Black-Crowned Dwarf Marmoset, Callibella humilis
BLACK-CROWNED DWARF MARMOSET
Callibella humilis
Geographic Distribution and Habitat
The black-crowned dwarf marmoset (Callibella humilis), also known as Roosmalen’s dwarf marmoset (after Marc Van Roosmalen, the renowned and embattled Dutch scientist/primatologist credited with this primate’s discovery), and sometimes called the black-crowned pygmy marmoset, is native to Brazil, residing in the Amazon Rainforest. Of all the primates who call this tropical biome “home,” black-crowned dwarf marmosets have the smallest geographic distribution.
Several rivers within the Amazon River Basin provide the boundaries for the species’ distribution. In the north, these monkeys are found on the east bank of the lower Madeira River; their range extends eastward to the west bank of the Aripuanã River (a tributary of the Madeira River) from the river’s mouth just southwest of the town of Novo Aripuanã; westward to the Manicoré River and its tributary, the Mataurá River where the range stretches to the east bank of the Uruá River within the state of Amazonas in northwestern Brazil; south to the village of Tucunaré, continuing to the middle-lower section of the Roosevelt River. An isolated population has been reported within the mid-Atininga River region, approximately 6.2 miles (10 kilometers) east of the Manicoré River and about 31 miles (50 kilometers) southwest of the presumed southernmost limit marked by the headwaters of the Mariepauá and Arauá Rivers. The Cerrado ecoregion between the Ji-Paraná and Roosevelt Rivers has also been suggested as the southernmost limit. More field studies are necessary to confirm the species’ southwestern and southern boundaries.
An adaptable species, black-crowned dwarf marmosets are frequently found near human settlements within edge habitats. Their presence in areas managed by humans—including multi-species forests, fruit orchards, and gardens nurtured by dark, fertile soil known as terra preta, or “Indian black earth,” tilled by human farm workers—is indicative of the species’ commensal relationship with humans. Thus, the monkeys’ reliance on human horticulture, cultivation, and agronomy is tied to the species’ survival—a somewhat unusual example where anthropogenic (human-influenced) activities greatly benefit a species. The exception appears to be the isolated population residing in the mid-Atininga River region. These black-crowned dwarf marmosets have been found in undisturbed primary forests and in swamp-like forests referred to as “igapó” in the Portuguese language.
A bit of a kerfuffle surrounds the taxonomic classification of this tiny primate, not unusual for the scientific community charged with this decision. The species was first described in 1998 and placed within the genus Callithrix. But after considering the primate’s distinct size (somewhere between that of the pygmy marmoset [Cebuella pygmaea] and other marmosets of the Amazon region) and careful analysis of morphological and genetic studies, a group of scientists (including Roosmalen) decided that the species deserved its own genus. Thus, the species was reclassified as Callibella. Other scientists, however, assert that the species should be transferred to the genus Mico. Despite the assertion of these dissenters, the International Union for the Conservation of Nature (IUCN) recognizes the species as Callibella humilis—for now.
Size, Weight, and Lifespan
As its common name suggests, the black-crowned dwarf marmoset is a tiny monkey; only the pygmy marmoset is smaller. Adult females are slightly larger than males, weighing about 6.5 ounces (185 grams) with a head-to-body length of 6.6 inches (16.7 centimeters) and a tail length of 8.7 inches (22 centimeters). Adult males weigh about 5.3 ounces (150 grams) with a head-to-body length of 6.3 inches (16.1 centimeters); their tail adds another 8.2 inches (20.9 centimeters) to their frame.
Lifespan has not been reported for this species. In general, marmosets live an average of 5 to 7 years in the wild with a maximum lifespan of 16.5 years for some species.
