Spider Monkey (Ateles geoffroyi)
by Katharine Milton, Ph.D.
Spider monkeys are the acrobats of New World primates. Unlike other New World monkeys, they do not travel about on all fours (quadrupedal) but rather are brachiators, swinging the body down and around the grasping hand and using the prehensile tail as a fifth hand to aid in their rapid locomotion. Like howler monkeys, spider monkeys have a very wide geographical distribution, being found from southern Mexico through most tropical regions of Central and South America. They are totally arboreal--but this does not mean that they, like other New World primates, cannot move about on the ground. Typically they are found swinging and feeding in the tallest and largest forest trees. There is some debate as to how many spider monkey species there are--some authorities want to place all spider monkeys in a single species whereas others recognize 5 species. When adult, spider monkeys, like howler monkeys, weigh from 8 to 9 kilograms.
The Barro Colorado Island spider monkey species is Ateles geoffroyi, the black-handed spider monkey. This species is covered with brilliant reddish hair, but the immediate facial area tends to be naked and the area around the eyes lightly pigmented, often with many tiny or large freckles or black dots. It is possible for human observers to recognize individual spider monkeys by their distinctive facial markings. Spider monkey males and females are about equal in size, in contrast to howlers, in which males are about 20% larger than females. When Barro Colorado Island was established as a nature preserve in 1924, there were no spider monkeys on the island as they had been exterminated from the area by hunters. In the late 1950's a number of young spider monkeys were purchased in the market in Panama City and released onto the island, provisioned with fruits to help them survive to adulthood. Five monkeys did survive--one male and four females and they were the founders of the present-day Barro Colorado spider monkey population, which now numbers some 24 animals and is in its third generation. The Barro Colorado Island spider monkeys have the same behavior, social structure and diet as any other spider monkeys even though they were raised without older "role models" to guide them.
Spider monkeys have an unusual social structure. They live in relatively closed social units called "communities", which are composed of some 18-30 members but, unlike howler monkeys, spider monkeys do not travel through the forest and feed as a cohesive social unit. Rather they have what is known as a "fission-fusion" social structure in which typically only a few spider monkeys are moving about the forest together at any one time. The entire group may come together late in the day, and, on BCI, are often all seen playing, grooming, or fighting around the laboratory buildings in the early evening but most of the day they are seen in small sub-groups of only a few individuals or even alone. Spider monkeys are dietary specialists in that the great majority of their food is ripe fruits. Because ripe fruits tend to be distributed on relatively few trees in the forest at any one time and because each tree may have only limited amounts of ripe fruit, spider monkeys apparently are forced to break up into subgroups each day to feed--each subgroup moving to a certain number of fruiting trees within the home range area of the total community each day. In this way, competition for ripe fruits should be kept low and all members of the community should get enough to eat. As fruiting trees tend to be fairly sparsely distributed throughout the forest, spider monkeys have a large home range. On BCI, it is estimated that male spider monkeys range over an area of 300 or more hectares in search of food while females range over a somewhat smaller area, estimated at 200 or more hectares. Like howlers, spider monkeys eat a wide array of different plant species, consuming foods from over 125 different plant species per year.
There is intense bonding between males of a spider monkey troop and if you see a group of spider monkeys in the forest, you may discover that all of them are males. As a general rule, males travel farther than females each day to feed and also carry out long, drawn-out episodes of social grooming--a behavior which many believe serves to lower tensions between individuals and helps them remain on good terms with one another. Individual spider monkeys often give a loud call when they encounter trees heavily laden with fruits; this call alerts other members of their community as to the location of this dietary resource and many individuals may temporarily congregate in the area to take advantage of this nutritious windfall.
Like human infants, young spider monkeys have a very long period of maternal dependence. On BCI, spider monkey infants, in contrast to the brilliant red adults, are black when born and tend to have large naked pink areas around the eyes. Their "baby face" clearly distinguishes them from adults who tend to be very tolerant of younger individuals. Infants remain tiny and fairly helpless until they are almost a year old. They are carried by the mother until more than two years of age and continue to associate and travel with the mother until they are at least 3.5 years old. Spider monkeys have relatively large brains for their body size and it would appear that as they mature, they need time to learn the types and locations of a large number of different fruit species in the forest as well as other behaviors critical for survival. The long period of maternal dependence should permit each young monkey to mature sufficiently to master all of the information that seems to be required to specialize successfully as an independent adult on a diet composed almost exclusively of ripe fruits.
Katharine Milton 1998