The northern muriqui (Brachyteles hypoxanthus) is an endangered muriqui (woolly spider monkey) species endemic to Brazil. It is unusual among primates in that it shows egaliterian social relationships. It is found in the Atlantic Forest region of the Brazilian states of Rio de Janeiro, Espírito Santo, Minas Gerais and Bahia. Muriquis are the largest species of New World monkeys. The northern muriqui can grow up to 4.3 ft tall. It feeds mainly on leaves and twigs, but will also eat fruit. It often hangs upside-down by its prehensile tail while eating.
Northern muriquis can be individually recognized by their natural markings and facial features, such as fur color and patterning, ear shape, and face shape and pigmentation. Their sex can usually be determined within a week or so of birth, based on the shape and positioning of their genitalia. The structure for Northern Muriqui’s are groups of male and females. Female Muriqui’s are more independent than males, “Females disperse from their natal groups at a mean age of six years.”  The female Muriqui also engage in sex with multiple partners, “A a pattern that in other primates has been attributed to various functions such as confusing paternity, reducing male aggression, or ensuring fertilization”  A fact to note about the male Muriqui, is that there almost no interaction between the male and infant. On the brief occasion that there is an interaction between the two, the infants would be the one to initiate. 
B. hypoxanthus is one of the world's most critically endangered primates.The species is classified as Critically endangered on the IUCN Red List of Threatened Species. It is threatened by hunting and (in common with most other primates of the region) destruction and fragmentation of its Atlantic Forest habitat. Among the scattered populations of Northern Muriqui only one population, living in Caratinga is considered, as of now, viable for the next 100 years. The Northern Muriqui suffers from a lack of knowledge on their species, along with most endemic species causing problems in conservation.  The estimated wild population of northern muriquis has increased from about 500 to 1,000 individuals in 2005, due to new discoveries and research in other forests 
Human evolution relevance
The northern muriqui has been argued to be important to understanding human evolution, since it is one of the few primates that has tolerant, nonhierarchial relationships among and between males and females, a feature shared with hunter-gatherer humans, but which contrasts with the ranked relationships of most other primates. Group aggression is also rare. The success of males fathering offspring links to the maternal investment they gain from their mothers and coresident female kin. This provides support to the grandmother hypothesis.
- Groves, C. P. (2005). "Order Primates". In Wilson, D. E.; Reeder, D. M. Mammal Species of the World (3rd ed.). Johns Hopkins University Press. p. 151. ISBN 978-0-8018-8221-0. OCLC 62265494. http://www.bucknell.edu/msw3/browse.asp?id=12100410.
- Mendes, S.L., de Oliveira, M.M., Mittermeier, R.A. & Rylands, A.B. (2008). "Brachyteles hypoxanthus". IUCN Red List of Threatened Species. Version 2011.2. International Union for Conservation of Nature. http://www.iucnredlist.org/apps/redlist/details/2994. Retrieved 19 January 2012.
- Strier, K.B., Chaves, P.B., Mendes, S.L., Fagundes, V., Di Fiore, A. (2011). Low paternity skew and the inﬂuence of maternal kin in an egalitarian, patrilocal primate, PNAS, 108, 18915–18919 doi:10.1073/pnas.1116737108
- Strier, Karen B, Jean P. Boubli, Carla B Possamai, and Se ́ rgio L. Mendes4. "Population Demography of Northern Muriquis (Brachyteles Hypoxanthus) at the Estac ̧ a ̃ O Biolo ́ Gica De Caratinga/Reserva Particular Do Patrimoˆ Nio Natural-Felı`ciano Miguel Abdala, Minas Gerais, Brazil."AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY (2006): 1-3.
- Strier, KB, SL Mendes, and K. "Web of Knowledge [v5.6]." Genetic Diversity and Population History of a Critically Endangered Primate, the Northern Muriqui 17.2 (2011): 53-69. Web.
- Possami, Carla. "Socio-Sexual Behavior of Female Northern Muriquis." Socio-Sexual Behavior of Female Northern Muriquis 69.7 (2007). Web.
- Oliveria Guimaraes, Vanessa. "Adult Male-infant Interactions in Wild Muriquis (Brachyteles Arachnoides Hypoxanthus)." N.p., Oct. 2001. Web. 26 Oct. 2012.
- Brito, Daniel, and Carlos Eduardo V. Grelle. Estimating Minimum Area of Suitable Habitat and Viable Population Size for the Northern Muriqui. Pdfcast.org. Springer, n.d. Web. 26 Oct. 2012. <http://pdfcast.org/pdf/estimating-minimum-area-of-suitable-habitat-and-viable-population-size-for-the-northern-muriqui-brac#>.
- Daniel, Brito. "Lack of Adequate Taxonomic Knowledge May Hinder Endemic Mammal Conservation in the Brazilian Atlantic Forest." Google Scholar. N.p., 2004. Web. 26 Oct. 2012.