Alouatta pigra
Mexican black howler monkey

Alouatta pigra is found in Belize, northern Guatemala, and southeastern Mexico ("Belize Zoo", 2007; Gonzalez-Kirchner, 1998). They are endemic to the largest tropical rain forest region of Mesoamerica, Selva Maya, which encompasses over 4 million hectares of land in these three countries (Estrada et al., 2004). They are the only howler monkey species on the Yucatan peninsula, and are found in a reduced area of the Yucatan (Estrada et al., 2004; Gonzalez-Kirchner, 1998). They inhabit the Mexican states of Campeche, Quintana Roo, and Chiapas (Estrada et al., 2004). Populations have been found and heavily studied at the Mayan sites of Calakmul and Yaxchilán, Mexico, and Tikal National Park, Guatemala (Estrada et al., 2004). They are widespread throughout Belize. ("Belize Zoo", 2007; Estrada et al., 2004; Gonzalez-Kirchner, 1998)

The habitat of Alouatta pigra is varied, but is typically tropical forests. They are known from lowland, deciduous, semi-deciduous forests, and evergreen forests (Belize Zoo, 2007; Gonzalez-Kirchner, 1998). They are known from pristine tropical rainforest to riverine forests, and can be normally found at low elevations in riparian forests of less than 400 m, although black howlers have been found at elevations higher than 500 m (Estrada et al., 2004). In riverine habitats they usually avoid riverbank areas and inhabit inland areas (Estrada et al., 2004). They prefer larger trees and usually inhabit the middle to upper canopy (Gonzalez-Kirchner, 1998). Although they have a wide habitat range, A. pigra has lower rates of success in areas disturbed by humans (Gonzalez-Kirchner, 1998). ("Belize Zoo", 2007; Estrada et al., 2004; Gonzalez-Kirchner, 1998)

Alouatta pigra is one of the largest monkeys in the Americas. It averages 70 cm long, excluding the tail, which is slightly longer than its body length ("Belize Zoo", 2007). It is the largest and most sexually dimorphic of the howler monkeys, with an average weight of 11.4 kg for males and 6.4 kg for females (Fleagle, 1999). They are characteristically covered by black hair, which is a bit longer near the throat, and they have white genitalia (Treves, Drescher, & Snowdon, 2003). Males have a pink scrotum, which distinguishes Alouatta pigra from Alouatta palliata (Horwich, 1983a). Young retain pale fur until 9 to 10 weeks of age (Treves, Drescher, & Snowdon, 2003). Like other members of Atelidae, A. pigra has a prehensile tail with gripping pads (Fleagle, 1999). ("Belize Zoo", 2007; Fleagle, 1999; Horwich, 1983a; Treves, Drescher, and Snowdon, 2003)

The average lifespan of Mexican black howler monkeys is 20 years ("Primate Info Net", 2004). ("Primate Info Net", 2004)

Mexican black howler monkeys live in groups of 4 to 11 individuals (Estrada, 2004; Gonzalez-Kirchner, 1998). Adults make up about two thirds of a group, and the sex ratio is almost equal, about 4:3, although these numbers can vary quite widely (Estrada et al., 2004). Groups are more terrestrial than those of other howler monkeys. Aside from sleeping, Mexican black howler monkeys spend most of their time resting (about 70%), followed by feeding (18%), moving (5%), and socializing (1.2%) (Behie & Pavelka, 2005). Another study showed that they spend about 77% of their time resting, followed by about 15% feeding, 5% moving, and less than 1% socializing (Behie & Pavelka, 2005). Their long periods of inactivity are to promote the digestion of leaves, as they lack many adaptations common to other folivorous mammals (Kitchen, Horwich, & James, 2004). (Behie and Pavelka, 2005; Estrada et al., 2004; Kitchen, Horwich, and James, 2004)

A typical group consists of one dominant alpha male, a few females, their offspring, and occasionally extra males (Treves, Drescher, & Snowdon, 2003). Subordinate males are probably involved in group defense (Kitchen, Horwich, & James, 2004). Occasional lone males have been recorded (Estrada et al., 2004). These solitary males generally are trying to start their own groups (although they have also been found to usurp alpha males from already established groups) and can be used as indications of nascent groups. Groups that are recently formed are generally much smaller than longer-established groups (Gonzalez-Kirchner, 1998). (Estrada et al., 2004; Gonzalez-Kirchner, 1998; Kitchen, Horwich, and James, 2004; Treves, Drescher, and Snowdon, 2003)

Generally, Mexican black howler monkeys have densities that range from 6.3 to 89.5 individuals per square km, lower than most other howler monkey species (Estrada et al., 2004; Gonzalez-Kirchner, 1998). Group sizes are also smaller than most other howler monkeys (Knopff & Pavelka, 2006). Current research focuses on which factors, including social considerations, their inactive lifestyle, or intragroup food competition, limit group size. One hypothesis is that infanticide limits group sizes (Knopff & Pavelka, 2006). It is not clear, however, if it is small group size that causes high rates of infanticide, or vice versa. Also, reproductive success decreases when group size increases, naturally limiting group size (Knopff & Pavelka, 2006). (Estrada et al., 2004; Gonzalez-Kirchner, 1998; Knopff and Pavelka, 2006)

