Spider monkeys and their close relatives (woolly monkeys, howler monkeys, and muriquis) are the largest nonhuman primates in South America.  Predation of these species by animals other than humans is not common, although there are reports of harpy eagles preying on woolly monkeys in Yasuní and similar reports in the literature for the other Atelines.

The white-bellied spider monkey is, for the most part, distributed throughout the Amazon in rain and deciduous forest.  Spider monkeys are entirely arboreal and are aided in locomotion and foraging by a prehensile tail.  The species is not sexually dimorphic; in other words, adults of both sexes are roughly the same size.  Spider monkeys appear to be ripe fruit specialists, although they will eat new leaves and flowers when fruit is not available.

Spider monkeys display a rare form of social organization referred to as fission-fusion sociality in which members of a large heterosexual community with a large home range commonly break into smaller parties of variable composition for daily foraging and travel.  Chimpanzees and some populations of muriquis are the only other primates with this unique social system.  Party size tends to decrease when food patches are smaller, or when fruit is less common in the environment. 

Given fission-fusion sociality, the potential distance between parties of the social group varies considerably.  Individual spider monkeys communicate information about location and food patches, among other things, with long calls.  Human observers can hear these calls from 500 meters and can often distinguish the calls of different individuals; it is assumed that spider monkeys have similar abilities.  Data on dispersal, or the movement of individuals from the group of birth prior to the onset of reproduction, is limited, although studies of other species of spider monkeys suggest that males remain in their natal range while females emigrate.  Likewise, conclusions about spider monkey mating patterns are incomplete.  It is thought that males do not monopolize receptive females and that females actively choose partners and may mate with multiple males.

• Chapman, C. (1988)  Patch use and patch depletion by the spider and howling monkeys of Santa Rosa National Park, Costa Rica.  Behaviour 105: 99-116.
• McFarland Symington, M. (1990) Fission-fusion social organization in Ateles and Pan.  International Journal of Primatology 11: 47-61.
• van Roosmalen, M. (1985) Habitat preferences, diet, feeding behaviour and social organization of the black spider monkey (Ateles paniscus paniscus Linnaeus 1758) in Surinam. Acta Amazonica 15: 1-238.

Lowland woolly monkeys -- also known as common or Humboldt's woolly monkeys  -- are widely distributed throughout western Amazonian South America and are divided into four, geographically distinct subspecies.  Woolly monkeys of Yasuní belong to one of the smaller, least studied subspecies.  Woolly monkeys live in primary forest, do not adapt to disturbed areas, and are very sensitive to hunting pressure. 

Like spider monkeys, woollies are entirely arboreal and highly frugivorous.  Di Fiore, however, found that woolly monkeys in Yasuní spend a significant part of their time budget foraging for insects, perhaps as a strategy for laying down fat reserves for the dry season when fruit is not as abundant.  The other small subspecies of woolly monkey, Lagothrix lagotricha lugens, is similar in its reliance on insect prey.  Woolly monkeys have a prehensile tail.

Woolly monkeys live in large multimale, multifemale groups of 20 to 40 individuals.  They are sexually dimorphic, with males weighing about 20% more than adult females.  Within social groups, inter-individual distance varies considerably throughout the day, although social cohesion is maintained by frequent vocalizations.  Unlike spider monkeys, daily travel and foraging behavior is coordinated and the group does not fission into independent subgroups.   Inter-group aggression is low in woolly monkeys, and the home ranges of different groups have a large degree of overlap.  In other words, woolly monkeys are not territorial.

The pattern of emigration from natal groups is unresolved in woolly monkeys, although it is likely that males are philopatric as in spider monkeys and muriquis.  However, solitary individuals observed in the Proyecto Primates study area are mostly males.   Adult males woolly monkeys are tolerant of mating by other males within their social group, and females often mate with more than one male during estrus.  Matings may be harassed by other adult females from the social group. 

• Nishimura, A. (1990)  A sociological and behavioral study of woolly monkeys, Lagothrix lagotricha, in the Upper Amazon.  Science and Engineering Reviews of Doshisha University 31: 87-121.
• Peres, C.A. (1994)  Diet and feeding ecology of gray woolly monkeys (Lagothrix lagothricha cana) in Central Amazonia: comparisons with other Atelines.  International Journal of Primatology 15: 333-72.
• Stevenson, P.R., Quiñones, M.J., Ahumeda, J.A. (1994) Ecological strategies of woolly monkeys (Lagothrix lagotricha) at Tinigua National Park, Colombia. American Journal of Primatology 32:123-14

Red howler monkeys are distributed through much of the western Amazon both in mature and disturbed forests; howler monkey density is often higher near major rivers.  Like their relatives the spider and woolly monkeys, howlers are a large primate for South America and have a prehensile tail.  Howler monkeys are sexually dimorphic, with a difference in body size between males and females larger than that found in woolly monkeys.   Predation by harpy eagles and jaguars has been noted. 

