- Suborder: Haplorrhini
- Infraorder: Simiiformes
- Family: Atelidae
- Subfamily: Alouattinae
Red howler monkeys are new world monkeys and part of the Atelinae subfamily (Di Fiore & Campbell, 2007). These species tend to sexual dimorphism but this also depends on the population being studied. Males tend to weight 6 to 7.6 kg while females tend to weight 4.5 to 6.3 kg (Thorington et al., 1979, Camacho and Defler, 1985; Smith and Jungers, 1997; Rodríguez, & Boher, 1998). They like most other new world monkeys have a prehensile tail. Their tails tend to be longer than their head and body length (Thorington et al., 1979; Bergeson, 1993; Richard-Hansen et al., 1999). Red howler monkeys has the name suggest are generally red but vary to red orange or golden orange (Hill, 1962; Thorington et al., 1979).
Like most others within the Aloutta genus have a large hyoid bone which functions as a way to deepen pitch, resonate and amplify their vocalizations (Thorington et al., 1979; Crockett and Eisenberg, 1988). Red howler monkeys however contain the largest hyoid and exhibit sexual dimorphism. Female hyoid bone volume is only 12.5 ml while males measure 69.5 ml (Sekulic, 1981; Crocket and Eisenberg, 1987).
Red howler monkeys can be found in the tropics of South America, including Bolivia, Brazil, Colombia, Ecuador, French Guiana, Guyana, Peru, Suriname, Trinidad and Tobago and Venezuela (Napier, 1976; Wallace et al., 1998). Due to their large ranger they are found in several different habitats including tropical rain forests, humid rain forests, dry rainforest, high terra firm forests, southern rainforest, lower montane forest, cloud forests, swamp forests, semi decidious forest and deciduous season forests and swamp woodlands (Hernández-Camacho and Cooper, 1976; Rudran, 1979; Braza et al., 1981; Gaulin and Gaulin, 1982; Cordero and Boher, 1988; Yoneda, 1990; Wallaxce et al., 1998; Youlatos, 1998; Palacios and Rodriguez, 2001; Lehman, 2004).
Red howlers monkeys diet also differs quite a lot depending on where they are located. For the most part they mainly consume fruits, fruit pulp and leaves. But will also consume roots, flowers, epiphytes, seeds, berries, drupes, petioles, leaf buds, bark, wood, vine, and other plant materials (Braza et al., 1983; Soini, 1986; Neves and Rylands, 1991; Julliot, 1996, Palacios and Rodriguez, 2001; Simmen et al., 2001). Due to the seasonality of the tropics and limits on fruit availability for some part of the year howler monkeys are folivorous while for other parts of the years they are frugivorious (Soini, 1986; Izawa and Lozano, 1990; Crocket, 1998; Stevenson et al., 2000; Simmen et al., 2001).
Red howler monkeys are diurnal and vary in activity depending on if it is the dry season or wet season (Braza et al., 1981). Braza (1981) found resting occurred slight more during the rainy season than in the dry season. Gaulin and Gaulin (1982) suggested this was do to the low nutritional diet from being folivorious and the difficulties in digestion with this type of diet.
Raptors such as harpy eagles are their main predators (Eason, 1989; Peres, 1990; Sherman, 1991).
Group sizes varies greatly with semi deciduous habitats containing larger groups and evergreen areas contain smaller troops (Crockett and Eisenberg, 1988). Rudran and Fernandez-Duque (2003) found troop sized averaged from 6 to 10.5 individuals and would vary from 4 to 18 individuals.
Groups tend to be uni male or multimale. Multi male groups tend to be larger. Generally there are more females than males. (Soini, 1986; Crockett and Eisenberg, 1988; Rudran and Fernandez- Duque, 2003). Generally there are only four breeding females per group (Crockett, 1996). All adult males are dominant over females (Neville et al., 1988).
Multi male groups generally have one dominant male who reproduces with all the females (Kimura, 1997; Pope, 1990; Crockett, 2003). These males tend to be larger than the other males (Kimura, 1997).
Both male and female red howler monkeys emigrate from their natal troops (Crocket, 1996). Once a female begins to breed in a group then she will remain in the group for the rest of her life (Pope, 2000). Males will join existing troops via challenging resident males. However they generally challenge this male with another male. This other male tends to be a relative (Crockett, 1984; Pope, 1990; Crockett and Pope, 1993; Agoramoorthy and Rudran, 1993).
Solitary females often have difficulty joining groups, group females often prevent solitary females from entering the group and thus females are more likely to be injured (Rudran, 1979; Sekulic, 1982; Crockett and Pope, 1988). Females often have to travel farther in order to establish new groups while males will often join troops adjacent to their natal troop (Pope, 2000). Females will emigrate from their natal groups at 2 to 4 years of age, while males emigrate 4 to 6 years of age (Crocket and Pope, 1993).
Vocalizations are incredibly important for howler monkeys and are often common in the early monring in order to stop other howler monkeys from approaching the troop during the day (Sekulic, 1982).
Red howler monkeys will often exhibit either a a multi male or single male polygynous breeding systems (Crockett and Rudran, 1987; Pope, 1990; Kumura, 1992). However females will also mate with several males in order to avoid infanticide as a result of male status changes within the group (Crockett, 2003).
Copulatory behaviors include tongue flicking, genital sniffing and inspection of genitals, and body licking (Izawa and Lozano, 1989; Agoramoorthy and Hsu, 1999). Females will first give birth approximately 4 to 5 years of age while males will not father offspring until they 7 years of age (Crockett and Eisenberg, 1987; Crocket, 1998). Gestation is approximately 191 days (Crockett and Sekulic, 1982; Crockett and Rudran, 1987).
Red howler monkeys also exhibit a large amount of infanticide this generally occurs during invasions of a group by external males and during dominance status/breeding status changes of the resident males (Crockett and Sekulic, 1984; Agoramoorthy and Rudran, 1995).
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