Squirrel Monkey

Squirrel monkeys are small, fast monkeys of the genus Saimiri, part of the same family as the capuchins. The name saimiri comes from the Tupi language of Brazil and is usually translated as small monkey, which is why many people also search for the saimiri monkey. In English they got the name squirrel monkey because of their size and their quick, darting way of running along branches. They are easy to recognize: a white mask around the eyes, a dark muzzle, a gray or olive back, bright yellow or orange forearms and hands, and a long tail with a black tip. That tail is not prehensile. Unlike spider monkeys or howler monkeys, a squirrel monkey cannot hang from its tail, so it uses it for balance when it jumps, and babies sometimes wrap it around their mother. They live in tropical forests from Costa Rica and Panama to Bolivia and the Amazon, almost always in big, noisy troops. They are active by day and spend nearly all their time in the trees. They are also famous for a less happy reason: in 1959 a squirrel monkey called Miss Baker flew on a United States rocket and came back alive, one of the first primates to survive a trip to space.

The IUCN currently recognizes seven species of squirrel monkey. The common squirrel monkey, Saimiri sciureus, also called the Guianan squirrel monkey, lives in Guyana, Suriname, French Guiana and northern and eastern Brazil, and it is the one most often seen in zoos. Humboldt's squirrel monkey, S. cassiquiarensis, covers the western Amazon in Colombia, Venezuela, Ecuador, Peru and Brazil, and the IUCN includes in it the Ecuadorian form, macrodon. The Bolivian squirrel monkey, S. boliviensis, lives in Bolivia, Peru and western Brazil and has a darker cap and a rounder white arch over each eye. Collins' squirrel monkey, S. collinsi, lives in the eastern Brazilian Amazon, and the golden backed squirrel monkey, S. ustus, lives south of the Amazon River in Brazil. The black squirrel monkey, S. vanzolinii, has darker fur on the head and back and the smallest range of any primate in the Americas, about 870 square kilometers of flooded forest in Brazil. Far to the north, the Central American squirrel monkey, S. oerstedii, lives only in Costa Rica and Panama. Scientists still argue about where to draw the lines between them, so some sources list five species and others eight.

A squirrel monkey is not much bigger than a large squirrel, which is part of where its name comes from. The head and body measure about 25 to 35 centimeters (10 to 14 inches), and the tail adds another 35 to 42 centimeters (14 to 17 inches), so the tail is longer than the rest of the animal. Males weigh about 750 to 1,100 grams (1.7 to 2.4 pounds) and females about 500 to 750 grams (1.1 to 1.7 pounds), which makes them among the lightest monkeys in the Americas after the marmosets and tamarins. In several species the males change shape before the mating season. Driven by a rise in hormones, they put on weight in the shoulders, arms and upper body and look much bulkier, a state researchers call the fatted male. Females prefer these bigger males, and the extra weight is lost again once the season ends. Their brain is very large for such a small body, about one seventeenth of their body weight, a higher share than in humans, where it is about one thirty fifth. That big head makes births difficult, because the baby is large compared with its mother.

The Central American squirrel monkey, Saimiri oerstedii, is the only squirrel monkey outside South America. It lives on the Pacific side of southern Costa Rica and in the far west of Panama, in lowland and foothill forests up to about 1,145 meters. There are two forms. The black crowned squirrel monkey lives on the Osa Peninsula, around Corcovado National Park, and across the border in Chiriquí, Panama, including the Burica Peninsula. The gray crowned squirrel monkey lives in the Central Pacific of Costa Rica, with Manuel Antonio National Park as its best known home. Both have a redder orange back than most of their South American relatives, and in Costa Rica people call them mono tití. Their social life is different too: troops are relaxed, with little fighting, and it is the young females who leave to join other groups. In its 2020 assessment the IUCN moved the species from Vulnerable to Endangered, with a decreasing population. Its range is small and broken into patches by farms, oil palm plantations, roads and tourism development, and monkeys die on power lines and roads. Rope bridges over the roads near Manuel Antonio, forest corridors and strict rules against keeping them as pets help them hold on.

