation of the trees. The group sleeps together, affording them greater protection against forest predators: carnivorous mammals such as mustelids (members of the weasel family), wild cats, raptors, arboreal snakes, and owls.
The name “marmoset” comes from the French word “marmouset,” meaning “dwarf,” or “little.”
Common marmosets live in social groups of 3 to 15 individuals, with an average of 9. These groups often include three family generations: one or two breeding females with one or more breeding males, related adults such as parents or siblings, and a breeding pair’s offspring. Females within a group are usually closely related (mothers/daughters and sisters); however, adult males are more often émigrés from an outside common marmoset group. Whereas many primates leave their birth group upon reaching adolescence, common marmosets remain with their family until they reach adulthood. At this time, males emigrate from the group to seek breeding females. Common marmoset males do not breed with females to whom they are closely related.
Social status is linked to breeding status, and a breeding pair is usually codominant (unlike with many other primate species, where a dominant male leads the group). However, if there is more than one breeding female in a group, one of the breeding females is dominant over the other. Dominance hierarchy for nonbreeding individuals is determined by age, with the older individuals receiving more respect and deference from younger individuals in the group. Again, gender is not a factor.
Common marmosets use a repertoire of vocalizations, visual cues, and olfactory signals to convey information about social status, emotional state, and intent to other individuals who might be members of the group or outsiders who pose a potential threat.
Alarm calls are either staccatos, a series of repeating calls that become higher with each call, or tsiks, a series of short, trickling calls that are continuously sounded, or sounded intermittently. These alarm calls warn other members of the group to flee a dangerous situation—or cue them to mob potential predators.
General vocalizations include trill and phee calls:
- Trill calls are low-pitched with a distinct vibrato sound and fluctuate in frequency. They are used to keep track of one another, particularly in low-visibility areas. All members of the group, regardless of gender, age, or status, use trill calls.
- Phee calls are loud, high-pitched whistles, sounded in a series of one to five notes lasting about 2 seconds each. They play a role in locating a lost group member, maintaining group cohesion, territorial defense, and in attracting a mate.
While vocal communication is important over longer distances, visual signals are important for close-range communication.
A partial open-mouth stare signals alarm, a frown indicates aggression, and a slit stare indicates submission. By flattening his ears close to his head, a marmoset is being submissive, expressing fear, or simply curiosity about a new object in his environment.
When dominant individuals want to elicit a certain action, such as displacing other individuals from a feeding site, they may give chase or resort to aggressive actions including nips, cuffs, lunges, grabs, ear-tuft flicks, or biting.
A scent gland on the common marmoset’s chest and on the genital/anal region allow the monkey to leave secretions (olfactory cues) to mark territorial boundaries, signify social status, or advertise reproductive status. A specialized organ in the nasal cavity (an adaptation of all New World monkeys) processes chemical signals, allowing the monkey to discern olfactory cues left by other animals.
Female common marmosets reach sexual (reproductive) maturity at about 1 year, 3-1/2 months of age (or 477 days). Male common marmosets reach sexual (reproductive) maturity a little sooner than females, at a little over 1 year old (or 382 days).
Females flick their tongues to solicit potential male suitors (breeding partners). After a gestation period of about 5 months, a female gives birth to her offspring: most often to a set of non-identical twins (twins are an uncommon occurrence with most primate species), but occasionally to triplets or to a single infant. She typically gives birth twice a year, at the end of the dry season and at the end of the rainy season. These seasonal births are a biological adaption. Because nursing a set of twins saps the energy of the mothers, giving birth during periods of relative food abundance removes some of their nutritional stress.
One of the defining social behaviors of common marmosets is cooperative infant care. Fathers take an immediate role, helping to carry their offspring. Older siblings, nieces, and nephews also pitch in.
Infants are considered to be weaned at 3 months of age. Although they are capable of self-feeding now, they do not yet gnaw their own tree holes to feed on exudates. Instead, they lick the gum, sap, or resin from holes created by older individuals.
Sadly, most scholarly and scientific articles equate a positive ecological role with the common marmoset’s value as a biomedical research model. Secondary is the primate’s appeal as a zoo inhabitant. Neither of these roles, however, considers the inherent value with which this primate regards his or her own life as a living, breathing sentient being—and a fellow primate to human primates.
The common marmoset is classified as Least Concern by the International Union for Conservation of Nature (IUCN, 2015). This lesser status is due to the species widespread distribution, the species highly adaptable nature, and population numbers that are not “sufficiently” declining.
Paradoxically, and alarmingly, the near complete destruction of their habitat in northeastern Brazil has severely threatened the species population there and has forced these monkeys to seek a more hospitable habitat. But no welcome mat has greeted them in the southeastern region of the country, where common marmosets have adapted to life at edge forests. Because the monkeys opportunistically visit the plantations in this area, farmers regard them as pests.
Since the 1960’s, common marmosets have been used in medical research in both the U.S. and Europe. In Europe, common marmosets are the most frequently used nonhuman primate in research laboratories.
After Brazil banned the export of common marmosets in the early 1970’s, however, scientists were forced to cultivate their own captive populations. Federally-funded National Primate Research Centers, academic institutions, and pharmaceutical companies in the U.S. and Europe have been key players in creating and maintaining captive breeding programs. But these are not species conservation programs.
In the name of science, common marmosets’ small bodies have been cut open and experimented on to discover how humans might be affected by periodontal disease, immunology, obesity, aging, congenital defects, and endocrinology, and other conditions.
Common marmosets are also captured and illegally traded as pets. Sadly, these pet common marmosets do not fare well. Poor diet and exposure to human diseases threaten their health. As they age and become destructive in their human households, common marmosets are often abandoned or killed by their human keepers. Less often, common marmosets are captured to become zoo inhabitants, where they are then bred as a popular primate attraction for zoo visitors.
The common marmoset 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.
Some of these monkeys live within protected areas. However, conservationists argue for further protection and want a revised land-use policy that reflects the need to protect common marmosets—before the species meets the dire fate of so many other primates whose survival is threatened.
Although there are zoos that practice legitimate conservation programs of endangered species, taking in injured or abused individuals and releasing back into the wild whenever possible, zoo conservation programs with regard to common marmosets (researched for this article) are dubious, at best.
Written by Kathleen Downey, November 2017