Appearance
These little munchkins are cloaked in a pelage of silky, soft, olive-brown fur that acts as insulation in the humid rainforest environment. The underside is yellowish and the long tail is nearly black with barely perceptible rings. A furry, black crown nearly overwhelms the monkey’s bare, scalloped ears and encroaches upon a pale pink, hairless face. Juveniles are identified by streaks of white hair on the crown of their head. Tufts of white fur extend from the temples and shoot out from above dark, expressive eyes, resembling wild, unkempt eyebrows. The muzzle is flat and nondescript. Hands and feet are similar to those of squirrels and are fitted with sharp claws, except for their big non-opposable toe and thumb which have finger/toenails.
Subtle differences in coat color have been reported in the isolated Atininga River population. An orange hue, rather than yellow, characterizes the furry underside of these marmosets. And white tufts of fur on the crown of juveniles are retained into adulthood.
Diet
Black-crowned dwarf marmosets are largely frugivorous omnivores—meaning that they eat primarily fruits, along with flowers and nectar. But animal prey such as snails, spiders, and insects complement the meal plan of these little primates. Plant exudates—tree gums or sap—provide a significant dietary staple.
Ripe fruits are yummy, particularly yellow mombin, aka hog plum (Spondias mombin), and a sweet grape-like fruit (Tapirira guianensis), both from the plant family Anacardiaceae (known as the cashew or sumac family). A type of wild sugar apple (Rollinia mucosa) and flowers are provided by the plant family Annonaceae (known as the custard apple family). A fruit referred to as ice-cream bean (Inga edulis) and a similar fruit (Inga ingoides) are provided by the legume family Fabaceae.
Jackfruit, another black-crowned dwarf marmoset favorite, is courtesy of the jack tree (Artocarpus heterophyllus) from the plant family Moraceae (often called the mulberry family). Palm fruits (Astrocaryum vulgare) are provided by the perennial flowering plant family Arecaceae. Another flowering plant popular with the species is Sterculiceae from the family Malvaceae. Wild cacao (Theobroma speciosum) from the plant family Malvaceae is also enjoyed. Perhaps the species’ favorite tree gum comes from the pioneer tree (Didymopanax morototon) within the Araliaceae family. A sprinkling of insects adds a crunchy condiment to their meals
Behavior and Lifestyle
These little forest dwellers spend the majority of their time in the trees, making them arboreal. While they can be found in various levels of the forest canopy, from the lower understory to the uppermost branches, they prefer areas with high tree density that offer plenty of camouflage from the Amazon’s apex predators: the fearsome harpy eagle (Harpia harpy) whose rear talons are about the same length as a grizzly bear’s claws!; Anaconda snakes (Eunectes); and jaguars (Panthera onca). Domestic cats and dogs also prey upon the species.
Active during daylight hours, black-crowned dwarf marmosets are diurnal. Much of their day is spent foraging. Their adaptability to various canopy levels allows them access to different food sources. The species’ small and slender build allows the monkeys to easily navigate intertwined branches and thick foliage as they advance quadrupedally (using all four limbs) with impressive speed and agility—whether foraging for food or evading a predator. Their long nonprehensile tail, while incapable of grasping, helps them to keep their balance as they leap from tree to tree. Their sharp claws assist in grasping tree limbs and allow them to cling vertically to trees, not unlike a common squirrel might do.
And it is while clinging to trees that these little primates engage in a specialized, morphological exudativory behavior: tree gouging. Like their even tinier cousins, pygmy marmosets, black-crowned dwarf marmosets use their sharp incisor teeth to gouge holes into tree trunks and branches to stimulate the flow of gums and saps (exudates). Just a small amount of gum or sap is released at each hole; thus, the marmosets typically spend no more than two minutes at one location before moving on to gouge another hole and repeat the activity. Gums and saps provide important sustenance, particularly during the dry season when fruits are scarce.
Only marmosets and slow loris species (Nycticebus), the latter being Asian prosimians, are known to routinely practice exudativory behavior. Other primates with a penchant for gums and saps prefer to wait patiently (or opportunistically?) for insects to come along and bore holes in the bark to release these exudates.