The diet of howler monkeys is composed of a larger percentage of leaves, especially new leaves, than that of their close relatives, although howlers do eat fruit.  Variation in diet composition varies according to forest type, climate and, of course, phenology of the forest (or the change in abundance of ripe fruit, flowers and new leaves throughout the year).  Day ranges are, on average, less than those of woolly and spider monkeys, perhaps reflecting poorer nutrition through a greater reliance of leaves.  Home ranges of different social groups do overlap, and there is little defense of territory between social groups.  The howler monkey long call, often heard in the morning, may facilitate intergroup spacing.  The red howler monkeys of Proyecto Primates, however, are cryptic and appear to be at low population density.

Red howlers typically live in stable, heterosexual groups of 4 to 17 individuals.  Even though these social groups can have more than one adult male, a single male often obtains the majority of copulations and sires most of the offspring.  Female howler monkeys emigrate prior to the onset of reproduction, and males, when they do emigrate, often move to groups closer to their natal group. 

• Crockett, C.M., Pope, T.R. (1988) Inferring patterns of aggression from red howler monkey injuries.  American Journal of Primatology 14:1-21.
• Milton, K. (1980)  The Foraging Strategy of Howler Monkeys: A Study in Primate Economics.  New York: Columbia University Press.
• Pope, T. (1992) The influence of dispersal patterns and mating systems on genetic differentiation within and between populations of the red howler monkey (Alouatta seniculis). Evolution 46:1112-1128.

The white-fronted capuchin is found throughout the western Amazon and has between 11 and 13 subspecies.  Like the howler monkey, capuchins are often able to adapt to more disturbed forests and are considered a pioneering species -- primarily because of their omnivorous diet.  The number of potential predators is greater than for the Atelines because of capuchin's smaller body size and includes smaller hawks and felids.

Capuchins are very flexible in their diet, depending on the local and seasonal availability of preferred foods.  They eat ripe fruit, seeds, insects, leaves, buds, shoots, and vertebrates.  They are known to open palm nuts by hammering them against tree trunks, a sound often heard in Yasuní.  Range size, degree of territory overlap between social groups, and day range length varies considerably among studies and depends on the availability of local resources.  In Yasuní, capuchins tend to travel with social groups of squirrel monkeys.

Social groups are heterosexual and contain between 7 and 30 individuals.  Male capuchins are larger than females and emigrate from their natal groups.  Mating is promiscuous, although the dominant male may be able to monopolize genetic reproduction.   Female capuchins form the stable core of the social group with observable matrilineal dominance hierarchies.   Unlike many other South American primates, grooming of other individuals is often observed.  Infanticide has been noted. 

• Janson, C.H. (1986) The mating system as a determinant of social evolution in capuchin monkeys (Cebus). In J.G. Else and P.C. Lee (eds): Primate Ecology and Conservation.  Cambridge: Cambridge University Press:169-180.
• Janson, C.H. (1988) Food competition in brown capuchin monkeys (Cebus apella): quantitative effects of group size and tree productivity. Behaviour 105:53-76.
• Podolsky, R. (1990) Effects of mixed-species association on resource use by Saimiri scireus and Cebus apella. American Journal of Primatology 21:147-158

The common squirrel monkey ranges throughout the Amazon basin in both mature and disturbed forests.  Like the capuchin, squirrel monkeys are arboreal and favor the middle and lower levels of canopy, and both species have been known to briefly descend to the ground.  Locomotion is also like that of the capuchin --  climbing and jumping -- primarily because of the lack of both a prehensile tail and a semibrachiating forelimb, as found in Atelines.  Similarly, the small body size of squirrel monkeys makes predation a potentially important environmental factor in that smaller raptors and felids can easily hunt them. 

The diet of squirrel monkeys is composed of both insects and fruit, the quantities of each depending on seasonal availability.  Home range size is large (between 100 and 500 hectares) and seems to depend on resource abundance and group size.

Social groups are large and cohesive; group composition includes multiple adult females and males and their offspring, often between 23 and 54 individuals.  For the species of squirrel monkey found in Yasuní, previous studies in Peru have observed a dominance hierarchy among females.  Females remain in their natal group while males emigrate.  However, even though females are related and have a dominance hierarchy, grooming behavior is not as common as in the capuchin.  Female squirrel monkeys do mate with more than one male, although the dominant male may be able to monopolize a female in estrus.