Mexican black howler monkeys uses arboreal quadrupedalism as their main mode of locomotion (Fleagle, 1999). (Fleagle, 1999)

Home Range

The size of the territories is dependent on the number of monkeys in the group. Territory sizes are from 3 to 25 acres and are announced by howling vocalizations. ("Belize Zoo", 2007)

Mexican black howler monkeys announce and defend group territories through howling vocalizations ("Belize Zoo", 2007). They are highly social animals Adult howlers are less social, while infant and adolescent howlers engage in most of the social interactions (Behie & Pavelka, 2005). Adults in a group will often howl when approached by one or a few solitary males who may be trying to take over the alpha spot in the troop (Kitchen, Horwich, & James, 2004). Other threats include these solitary males trying to commit infanticide and thus lowering the reproductive success of the current alpha male (Kitchen, Horwich, & James, 2004). Howling can last for over an hour and is low frequency and loud: about 88 decibels at a 5 m distance (Kitchen, Horwich, & James, 2004). The energy used in howling is still less, and the risk smaller, than in physical confrontation. When more than one male is present howling occurs in staggered rounds and, as a result, howling can be used to indicate the number of males present (Kitchen, Horwich, & James, 2004). This information is useful to a group in deciding whether or not to approach another group and initiate a physical confrontation (Kitchen, Horwich, & James, 2004). The howls of females are aurally different from those of males, but they also participate in group howling. ("Belize Zoo", 2007; Behie and Pavelka, 2005; Kitchen, Horwich, and James, 2004)

Mexican black howler monkey diet is varied. They are strict herbivores, consuming many plant parts, including flowers, fruits, and leaves ("Belize Zoo", 2007). All Alouatta species are highly folivorous. Alouatta pigra is one of the more frugivorous species in this genus. Their preferred food is fruit and they usually eat leaves only in conditions where frugivory is unfavorable (Behie & Pavelka, 2005). This allows them to have a flexible, adaptable diet that can adjust in cases of habitat change. Generally, a little more than 36% of their diet consists of fruits, followed by about 30% mature leaves, 25% new leaves and buds, and 5% flowers (Behie & Pavelka, 2005). Although they consume a significant amount of leaves, Alouatta pigra has a primitive digestive tract more suitable for frugivory; breaking down leaves therefore requires a longer digestion time, which explains the long inactive periods in their activity budget (Kitchen, Horwich, & James, 2004). ("Belize Zoo", 2007; Behie and Pavelka, 2005; Kitchen, Horwich, and James, 2004)

Mexican black howler monkeys can howl to frighten away predators or alert others of a predator's presence. Mothers aggressively protect their young for a year against predators and other dangers (Kitchen, Horwich, & James, 2004). The greatest predatory threat to young black howlers is infanticide by adult males. Otherwise, there are few natural predators for Mexican black howler monkeys. Occasionally they are preyed upon by jaguars, pumas, and harpy eagles (Knopff & Pavelka, 2006). Other cats, large birds of prey, and large arboreal snakes are also potential predators (Treves, Drescher, & Snowdon, 2003). Humans have been known to illegally catch individuals for pets, and they are occasionally eaten by humans (Kitchen, Horwich, & James, 2004). (Kitchen, Horwich, and James, 2004; Knopff and Pavelka, 2006; Treves, Drescher, and Snowdon, 2003)

Mexican black howler monkeys are often found living in close conjunction with Ateles geoffroyi and other spider monkeys (Estrada et al., 2004). They are also important for the role they play in seed dispersal (Van Bell, 2006). Their frugivorous diet has helped disperse the seeds of some rainforest plants. Although they do not shy away from human contact and their presence is quite obvious due to their howling, Mexican black howler monkeys rarely interfere with human crops. (Estrada et al., 2004; Estrada, 2006; Van Bell, 2006)

Mexican black howler monkeys occasionally raid crops, although this behavior is rare (Estrada, 2006). (Estrada, 2006)

Mexican black howler monkeys are important members of the ecosystems in which they live. Their howling is a unique element of their Mesoamerican forests and their presence can attract ecotourism.

Mexican black howler monkeys are endangered by hunting and habitat destruction. The forests in which they live and feed are often converted into pastures or agricultural plots. Human influence as a whole is causing significant fragmentation of A. pigra habitats (Estrada, 2006). Other factors resulting in declining populations are: disease, abnormal rainfall, human capture for pets, and natural disasters such as hurricanes (Gonzalez-Kirchner, 1998). In areas affected negatively by humans, Mexican black howler populations become smaller both in numbers and in area (Gonzalez-Kirchner, 1998). They have been listed by the IUCN as an endangered species since 2003. They were previously (as recently as 2000) considered lower risk/least concern (Cuarón et al., 2003). The U.S. Fish and Wildlife Service lists Alouatta pigra as a threatened species. In the next 30 years, the total population is predicted to decrease 74% (Cuarón et al., 2003). Primates from this region have been hunted and captured as pets, causing their populations to decline further (Estrada et al., 2004). (Cuaron et al., 2003; Estrada et al., 2004; Estrada, 2006; Gonzalez-Kirchner, 1998)

Alouatta pigra is commonly known as Mexican black howler monkeys, Guatemalan howler monkeys, and saraguatos.