When not foraging, or enjoying a rest break from foraging, marmosets engage in social interactions with one another.
Overnight, these monkeys sleep within thick tangles of vegetation or inside of tree hollows. They assume a curled-up position with their tail wrapped around their body so as to blend in with their surroundings, thereby mitigating the chance and lessening the worry of a predator attack.
Because of their anatomical characteristics, marmosets are considered to be the most primitive monkeys, along with their genetically close cousins the tamarins.
Small family groups of two to eight individuals are the norm for black-crowned dwarf marmosets. During his early field studies, however, primatologist (and species’ discoverer) Roosmalen reported group aggregations, or “clusters” of more than 30 individuals.
These are highly social animals who form strong family bonds with one another. Typically, a group comprises a dominant breeding pair and their offspring—the norm for most marmoset species. But it is not uncommon for a family group of black-crowned dwarf marmosets to include other reproductive females along with their offspring.
Upon reaching sexual maturity, individuals may leave their natal (birth) group to seek out a mate and begin their own family group. Such dispersal helps prevent inbreeding and promotes genetic diversity within a species. Whether “leaving home” is characteristic of males or females (or both) is not reported for black-crowned dwarf marmosets. However, in other marmoset species, subordinate, reproductive females who copulate with males outside of their birth group are likely to join one of these outsider groups to attain a dominant breeding rank.
Described as the “last refuge” for a multitude of the world’s animals, the Amazon Rainforest is home to jaguars, pink river dolphins, sloths, poison dart frogs, 370 species of reptiles, 3,000 freshwater fish species, and more than 1,000 species of birds including hummingbirds, channel-billed toucans, hoatzins macaws, and, of course, harpy eagles.
Sympatric primate species include the Marca’s marmoset (M. marcai) and howler (Alouatta), black spider (Ateles), tamarin (Saguinus), and capuchin (Cebinae) monkeys.
Home range has not been reported for the species. Clues may possibly be taken from the black-crowned dwarf marmoset’s tinier cousin, the pygmy marmoset, whose daily travels average a mere 0.001 square mile (0.003 square kilometer).
Marmosets communicate through vocalizations, postures, social behaviors, and scent. Regrettably, scant information is available specific to black-crowned dwarf marmoset communication. We again must look to other marmoset species, with the pygmy marmoset as our foremost candidate, for clues.
These tinier cousins rely on an intricate system of vocalizations to communicate with one another. Their contact calls include trills, used for short-distance communication during feeding, foraging, and traveling; J-calls, a series of fast notes repeated by the caller at medium distances; and long calls, used when a group is spread out over distances greater than 33 feet (10 meters) or in response to a neighboring group. Alarm calls—high-pitched whistles—are emitted to alert group members of a predator threat, giving everyone a chance to get themselves to safety. (Both long calls and alarm calls have been reported in black-crowned dwarf marmoset populations.)
Pygmy marmosets modify the structure of their calls in response to changes in their environment, such as high humidity, which can impact the distance a call is carried or the audible nature of a call.
Posturing such as strutting and back arching, sometimes accompanied by piloerection, are performed in response to threats.
Wildlife biologists theorize that, as with pygmy marmosets, vocal and visual cues (postures) play important roles in black-crowned dwarf marmoset societies. A sonogram analysis of the species’ long call structure, reported in 2013, appears to confirm this theory—revealing similarities in note duration and frequency with the long call of the pygmy marmoset, and also with that of the silvery marmoset (Mico argentatus).
Unlike pygmy marmosets (and other marmoset species), who mark their territorial boundaries with secretions from scent glands on their chests and anogenital area—and then aggressively defend their range boundaries—this behavior has not been observed and is thought not to be characteristic of black-crowned dwarf marmosets. A lack of antagonistic (hostile) behavior in the species further strengthens this assumption, held by the species’ authority, Roosmalen, and other like-minded wildlife biologists.
