Golden-Backed Squirrel Monkey, Saimiri ustus
GOLDEN-BACKED SQUIRREL MONKEY
Geographic Distribution and Habitat
The golden-backed squirrel monkey (Saimiri ustus), also known as the bare-eared squirrel monkey, is a new world monkey species that is endemic to the Amazon basin, primarily found in Brazil, south of the Amazon River. They prefer seasonally flooded habitats and swamp forests along the banks of the river; however, they can also be found in moist tropical forests that do not get flooded. While pristine untouched forests are everyone’s dream, golden-backed squirrel monkeys can tolerate habitat modifications and can also be found in disturbed habitats such as forest fragments and forests that have regrown after extensive clearing.
The golden-backed squirrel monkey is a species that belongs to the genus Saimiri. The number of species within the genus Saimiri (squirrel monkeys) has been widely investigated and was historically divided into two species based on geographic distribution: the Central American squirrel monkey, Saimiri oerstedii, and the common squirrel monkey, Saimiri sciureus, in South America. However, recent increased genetic analysis has shed light on the molecular, morphological, and behavioral differences, and the genus has been reclassified into seven species:
- Black-crowned Central American squirrel monkey (Saimiri oerstedii)
- Guianan squirrel monkey (Saimiri sciureus)
- Black-capped squirrel monkey (Saimiri boliviensis)
- Golden-backed squirrel monkey (Saimiri ustus)
- Black-headed squirrel monkey (Saimiri vanzolinii)
- Humboldt’s squirrel monkey (Saimiri cassiquiarensis)
- Collins’ squirrel monkey (Saimiri collinsi)
The black-crowned Central American squirrel monkey is isolated in Central America in a relatively small area of lowland forests that extend from the central Pacific coastline of Costa Rica through the Pacific Coast of western Panama. The remaining six species occur in the lowland forests of the Amazon Basin of South America, ranging from Guyana through Paraguay, although the golden-backed squirrel monkey’s primary range is in Brazil, south of the Amazon River.
Size, Weight, and Lifespan
The golden-backed squirrel monkey shows sexual dimorphism in body size, with males being about 13% larger than females. On average, males can weigh between 1.4 and 2.6 lbs (620–1200 g) and females weigh around 1.6–1.9 lbs (710–880 g).
Members of the genus Saimiri may live for up to 30 years in captivity. However, in the wild, individuals may live for approximately 15–20 years.
The golden-backed squirrel monkey derives it alternate common name, the bare-eared squirrel monkey, from the lack of hair on its ears. The forearms, hands, feet, and back are covered with orange or yellowish hair, whereas the head is brownish-gray. Their face has a edistinctive black snout and white masks around the eyes, forming an arch. In fact, the arch shape of the hairpatch above the eyes has been used by some to identify different species. The golden-backed squirrel monkey, Central American squirrel monkey, and South American squirrel monkey possess a pointed arch of whitish hair above each eye forming a deep “V,” lending them the classification of “gothic arch” squirrel monkeys. The black-capped squirrel monkey and black squirrel monkey possess a rounded, shallow semicircular pattern above the eyes and are classified as “roman arch” squirrel monkeys.
Hindlimbs are longer than forelimbs, which is likely an adaptation suited for frequent leaping, their primary mode of locomotion. While squirrel monkeys can locomote through quadrupedal walking and running along the surfaces of branches, they prefer to leap between them and rarely descend to the ground to forage. Although they are born with long tails adapted to grasp or hold objects, the tail’s grasping ability is lost as they grow older, and the long tail in adults is primarily for balancing purposes.
Golden-backed squirrel monkeys are categorized as omnivores and eat both fruits and insects; however, they prefer eating insects more than fruits. So much so that they can spend up to 80% of their day foraging just for insects. Squirrel monkeys have also become specialized in finding insects by unrolling both dead and living leaves to reveal insect prey hidden within them. They may also occasionally eat flowers, nectar, seeds, bird eggs, and small vertebrates like frogs or lizards. Their diets are extremely flexible, and individuals are able to alter their diet in response to changing environmental conditions. For example, during the dry season, due to shortages of appropriate fruiting trees, they can depend entirely on insects or other small animal prey.
Golden-backed squirrel monkeys are diurnal (active during daylight hours) and arboreal, spending most of their time in the trees foraging for insects and fruits. While foraging, they tend to move frequently from spot to spot, searching the foliage for available food rather than spending a long time in one location. They are early risers and become active before sunrise. If food is limited, they spend the first hour or two foraging for breakfast. However, if food is abundant, they forage more leisurely and may even take a few power naps in the middle of the day. Foraging continues till a few hours after sunset. During the early and later parts of the day, individuals group-travel and rest together as a group. However, during the daytime, the groups split into small foraging parties of 2–8 individuals to move through the forest.
At night, the monkeys sleep on top of branches, hunched over in a huddled position with the tail curled over the shoulder. Two groups may sleep close to one another in the same home range.
Around mating season, males undergo dramatic physical changes, wherein they fatten up and become big, thereby attract more females.
Squirrel monkeys seek out monkeys of other species, such as capuchins, to form mixed-species groups that improve their foraging efficiency and also provides them with protection.
Golden-backed squirrel monkeys enjoy eating insects and are able to unroll leaves to reveal hidden insects.
