Mantled howler monkeys are found in southern Mexico (the states of Veracruz, Campeche, Chiapas, Oaxaca, and Tabasco), from Honduras in Central America to Colombia and western Ecuador in South America, and possibly in southern Guatemala where some unconfirmed sightings have been reported (Reid 1997, Cortes-Ortiz et al. 1996). This range includes most forest habitats that lie between sea level and 2500 meters (Reid 1997).
Mantled howler monkeys inhabit lowland and montane rain forests, including primary and regenerated forest habitats. Stoner (1996) researched two troops of mantled howler monkeys in northeastern Costa Rica to determine habitat selectivity within a lowland rainforest. She concluded that there was troop specific habitat selection occurring within the same area and significant intraspecific variability in foraging patterns. Following two study groups, she noted that one troop had a high utilization of primary forest (80%) over secondary forest, while the other preferred undisturbed riparian habitat (60%) to primary forest (30%).
Mantled howler monkeys are large and stocky with black fur, and most individuals have long, yellow or brown fur saddles (Reid 1997, Glander 1983). Long guard hairs are present on their flanks, earning the common name "mantled" howler monkey (Glander 1983). The face is naked, black and bearded, and the prehensile tail has a naked pad on the underside near its base. Males have a prominent white scrotum, weigh 6 to 7 kg, and usually have a longer beard than female individuals (Reid 1997, Glander 1983). Adult females typically weigh 4 to 5 kg, and newborn infants weigh 0.4 kg and appear silver to golden brown (Glander 1983). Adult tail size ranges from 520 to 670 mm, and body length ranges from 380 to 580 mm (Reid 1997). The dental formula is (I 2/2, C 1/1, PM 3/3, M 3/3) X 2 = 36.
Mantled howler monkeys are polygynous.
Females reach sexual maturity at approximately 36 months, and are typically 42 months old by their first birth (Glander 1980). They undergo a regular estrus cycle, with an average duration of 16.3 days, and display sexual skin changes, particularly swelling and color change (from white to light pink) of the labia minora. Although the ovulation time is unknown, it may be predicted by the alpha male's behavior, as he copulates only at the midpoint of sexual skin swelling and remains uninterested at other times. Breeding occurs year-round, without any apparent seasonal variation, and a female generally participates in multiple matings before conception. The age of sexual maturity for males is 42 months, but males attempting to breed in non-maternal groups may be delayed until social status is gained, generally only if they are older than 6 years of age (Glander 1980). The typical copulatory sequence begins when a female approaches a male (typically within 1 meter) and begins rhythmic tongue movements while facing him (Young 1982). The male responds with rhythmic tongue movements, and within one minute the female turns and elevates her rump. Meanwhile the male has achieved an erection and immediately mounts the female, performs 5-20 pelvic thrusts over the duration of 20-60 seconds. The animals then turn sideways and disengage, and usually sit quietly for several minutes before slowly walking away (Young 1982). Pregnancy lasts an average of 186 days (6 months), and the interbirth interval is about 22.5 months (Glander 1980). Once the infant is 4 months old, it no longer has an influence on the mother's future reproduction, and if an infant dies within this 4-month period the interbirth interval shortens. The highest reproductive success occurs in the middle ranking females, with the alpha position lower possibly because of competitive pressures, and infant mortality appears to be lower when the timing of births in a group of females is clustered (Glander 1980).
Behavior of young mantled howler monkeys varies with age and size. Social exploration and play follows an inverted "U" pattern when plotted against increasing age, peaking at an early juvenile stage and decreasing with the increasing demand for energy in digestion and foraging (Baldwin and Baldwin 1978). Infants spend the first weeks of their life clinging to their mother and nursing, not mouthing leaves until 3 weeks of age and not leaving her side until the 5th week of life (Lyall 1996). In the 10th and 11th weeks, infants begin exploratory feeding and will spend a significant portion of their time independent of their mother.
In general, mantled howler monkeys have an energetically conservative lifestyle, perhaps related to diet and feeding style since they spend the majority of their time foraging (Nagy and Milton 1979). Locomotion occurs within the rainforest canopy, and includes quadrupedalism (walking, running, and galloping on supports), bridging (crossing gaps by stretching), and climbing (Gebo 1992). There is sexual variation in locomotory style, where males climb less, leap more, and prefer the high canopy as compared to females. Adult males have an average lifespan of seven years, and females typically live eleven to twelve years (Otis et al. 1981). Mantled howler monkeys maintain linear hierarchies, and demonstrate complex interactions within the social group. For example, grooming behavior reflects social hierarchy, as dominant individuals groom subordinates (Jones 1979).
Group size ranges from 10 to 20 members, generally 1 to 3 adult males and 5 to 10 adult females (Reid 1997). Females form the stable social unit, rarely leaving once established within a group (Scott et al. 1978). The sex ratio seems to find a critical minimum of four females for every male (Scott et al. 1978). As this ratio drops below four, older males may use aggression to drive younger males away, or wander off themselves. In this way, immigration seems to play a more important role than reproduction in increasing group population size (Glander 1980). Spacing between neighboring troops seems to be dependent, at least in part, upon vocal communication between males (Whitehead 1987). Calls include woofs, grunts, barks, and howls, with the characteristic howls occurring at dusk and dawn, and in response to any sort of disturbance (Reid 1997).
