They’re Adorable. And Endangered. Meet the World’s Smallest Monkey: the Pygmy Marmoset

It was after midnight in Quito when Stella de la Torre came for me, waiting under a streetlamp next to the black minivan that would ferry us to the Amazon, to see the smallest monkey in the world. I bundled in, and we departed, driving through empty streets and pitted roads until the blackness around us began to soften. 

The world’s smallest monkey is called the pygmy marmoset, and it is, predictably, very tiny, just a little scoop of fur. It likes to eat the sticky resin found in tree bark, known as gum, and is therefore called a gummivore. Captured pygmy marmosets can cling onto the finger of a person the way a koala might hang on a tree. I had never seen one before, and the notion of a wee, gum-eating monkey seemed far-fetched. Even improbable life finds its way, though, and pygmy marmosets have developed an elaborate survival system based on talking to each other constantly. They are unusually cooperative; infants are often born in pairs and nursed by their mothers, carried around by their fathers, and watched over by their siblings. They live in family groups of around half a dozen until the younger adults venture out to find a mate. Different groups have distinct dialects, with trills varying by length and frequency. As infants, pygmy marmosets babble the way human babies do, both to get their parents’ attention and to learn the group dialect. They don’t often deign to eat fruit, but they do hunt insects, and as they forage, they chatter to each other through the trees.

De la Torre, an ecologist at the San Francisco University of Quito, in Ecuador, is arguably the world’s pre-eminent pygmy marmoset expert, and she has documented their method of communication. Their brief, high-pitched alarm whistle, for example, sounds almost like a cricket or a bird, and is shriller than the staccato J-calls they use across distances or the trills they use when in each other’s proximity. Amid the constant roar of rainforest life, it’s not easy to distinguish, but de la Torre, a slip of a woman with radiant eyes, has developed exquisitely sensitive ears over a lifetime of study. She also helped reclassify the pygmy marmoset into two distinct species: the northern Cebuella pygmaea, scattered roughly across Ecuador and Colombia, from the Cebuella niveiventris, found mostly in Peru, Ecuador and Brazil.

Behold the World’s Smallest Monkey: The Pygmy Marmoset

Within those ranges, pygmy marmosets only live in certain concentrated places, and this—along with the fact that each is no bigger than an apple—makes them difficult to find. Their habitats, known as gallery forests, border rivers in the Amazon basin that are seasonally flooded, creating flora adapted to the constant washing in of nutrients from river waters. Gallery forests’ proximity to water makes them attractive to people who want to build houses with yards that slope down to the river. This development, along with agriculture and the oil industry, has slowly carved into the places where pygmy marmosets live.

Over the past decade, de la Torre has watched her research subjects disappear. At one of her study sites on the Tiputini River—in Ecuador’s Yasuní National Park and Biosphere Reserve, near the field research station of the San Francisco University of Quito—de la Torre observed that of the seven pygmy marmoset groups she surveyed in 2012, she could find only one group in the same area a decade later. Even the protections that preserve the park where de la Torre works contain exceptions for oil companies and Indigenous communities, and wildlife hunting is still rampant. In 2017, three of de la Torre’s pygmy marmoset groups disappeared mysteriously. From one day to the next, they simply vanished. 

Data on pygmy marmosets is not easy to come by, and de la Torre told me their population size isn’t known, nor is the extent of the great disappearance she began to document before the Covid-19 pandemic disrupted her fieldwork. Although pygmy marmosets are on Ecuador’s red list of endangered species, there is some disagreement among primate specialists about what might be happening with their populations in other countries. 

Now de la Torre was heading out to check on the remaining monkeys in Ecuador. The car brought us down the mountains, where the rising sun traced verdant trees jutting at Seussian angles, to a motorized canoe, which deposited us at a van that brought us to another boat, whipping down brown waters as herons flew ahead and swallows darted by. Every so often, we would pass a cluster of yellow or white butterflies on the riverbanks, and they would flutter up together, twirling around each other like dazzling whirlwinds. Behind them, the forest rose up, dense and inscrutable, reticent to divulge its life. 