• Boinski, S.  (1994)  Affiliation patterns among male Costa Rican squirrel monkeys.  Behaviour 130:191-209.
• Mitchell, C.L., Boinski, S., van Schaik, C.P.  (1991) Competitive regimes and female bonding in two species of squirrel monkey (Saimiri oerstedi and S. sciureus). Behavior, Ecology and Sociobiology 28: 55-60.

Taxonomic work is incomplete in this genus which is widely distributed throughout the Amazon in a variety of different habitat types, including disturbed forest.  It is thought that there are between 5 and 7 species of night monkeys, and the species found in Ecuador -- the noisy night monkey -- is limited to the upper Amazon basin. 

This genus is arboreal and nocturnal, presumably as a strategy for avoiding predators.  Daytime is typically spent in tree holes, although several studies in more temperate forests, including a new one in Argentina by Fernandez Duque has found that night monkeys forage early and late in the day as well as at night.  Daytime foraging may be related to cold during the night.

Night monkeys are mainly frugivorous, although they also eat flowers, insects, leaves and nectar.  Day range length is short, especially on nights without moonlight, and territory size is small.

Groups are small, generally containing between 2 and 5 individuals.  Groups are composed of a monogamous pair and their offspring, and they defend territories from other social groups.   Both sexes emigrate from their natal groups before the onset of reproduction.  Males and females are monomorphic, or similar in body size, and males assume much of the parental care, often carrying the infant from the day of birth. 

• Aquino, R., Encarnacion, F.  (1986)  Characteristics and use of sleeping sites in Aotus (Cebidae: Primates) in the Amazonian lowlands of Peru.  American Journal of Primatology 11: 319-331.
• Wright, P. (1984) Biparental care in Aotus trivirgatus and Callicebus moloch.  In M. Small (ed): Female Primates: Studies by Women Primatologists.  New York: Alan R. Liss: 59-75.
• Wright, P. (1989)  The nocturnal niche in the New World.  Journal of Human Evolution 18: 635-658.

As with the night monkey, taxonomic affinities among the roughly 13 species of the titi monkey are unclear.  Titi monkeys are found in a wide variety of forest types and seem to prefer dense vegetation.

Titi monkeys are primarily frugivorous and some species are seed predators.  Group territories are small (roughly 5 to 12 hectares) and daily range length is typically under 800 meters.  Larger groups tend to have longer daily range lengths.

Group size is small in titi monkeys, between 2 and 5 individuals, and groups consist of a monogamous pair and their offspring.  Like other monogamous species, titi adults are roughly the same weight and size.  The adults in titi social groups are highly territorial and give loud duets, typically at dawn, when other pairs are calling from their territories.  A close pair bond is often found between the adult male and adult female in a group.  They often entwine their tails during duets and as well as during other times of the day (such as during rest), and grooming takes up almost 10% of the daily time budget.  Male titi monkeys participate in parental care of offspring and  both sexes emigrate from their natal social group.

• Kinzey, W., Robinson, J. (1983)  Intergroup loud calls, range size and spacing in Callicebus torquatus. American Journal of Physical Anthropology: 60: 539-544.
• Wright, P. (1986)  Ecological correlates of monogamy in Aotus and Callicebus.  In Else, J., Lees, P. (eds): Primate Ecology and Conservation.  Cambridge: Cambridge University Press:159-167.

Saki monkeys are found in primary and disturbed rainforest as well as cloud forest.  They tend to use middle to upper canopy and move by running, leaping and clinging.  In Yasuní, Pithecia have been observed in mixed species groupings with capuchins, squirrel monkeys, spider monkeys, or woolly monkeys.  Predation by raptors and felids has been observed in other field sites.  It is not known how the two species of Pithecia found in Yasuní partition the forest in areas where they are sympatric.

Like many neotropical primates, sakis are frugivorous, although they tend to eat harder fruits and the softer seeds within these fruits.  Leaves and insects complete the observed diet.  Range size in this species is unknown but is possibly larger than that of other better studied species of sakis.   Group ranges overlap in some species.

Social groups of monk sakis have between 2 and 8 individuals.  The social organization of saki monkeys is assumed to be monogamous, but since so little research has been completed on this genus, some variation in grouping patterns may exist.  Infant care is primarily by the mother, though males and more mature offspring occasionally carry infants.  Mothers share food with their offspring. 

• Kinzey, W. (1992) Dietary and dental adaptations in the Pitheciinae.  American Journal of Physical Anthropology 88: 499-514.
• Peres, C. (1993)  Notes of the ecology of buffy saki monkeys (Pithecia albicans, Gray 1860): a canopy seed-predator.  American Journal of Primatology 31: 129-140.
• Sioni, P. (1986)  A synecological study of a primate community in the Pacaya-Samiria National Reserve, Peru. Primate Conservation 7:63-71.