Squirrel monkeys occupy a broad stretch of the American tropics. In Central America they live only on the Pacific side of southern Costa Rica and western Panama. In South America they range from the Llanos and the Amazon of Colombia and southern Venezuela through Guyana, Suriname and French Guiana, and across the Amazon basin in Ecuador, Peru, Brazil and Bolivia. They are mostly lowland animals. The Bolivian squirrel monkey lives between about 50 and 500 meters, and Humboldt's squirrel monkey reaches about 1,500 meters on the slopes of the Andes. Unlike many monkeys, they do well in forest that is not untouched. They are most common along rivers, in seasonally flooded forest such as the várzea of the Amazon, in gallery forests that cross the savannas, at forest edges and in secondary forest, where insects are plentiful. In the flooded forests of the Amazon several species live side by side, separated by large rivers: the Amazon itself, the Negro and the Japurá act as barriers that troops do not cross. This is why the black squirrel monkey can survive in a pocket of flooded forest at the meeting of the Solimões and Japurá rivers, inside the Mamirauá reserve.

Food is at the center of almost every waking hour for a squirrel monkey. They are omnivores, but above all they are insect hunters, and in many studies they spend most of their foraging time searching for animal prey. They scan leaves, curled dead leaves, bark and vine tangles for grasshoppers, caterpillars, beetles, katydids and spiders, and they snatch flying insects out of the air with their hands. Ripe fruit gives them quick energy, and they visit fruiting trees such as figs, often in the company of other monkeys. When fruit is scarce in the drier months, flowers and nectar become more important. Small vertebrates such as lizards, tree frogs and sometimes bird eggs or nestlings are eaten when the chance appears and add extra protein. Because a squirrel monkey cannot grip with its tail, it relies on sharp eyesight, fast hands and agile jumps to catch prey among thin branches. A troop spreads out as it moves, and each monkey searches its own patch of leaves while staying in contact by calls. Their foraging is restless and constant, and a troop can travel a couple of kilometers or more in a single day while working through the forest in search of food.

From sunrise to sunset, squirrel monkeys are rarely still. Troops that commonly number between 20 and 75 individuals, and sometimes more than 100, move together through the forest, foraging at every level from the understory to the upper canopy. Communication keeps the group together. Researchers have identified a wide range of calls: soft peeps and twitters that keep troop members in touch while they search for food, chucks between friends, and loud alarm calls that send everyone diving for cover when a hawk or eagle appears overhead. In the Amazon they often travel with capuchin monkeys. The capuchins lead the way to fruit trees, and the squirrel monkeys gain extra eyes watching for predators and catch the insects that the bigger monkeys disturb. Squirrel monkeys also have a curious habit called urine washing: they rub urine on their hands and feet, which may help them leave scent marks, keep a grip on branches or cool down. Their predators include birds of prey, snakes such as boas, small wild cats and tayras, and young monkeys are the most at risk. At night the troop sleeps high in trees, often huddled together for warmth on branches near the trunk or in dense vegetation.

Squirrel monkeys breed once a year. Mating happens in a short season, timed so that babies are born at the start of the rainy season, when insects and fruit are most plentiful. Inside a troop most births come within a few weeks of each other. After a pregnancy of about five months, between 145 and 170 days, the mother has a single baby, which is large for her size. For the first weeks it rides on her back, clinging to her fur, and it starts to explore on its own after about a month. In some species other females help, carrying the baby for a while or keeping it close. Young females are ready to breed at about two and a half years, while males take longer, about four or five. Social life is not the same in every species, which surprised the researchers who compared them. In the Bolivian squirrel monkey, females stay in the troop where they were born, form strong alliances with relatives and dominate the males, who leave. In the Guianan squirrel monkey both sexes leave, and males are dominant. In the Central American squirrel monkey, troops are peaceful and it is the females who move away. In all of them, a big troop means more eyes watching for danger.

The conservation status of squirrel monkeys depends on the species. The IUCN lists four of the seven squirrel monkeys as Least Concern: the Guianan, Humboldt's, Bolivian and Collins' squirrel monkeys, thanks to their wide ranges in the Amazon and the Guianas. The golden backed squirrel monkey is Near Threatened because the forests south of the Amazon are being cleared for farms and cattle. Two are Endangered. The black squirrel monkey lives in a single small area of flooded forest in Brazil, where logging and more extreme floods and droughts put it at risk, and the Central American squirrel monkey has lost much of its forest in Costa Rica and Panama. Even the common species have decreasing populations. Deforestation for cattle ranching, soy and oil palm keeps shrinking and splitting their habitat. In the 1960s and 1970s, very large numbers of squirrel monkeys were caught in Peru and Colombia and shipped to the United States and Europe for the pet trade and for medical research. Laws and CITES now control that trade: the Central American species is on Appendix I, which bans its commercial trade, and the rest of the genus is on Appendix II. Illegal capture for pets still happens in some areas.