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At dawn the next day, we made our way from the research station to a canoe we took upriver, where we docked and scrambled up a hill of mud. After we entered the forest, picking our way along a narrow path slick with leaves and framed by fronds, we came upon a clearing and the thick, amber-colored trunk of a tree that extended above the canopy and into the sky. Then we waited. “Pygmies are lazy,” de la Torre said. “They like to sleep.” 

The sky was gray, and rain pattered down through the leaves. Pygmy marmosets usually head to their feeding tree shortly after they rouse in the morning, and then they play and forage; they often have another burst of activity around dinner shortly before sundown. De la Torre told me that the pygmies take longer to come out on rainy days. But she heard the babbling of an infant, somewhere in the trees above us. We were in the right spot. 

Usually, the first member of a group of marmosets to venture out is an adult male. De la Torre listened, tracking as the pygmy marmosets made their way from their sleeping tree toward the amber trunk, which was pocked with their gouges and oozing clear resin. Since the pygmies are so very tiny and defenseless, they move quickly, skittering along the branches and ducking under leaves. It’s quite possible to stand under a group of pygmies and never know they’re there, unless, like de la Torre, you can decipher their calls.

The pygmies always come back to their feeding tree, though. Pygmy marmosets use their teeth to gouge holes in trees that secrete gum, or resin—not to be confused with sap, which runs through a tree’s vascular system. The monkeys consume the starchy, carbohydrate-rich stuff with their tongues. In this, they are uncommon. Only a small number of the world’s animals, including lemurs and tamarins, have similar gustatory proclivities. One strange thing about pygmy marmosets, de la Torre has documented, is that their choice of tree doesn’t seem to rely on the abundance of its resin. Each group of monkeys has trees it returns to every day, and almost none of them share the precise set of favorites. It might be that the gum preferences of a group of pygmy marmosets can’t be explained by anything other than taste.

The feeding tree was a Parkia balslevii. Although the group had gouged little holes in a few other nearby parkias, this trunk was thicker than the rest, and its bark was rippled with the scars of their gouging. The pygmy marmosets were moving toward their feeding tree from the one where they had slept, pressed together up high between the branches. A scampering adult pygmy marmoset barely leaves a trace in the leaves and vines. De la Torre could tell that they were moving, but we couldn’t see anything in the jumbled knots of vine under the canopy. 

Suddenly, among the leaves, I made out a little tail, fuzzy and curled into a hook. From around the curve of the tree came the first pygmy marmoset, skimming along the surface, limbs and tail splayed against the trunk. Its head bobbed up and down, and it moved in quick jerks. Another marmoset scampered down a branch and stopped suddenly to look at us. It was so small, and colored so indistinguishably from the tree, that even sitting a few yards in front of us it was easy to miss. It kept its body hunched, and its face was etched like miniature porcelain against the halo of fuzz that gathered at its neck. In Spanish, the pygmy marmoset is called leoncillo, or little lion. Lilliputian lions, whose wildebeests are passing butterflies.

Then, they were there all at once. Now there was one on the tree, nuzzling the trunk as it licked up the jelly-like resin; now there was another scurrying down the side. Two juveniles darted across a branch, and the younger female began to hunt flying insects, moving in a slow crouch. “The adult male is missing,” said Ramiro Sanmiguel, our forest guide. “It’s raining so he’s still up high, maybe with the babies.” 

After a few minutes, we spotted him, moving quickly up a skinny tree behind the amber one. He was tense, Sanmiguel observed, and he wasn’t carrying infants. The marmoset made his way to the parkia and jumped onto it, jutting his face into the bark to eat. He was 30 feet up the trunk, much higher than the others had been. (In general, pygmies try to stay out of sight of birds of prey.) If he had left the infants somewhere, they would be nearby. The rain kept pattering down, and the pygmies began to call each other in a long chirrup. 

