Woolly Spider Monkey (Brachyteles arachnoides)

The woolly spider monkey, or muriqui, has a prehensile tail, like most of the large cebids. This species is sexually monomorphic. The woolly spider monkey lacks a thumb, and has long limbs like the spider monkeys. The average body mass for this species is between 12 to 15 kilograms. The canines of males and females are not dimorphic and are generally small in size (Rosenberger and Strier, 1989). The premolars and molars are adapted for folivory, having high shearing crests (Zingeser, 1973; Rosenberger and Strier, 1989). The body hair of this species is buff-gold in color, with the extremities being black in color (Kinzey, 1997). The males have large testicles, and Milton (1985) proposed that competition among males for mates is regulated to sperm competition as opposed to individual aggression.

The woolly spider monkey is found in the country of Brazil, and lives in the Atlantic coastal forest along the Eastern coast.

The woolly spider monkey is primarily a folivorous species, but also consumes fruit, seeds, flowers, nectar, pollen, bark, bamboo, and ferns. It prefers to eat fruits which are more ripe. Leaves are eaten for bulk and protein, and the woolly spider monkey prefers immature leaves over mature ones (Kinzey, 1997). Strier (1992b) suggests that folivory is a result of seasonal fruit shortages and the woolly spider monkey needs to eat leaves to survive. Group size for this species is between 15 to 25 individuals.

The woolly spider monkey moves through the forest in both a quadrupedal and suspensory fashion, and suspension is an important feeding posture (Fleagle, 1988).

The woolly spider monkey has a multimale-multifemale social system, with a trend for having slightly more females than males. Groups may split up into smaller groups. When a group splits the resulting subgroup is heterosexual (Strier, 1990). The mating system of this species is described as being polygamous with individuals being promiscuous (Milton, 1985; Strier, 1986, 1991). Males of this species are philopatric and females disperse from the groups to which they were born in. Embracing is an important behavior in maintaining social bonds. This species does not exhibit much aggression towards group members (Strier, 1992a). males of this species spend a large amount of time close together without aggressive encounters, which is uncommon among the primates (Strier, 1990). Woolly spider monkeys do not perform social grooming on one another (Strier, 1992a). Strier (1990) found that the woolly spider monkey does not defend territories. Related males cooperate together and spend time close to each other, and Strier (1997) suggested that this cooperation amongst related males may have a purpose in preventing unrelated males from having access to group females.

neigh: This call sounds like a "neigh", and each individual has there own variation. Neighs are used when the group is together, and they are softer when individuals are close and louder when the distance increases. This sound is also used by the woolly spider monkey during intergroup encounters.

bark: This noise made by the woolly spider monkey is used as an alarm call, or when surprised by an unusual presence.

screech: This call has a higher pitch and lasts longer than screams; it is used by juveniles when left by mothers to travel across difficult tree-crossings alone (Strier, 1992a). Mothers may respond to this call by using their bodies as a bridge (Strier, 1992a).

chirp: This call is given by both sexes of adult and immature woolly spider monkeys (Strier, 1992a). Soft chirps are given when a group is foraging and feeding together and when an individual finds a tasty food item (Strier, 1992a). The first individual to a large patch of fruit may emit louder chirps (Strier, 1992a).

anogenital sniffing: This is done by both males and females, mostly males, to check the readiness of the other for copulation. In the females there is no outward sign of estrus (no change in physical appearance).

urine-wash: Both males and females wash their hands in urine, and then transfer the urine to there hands and feet; the urine is deposited upon the substrate over which they travel (Milton, 1984).

grin: The females does this, while looking over her shoulder, to the male wihile he is copulating with her from behind (Strier, 1992a).


The woolly spider monkey gives birth to a single offspring. The male normal mounts the female from behind, but missionary, or face-to-face, style copulation has been observed (Strier, 1992a). After ejaculation a vaginal plug forms, and will stay there unless removed (Strier, 1992a). Sometimes the female will remove this plug after it forms, but sometimes she leaves it in (Strier, 1992a). Females will have more than one sexual partner over a few day period in which she is in estrus (Strier, 1992a).

Burton, Frances. 1995. The Multimedia Guide to the Non-human Primates. Prentice-Hall Canada Inc.

Fleagle, John G. 1988. Primate Adaptation and Evolution. Academic Press.

Kinzey, W.G. 1997. Brachyteles. in New World Primates: Ecology, Evolution, and Behavior. ed. Warren G. Kinzey, Aldine de Gruyter, New York.

Milton, K. 1984. Urine Washing Behavior in the Woolly Spider Monkey. Zeitschrift fur Tierpsychologie. Vol. 67, 154-160.

Milton, K. 1985. Mating Patterns of Woolly Spider Monkeys, Brachyteles arachnoides: Implications for Female Choice.Behavioral Ecology and Sociobiology. Vol. 17, 53-59.

Rosenberger, A.L. and Strier, K.B. 1989. Adaptive Radiation of the Ateline Primates. Journal of Human Evolution. Vol. 18, 717-750.

Strier, K.B. 1986. Reproducao de Brachyteles arachnoides (Cebidae Primates). In A Primatologia no Brasil-2. ed. M.T. de Mello. Brasilia: Instituto de Ciencias Biologicas.

Strier, K.B. 1990. New World Primates, New Frontiers: Insights from the Woolly Spider Monkey, or Muriqui (Brachyteles arachnoides). International Journal of Primatology. Vol. 11, 7-19.

Strier, K.B. 1991. Demography and Conservation of an Endangered Primate, Brachyteles arachnoides. Conservation Biology. Vol. 5(2), 214-218.

Strier, K.B. 1992a. Faces in the Forest, the Endangered Muriqui Monkeys of Brazil. Oxford University Press.

Strier, K.B. 1992b. Atelinae Adaptations: Behavioral Strategies and Ecological Constraints. American Journal of Physical Anthropology. Vol. 88(4), 515-524.

Strier, K.B. 1997. Subtle Cues of Social Relations in Male Muriqui Monkeys (Brachyteles arachnoides). in New World Primates: Ecology, Evolution, and Behavior. ed. Warren G. Kinzey, Aldine de Gruyter, New York.

Zingeser, M.R. 1973. Dentition of Brachyteles arachnoides with Reference to Alouattine and Atelinine Affinities. Folia Primatologica. Vol. 20, 351-390.

Last Updated: October 4, 2003.
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