Pygmy marmosets are found throughout the upper Amazon basin and appear to prefer forest along the edges of rivers or in varzea, although they do live in terra firme at lower population densities.  Pygmy marmosets travel through the forest by running along substrate and clinging to trunks.  This species is the smallest primate in the neotropics, weighing between 85 and 141 grams, and are probably preyed upon by a variety of predators include raptors.

Pygmy marmosets feed on both insects and on gum from holes they create in trees such as Inga, Parkia, and Spondias.  The size of social groups is usually between 2 and 9 individuals, and the home ranges of these groups is small.  Because the distribution of gum producing trees changes over time, ranges of social groups change with time as well.  Day range length is typically between 30 and 100 meters.

The social system of pygmy marmosets appears to be monogamous, and the adult female is dominant to all individuals in the group.  In general, females are slightly larger than males.  Infants are typically twins in this species, and all members of the social group appear to assist in infant care. 

• Soini, P. (1988) The pygmy marmoset, genus Cebuella.  In Mittermeier, R., Rylands, A., Coimbra-Filho, A. and da Fonseca, G. (eds): Ecology and Behavior of Neotropical Primates, Vol. 2.  Washington D.C.: World Wildlife Fund.
• Soini, P. (1993) The ecology of the pygmy marmoset, Cebuella pygmaea: some comparisons with two sympatric tamarins.  In Rylands, A. (ed): Marmosets and Tamarins: Systematics, Behaviour and Ecology.  Oxford: Oxford University Press.

Golden-mantled tamarins are distributed in a small section of the Napo region of Ecuador and northeastern Peru.  Tamarins, in general, are arboreal and favor the mid and lower canopy of tropical forests.   This species has been subject to only one long-term study of which publication is currently in preparation.  Predators include raptors and snakes.

Tamarins eat mainly fruit, nectar and insects but also consume gum from either natural holes or holes created by pygmy marmosets (see above), especially during the dry season.   For the genus, home range size is between 20 and 40 hectares, home ranges of adjacent groups overlap, and  day range length is long -- averaging over 1200 meters.

Social groups are small, containing between 4 and 10 males and females.  It appears that both sexes emigrate from their natal groups.  In terms of mating behavior, usually only one female in a social group reproduces.  The ovulatory cycles of other adult females in the social group are usually suppressed.  Females commonly give birth to twins which are energetically expensive in terms of lactation and transportation; consequently,  breeding females need help with infant care.  Infant care is communal, and adult male assistance is especially important in terms of infant survival.

• Garber, P. (1993) Feeding ecology and behavior in the genus Saguinus.  In Rylands, A. (ed): Marmosets and Tamarins: Systematics, Behaviour and Ecology.  Oxford: Oxford University Press.
• Tardif, S., Harrison, M., Simek, M. (1993) Communal infant care in marmosets and tamarins: relation to energetics, ecology and social organization.  In Rylands, A. (ed): Marmosets and Tamarins: Systematics, Behaviour and Ecology.  Oxford: Oxford University Press.

This short list focuses on the description, distribution, behavior and ecology of South American primates and are included in addition to the journal articles noted above.  A few general college-level text books on general primate behavior and ecology are included for information. 
 

• Ciombra-Filho, A.F., Mittermeier, R.A. (1981)  Ecology and Behavior of Neotropical Primates, Volume I. 
• Eisenberg, J.F., Redford, K.H. (1999) Mammals of the Neotropics: The Central Neotropics, Volume III: Ecuador, Peru, Bolivia, Brazil.  Chicago: University of Chicago Press.
• Emmons, L., Feer, F.  (1997) Neotropical Rainforest Mammals: A Field Guide, Second Edition.  Chicago: University of Chicago Press.
• Kinzey, W.G. (1997)  New World Primates: Ecology, Evolution and Behavior.  New York: Aldine de Gruyter.
• Mittermeier, R.A., Rylands, A.B., Coimbra-Filho, A., Fonseca, G.A.B. (1988)  Ecology and Behavior of Neotropical Primates, Volume II.  Washington D.C.: World Wildlife Fund.
• Norconk, M.A., Rosenberger, A.L., Garber, P.A. (1996)  Adaptive Radiations of Neotropical Primates.  New York: Plenum Press.
• Robinson, J.G., Bennett, E.L. (2000)  Hunting for Sustainability in Tropical Forests.  New York: Columbia University Press.
• Smuts, B.B., Cheney, D.L., Seyfarth, R.M., Wrangham, R.W., Struhsaker, T.T. (1987)  Primate Societies. Chicago: University of Chicago Press.
• Strier, K.B. (2000)  Primate Behavioral Ecology.  Boston: Allyn and Bacon.
• Terborgh, J. (1983)  Five New World Primates: A Study in Comparative Ecology.  Princeton: Princeton University Press