A few minutes later we found them again, in the crook of a climbing branch that wrapped around a nearby, vine-covered tree. They were huddled together, at least five of them, their bitty heads clustered and their bodies pressing against each other. When the rain cleared and they dispersed into the tree canopy to hunt, a golden-mantled tamarin monkey came ambling down to their feeding tree and ate some of the resin. Tamarins are smaller than a common house cat, but compared with the pygmy marmosets they looked imposing. Resin is not the tamarins’ preferred food, but in the absence of other things, as during dry seasons, they’ll make do with what’s available. “The tamarins steal,” de la Torre told us. “It’s a problem.”

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De la Torre has a recurring dream of falling out of a canoe into the Amazon and being surrounded by black caimans, alligators that can reach mythological proportions. In the dream, she is frightened, but when she was on an actual trip once and encountered a gargantuan anaconda in a lake where she and her sister were paddling, she insisted on swimming in the water so that she would not be afraid. She feeds neglected dogs, and keeps bees, and gives stray cats a place in her home. 

When de la Torre began her doctoral work in the mid-1990s, at the University of Wisconsin-Madison under the renowned primatologist Charles Snowdon, she thought she would study how tamarin habitats affect their vocalizations, but she soon switched to pygmy marmosets. Their smaller home ranges—often less than 2.5 acres—made it easier to collect data. 

Over the course of her fieldwork, de la Torre observed the way pygmy marmoset groups developed specific dialects that were learned by the infants and that depended in part on the ambient noise of the surrounding forest. Scientists unaffiliated with de la Torre recently used artificial intelligence to determine that marmosets—a larger cousin of pygmy marmosets—use vocal cues for each individual in a group, akin to calling each other by name.

That proclivity for conversation is useful in such a species whose feeding habits depend on making collective use of a tree. De la Torre told me that in her years of studying pygmy marmosets, she has never seen one display aggression toward another member of the group. “They are very clever, and they are cute in the sense that they have these very nice behaviors between each other, like grooming and playing,” she said. “They are more cooperative than other primates, and I sometimes wonder if this extreme cooperation is something that is not necessarily benefiting them in the current time.” 

Largely defenseless in forests of potential predators, the monkeys occupy a dietary niche that makes them vulnerable to the incursion of oil companies and settlers on their riverside habitat. De la Torre told me that groups try to survive by eating the resin of less desirable trees and expanding their home ranges, but that they often can’t outrun the encroachment. Once exposed, living in fragmented forests close to humans, the monkeys can more easily come into contact with diseases, including human ones. “Malaria could kill them,” de la Torre said. “Dengue. Probably Covid as well. Yellow fever is known to kill them. Herpes is known to kill them.” She suspected that a yellow fever outbreak might have been the culprit for the mysterious disappearance of her study groups. “If we don’t first of all find out what is really going on with them, and then if we don’t do things to preserve their habitats and to try to reduce our impact, I’m really scared that they will at some point be gone from here.”

After an interlude, two juveniles came prancing down the vines, hopping along to alight on the parkia. They began to chase each other, darting in and out among the leaves, quick as a passing shadow. Finally, the adult male made his way down to the elbow of a descending bough to survey us. His delicate cheekbones were high and widely set, although his face was barely bigger than a thumbprint. As he kept watch, some of the others darted along the amber trunk, and, after some time, he joined them. When they had finished, de la Torre heard the golden-mantled tamarins returning through the canopy. We watched as they appeared on the amber trunk, one by one, catlike and resplendent in their orange coats.

When the tamarins were done eating, one of them appeared in the tree where the pygmy marmosets had been playing. With its face pointed downward, it let out a tsk, tsk, tsk. There was a stillness in the air. It took a moment to see, just a few feet down the trunk, one of the juvenile pygmies, frozen and peering up at the tamarin’s white and black face. They stared at each other. Then the tamarin departed, loping through the leaves. After a moment, the pygmy marmoset crept toward the spot where the tamarin had been, and we saw a few droplets of its urine patter down into the undergrowth. 