The mating system for this species is not reported; however, marmosets are generally monogamous (selecting one sexual partner for breeding). In marmoset family groups with two reproductively active females, the subordinate female is most often the daughter of the dominant breeding female. Unfortunately, a subordinate female does not always give birth to viable offspring—no doubt informing her decision to disperse from her natal group to seek out a mate with whom she can successfully reproduce. However, the presence, according to field studies, of offspring by multiple reproductive females within black-crowned dwarf marmoset groups suggests that the species may be more fecund than other marmoset species.
Between 18 and 24 months of age, black-crowned dwarf marmosets attain sexual maturity; that is, males are able to sire young and females are able to bear young. After a gestation period of 140 to 145 days, a female gives birth—usually to a single infant, though on occasion, two. (While twin births are common among other marmoset species, a single birth is the norm for black-crowned dwarf marmosets.) Newborns are born with their eyes open and weigh a mere 0.5 ounces (15 grams) and measure only 5.9 inches (15 centimeters) in length. As you might imagine, they are completely helpless and so cling to their mothers immediately. Later, they ride on her back as she travels.
With other marmoset species, all members of a group, including the father, assist with child care: offloading mom by carrying her infant, protecting the vulnerable youngsters, and teaching them essential life skills. This cooperative breeding system helps to ensure the family’s safety and survival.
Conversely, back in 2003, Roosmalen posited that a cooperative breeding system is likely not characteristic of black-crowned dwarf marmosets. He based his postulation, in part, on having only observed mothers carrying their infants.
At three or four months of age, youngsters begin to venture from their mothers for short periods of time and explore their surroundings. At six months of age, they are considered fully weaned and are able to eat solid foods. They are also becoming more self-sufficient and independent.
Unlike most mammals, reproductively active marmosets are able to conceive just two to four weeks after giving birth. Thus, mothers might be caring for two generations of offspring.
Thanks to their largely frugivorous lifestyle, black-crowned dwarf marmosets help to replenish their forest habitat. By distributing the seeds of the many fruits that they eat—via their feces—in the course of their daily travels, they nurture new plant growth throughout their environment.
Black-crowned dwarf marmosets are classified as Least Concern by the International Union for Conservation of Nature (IUCN, January 2015), appearing on the IUCN Red List of Threatened Species. Despite its restricted geographic distribution, the species faces no significant threats, hence the low classification ranking.
Nevertheless, continued destruction of forested habitat, for conversion into agricultural tracts, puts the species at risk. Black-crowned dwarf marmosets are not believed be hunted, likely because their small size would not yield much “bushmeat.” But there is concern that the monkeys are sometimes kept as pets. Sadly, these kidnapped infants are not likely to survive. As with other wildlife, these tiny primates require specific care that cannot be met as a kept pet.
The species has evaded the fate of the common marmoset (Callithrix jacchus), who has been used as a test subject in biomedical research (primarily in studies of infectious disease, immunology, and neuroscience) since the early 1960s.
Total population is unknown. Roosmalen put forth an estimate of 10,000 individuals back in 2003. Some wildlife biologists suggest that, because the species’ range is actually three times greater than what Roosmalen initially estimated (though still the smallest distribution of all primates in the region), the actual population is likely three times what Roosmalen suggested. But other wildlife biologists don’t appear to be convinced of this greater estimate and suggest a total population of only a few thousand individuals.
Black-crowned dwarf 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.
The species’ Least Concern classification does not lend itself to any dedicated conservation efforts for these monkeys.
Two protected areas are located within the species’ range, according to the IUCN. The Juma Sustainable Development Reserve (JSDR), or Reserva de Desenvolvimento Sustentável Juma in the Portuguese language, is a sustainable development reserve in the state of Amazonas. The Indigenous Territory Pinatuba, or Terra Indígena Pinatuba in the Portuguese language, is inhabited and exclusively possessed by a population of indigenous people identified as the Mura, also located in the state of Amazonas at the mouth and lower reaches of the Mataurá River.
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Written by Kathleen Downey, Jun 2024