Group and social structure for golden-backed squirrel monkeys are not well studied; however, some information can be derived from the research on other populations of squirrel monkeys that also inhabit the Amazonian basin. The size of squirrel monkey groups is variable across species, ranging from 15 to 75 individuals. Black-capped squirrel monkeys form the largest groups (mean size is 54 members), while South American squirrel monkeys display the smallest mean group sizes, at 23 individuals per group. Adult males in South American species usually travel on the periphery of the group but integrate with the adult females during the mating season. While the dominance pattern in a group varies according to the species, most species residing in South America, such as the Guianan squirrel monkeys and the black-capped squirrel monkeys, have been found to have females as the dominant sex with males being subordinate. It is likely that a similar group structure and dominance pattern may exist in the golden-backed squirrel monkeys, but more research is needed.
Squirrel monkey groups form mixed-species groups by joining capuchin monkeys. It has been reported that squirrel monkeys seek out these associations and derive more benefits from capuchins than the other way around. Due to their bigger size and better predator-detecting abilities, capuchins provide a security blanket to the smaller-sized squirrel monkeys. In fact, squirrel monkeys benefit so greatly from the ever-vigilant capuchins and the alarm calls they emit, that they respond more readily to the capuchin alarm calls than they do to the alarm calls of their own group members. Additionally, squirrel monkeys gain access to fruits dropped by capuchins that they would otherwise be unable to open such as tough-husked palm nuts. Capuchins for the most part tolerate the presence of the squirrel monkeys and pay little attention to them.
The vocal repertoire of squirrel monkeys has been heavily researched over the past decades and, so far, six main groups of calls have been classified. “Chucks” are most commonly used and help with coordinating troop movement and maintenance of troop cohesion, especially during foraging. “Peep” and “yap” serve as alarm calls and occur during situations such as hawks or vultures soaring overhead. Any activity is momentarily paused and the monkeys spring instantly to a higher height. Chucks can be combined with other calls such as twitters and peeps, to initiate group contact when separated, mother-infant interactions, or separation from the group. “Cackles” are low-frequency calls that express displeasure or annoyance. For example, food stealing, rough-play, or jostling by other animals will prompt a cackle. “Keckers” are loud sounds most frequently given by adult males. A quarrel within the group or the sudden appearance of a strange individual will cause a general increase in activity or excitement and will be accompanied by a kecker. While infant squirrel monkeys are not very vocal for their first few weeks of life, spending much of their time sleeping on their mother, it has been found that neonatal infants are actually capable of a wide range of vocalizations. As the infant develops and its contact with other group members increases, the vocalizations increase as well.
Squirrel monkeys also use visual displays as communicative means. The most distinctive display behavior is the genital display, which is performed more commonly by males. This display most often occurs either when a new male enters the group and may indicate intermale competition or while trying to woo females during the mating season.
Reproduction in golden-backed squirrel monkeys is also not well studied. However, information on other populations of squirrel monkeys can provide insights. Mating and births are highly seasonal, with mating usually occurring in the dry summer season and lasting for a short period of time, 1 week to 2 months each year. Females are able to first reproduce at an age of 3 years—and males at 5 years. The average gestation length is 5 months and time between births can vary between 1 to 2 years. A single offspring is born. While some females may give birth yearly, not all females in a group do so.
Squirrel monkeys do not show any sort of behaviors that precede copulation: there are no special vocalizations or displays that indicate readiness or willingness to mate. That said, they display one of the most unique reproductive changes amongst all nonhuman primates. Prior to and during the short mating season, males undergo dramatic morphological and physiological changes related to body size, wherein they gain up to 22% of their body weight. This seasonal fattening of males results from accumulation of water between the skin and muscles, particularly along the shoulders, back, and arms, almost resembling a body builder with big biceps. The fattening appears to be in response to changing hormone levels within individual males, and males that become the fattest are those with the highest testosterone levels. Being big comes with an advantage since the beefiest male in the group is preferred as a mating partner by a group of females.
While foraging for fruits, the arboreal golden-backed squirrel monkeys transport the fruits and their seeds away from where they procured the fruit, thus playing a role in seed dispersal for plant species over large distances. Additionally, while searching amongst leaves and branches, they expose insects—so much so that several birds follow groups of these monkeys and capture the escaping insects. Because of their affinity toward eating insects, these monkeys also control the populations of insects and small invertebrates.
The International Union for Conservation of Nature lists the golden-backed squirrel monkey as Near Threatened (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.
Natural threats include predation by raptors, coyotes, and venomous snakes. In fact, 50% of infants do not survive to 6 months of age due to predation by raptors.
Although very little is known about the golden-backed squirrel monkey in the wild, its distribution range coincides greatly with the areas undergoing deforestation, south of the Amazon. Although the species can adapt to living in fragmented forests, rapid loss of habitats and massive deforestation due to agricultural activities such as logging, cattle and soybean plantations, and transportation activities such as roads and railways, may lead to population decline and extirpation from their previous range. Additionally, ongoing and planned hydroelectric projects will flood forests near the rivers, the preferred habitats of this species, making it uninhabitable to survive.
The golden-backed squirrel monkey 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.
While golden-backed squirrel monkeys occur in several areas that are considered protected, such as national parks and biological reserves in Brazil, there are no existing conservation plans specific to the golden backs.
Additional research and monitoring of populations in the wild and genetic studies are required to understand the population size, distribution and trends, taxonomy, and threats. Conservation efforts such as building corridors for movement of the monkeys between fragmented forests and limiting projects in core areas of the forests are needed to limit the impact of industrial growth on population decline.
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Written by Divya Pawar, August 2022