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There is no want of ostentatious life in the rainforest, among the wide-winged macaws alighting in the overstory and the militaristic millipedes clambering underfoot. But it is a strange thing to see a pygmy marmoset. They’re curious in the way of primates, and the younger ones especially can’t seem to help but make inquiring overtures to interlopers. Even as they dart up and down branches and poke their strawberry-sized heads around the vines, they so perfectly match their surroundings that they can appear nearly like lumps on a branch. One very much gets the impression that they might prefer to not be seen at all. 

De la Torre had heard that a group of pygmy marmosets she followed in the territory of the Indigenous Secoya community had vanished, but that there might be other groups across the river. After driving through miles of palm plantations, we arrived at an area where the community had decided to try to preserve the forest. An old friend of de la Torre’s named César Piaguaje picked us up in his canoe and took us to his house, where his family greeted us. Then, we went out to a tree where a pygmy marmoset that Piaguaje’s daughter had kept as a pet established a family after he returned to the forest. (It was once popular to make pygmy marmosets into pets, but they often die in captivity, and the Secoya community recently did away with the practice.) 

Juvenile pygmy marmosets like to venture out to the edges of their family’s home ranges, in part to explore, but also to try to find a mate and establish a new group. Families vary in size, but when the juveniles can’t find mates, they stay behind as more and more babies are born; the family group near Piaguaje’s house had reached 15 members. 

Piaguaje led de la Torre to the tree, where she gathered data on the marmosets’ preferences using a tool called a durometer, which measures hardness. Then Piaguaje took her farther along, through bamboo stands near the riverbanks. “I think they could be here,” he said, machete swaying through the brush. “But what would they be eating?” He stopped in front of a vine-covered trunk. “A few months ago, I saw them in this tree. After that, I never saw them again.”

“It’s worth checking once in a while because it is possible they could come back,” de la Torre said.

“I could come in the mornings,” Piaguaje said, nodding.

“The four or five trees that I measured aren’t producing resin—they are dry,” de la Torre said. This meant that the pygmy marmosets would have nothing to eat. 

Later, de la Torre and Piaguaje went together to check on spots along the river where people had reported seeing pygmy marmosets, but there was nothing conclusive. Aside from a group Piaguaje had found near his son’s house, it wasn’t clear where any other group might be found.

Sitting in the shade of Piaguaje’s lodge, I asked him about the monkeys. He was proud they were there; his part of the community hadn’t given in to palm oil planters, and his granddaughter belonged to a recently formed group of forest guardians who patrolled to stop settlers from other parts of the country from felling the forest to build housing compounds and plant palm.

Piaguaje told me that, according to Secoya mythology, in the beginning the creator made a pair of beings who didn’t want to have much to do with anyone else. They weren’t yet people, they weren’t yet fully formed, and so the creator decided that these beings would be pygmy marmosets. Living among themselves, tiny and quick-footed, the reticent pygmy marmosets could have the privacy they so cherished.

When we returned to the amber-colored parkia tree, the pygmy marmosets were still talking. The male came back to the tree, slinging an infant on its back. The infant, big enough to walk and eat resin, would soon be too large to be schlepped from here to there. It babbled in the tree, making twittering exclamations interspersed with imitations of different adult calls. Eventually, the family would move together to the tree where they would sleep, nestled together in their group of seven. 

We left the tree and traveled on, crossing the fragmented forest, visiting places where pygmy marmosets had once lived and places where they still did. At the end of the week, I took the car back up to the highlands with de la Torre. As I headed toward the airport, passing glacial lakes and volcanoes, I was suddenly struck with awe at my fortune at being one of the bizarre fruits life has born, along with the pygmy marmosets, and at having been a part of their conversation, however fleetingly.