Within a social hierarchy, ranking determines a large extent of behavior and energy expenditure. For exampe, Jones (1996) demonstrated that increasing age or size in females eventually entails decreasing reproductive value and an increase in social behavior (in particular foraging for food). Younger females foraged significantly less than expected by their total numbers and left the discovery of ephemeral foods to older females of the social group, suggesting a selfish method of conserving reproductive or competitive energy by younger, higher-ranking females.
Natal emigration is performed by both sexes, with 79% of all males and 96% of the females leaving their original social group (Glander 1992). Males may remain solitary as long as 4 years, not entering a group until the alpha male is successfully challenged. The tenure as alpha male is around 46 months, but once in a breeding position within a group, males do not secondarily disperse to breed in other groups. Females generally don't spend more than a year in a solitary position, and join a group (with the help of a male) when they are able to rise to alpha status, where they remain for the duration of their lives. Glander (1992) hypothesized that competition with close relatives over limited food resources might encourage the high levels of natal emigration observed. By stipulating that the dominance hierarchy is most obvious during feeding, he argues that competition for food with distant relatives would be favored against a situation with close kin. This seems supported by the fact that surviving extra-group individuals join groups that don't contain kin.
Food resources consist mainly of leaves, fruit, and flowers, and vary seasonally with resource availability (Glander 1981). Flower availability is typically high during the dry season, and fruits are abundant during the wet season. Their diet is species-selective, and they graze almost exclusively on trees (Estrada and Coates-Estrada 1986). Food choice is influenced not only by digestibility and nutrient value, but also by secondary compounds (i.e. tannins) that may be undesirable (Glander 1981). While foraging, they spend nearly equal portions of their feeding time eating leaves as they do fruit, preferentially eating young leaves with higher protein to fiber ratio and lower tannin content (Estrada and Coates-Estrada 1986, Glander 1981). Sampling behavior may be important in adding new species to a social group's food base, and in testing seasonal changes in plant secondary compounds (Glander 1981).
This leaf grazing (folivorous) diet gives them access to a niche relatively unexploited by other mammals, so that they are often the most important arboreal mammalian folivore in a particular area. However, other competitors may impose significant pressure on available leaf resources, such as the leaf-cutting ant Atta cephalotus in Los Tuxtlas, Mexico. For a comprehensive list of plant species (34 species in 21 families) fed upon in a Mexican tropical rain forest setting, see Estrada and Coates-Estrada (1986). For a list of Costa Rican plant species, see Stoner (1996) and Glander (1981). Their feeding rate was calculated to be 53 g dry matter per kg each day (Nagy and Milton 1979). In other terms, an average sized individual would need to ingest 15% of its body size in fresh food per day during the dry season.
A small number of mantled howler monkeys are utilized for scientific experimentation.
The mantled howler monkey is classified as endangered in the United States Endangered Species Act of 1991 and vulnerable in CITES Appendix I. Under pressure from rainforest fragmentation, many howlers have moved into shaded cacao and coffee plantations where forest habitat has disappeared, especially in southern Mexico (Estrada et al. 1999). Continual decline may have severe implications for rainforest habitats, as howler monkeys play an important role in seed dispersal, and their fresh dung constitutes an important resource for dung beetles. Dung beetles use the howler excrement for both food and oviposition sites, and so dung beetle abundance is closely correlated with howler monkey abundance. It has been suggested that dung beetles are good biodiversity indicators, but they are also intrinsically important in their role as secondary seed dispersers and nutrient cyclers. A population analysis conducted by Cortes-Ortiz et al. (1996) in southern Mexico estimated that a total of 1,352 individual mantled howler monkeys inhabit protected areas, while 10,249 remain in unprotected habitat. Possible threats include habitat fragmentation and natural disasters. Cortes-Ortiz et al. (1996) suggest better managing of protected areas, establishing new reserves, developing translocation programs, managing populations within fragmented areas, controlling human traffic, promoting education and collaborating with other institutions.
Some evidence suggests that mantled howler monkey populations are becoming robust once again. The population on Barro Colorado Island is one example, where it suffered a severe decline resulting from a yellow fever epidemic (Collias and Southwick 1951), but returned to a robust size by the late 1970's (Nagy and Milton 1979). Also, evidence from La Pacifica, Costa Rica indicates that regenerating forest is becoming habitable to mantled howler monkeys, where dispersal and the formation of new social groups has been occurring (Glander 1992).
Sean Schoville (author), University of California, Berkeley, James Patton (editor), University of California, Berkeley.
living in the southern part of the New World. In other words, Central and South America.
Referring to an animal that lives in trees; tree-climbing.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
uses smells or other chemicals to communicate
ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
an animal that mainly eats leaves.
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
An animal that eats mainly plants or parts of plants.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
having the capacity to move from one place to another.
the area in which the animal is naturally found, the region in which it is endemic.
having more than one female as a mate at one time
rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.
reproduction that includes combining the genetic contribution of two individuals, a male and a female
associates with others of its species; forms social groups.
uses touch to communicate
reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.
breeding takes place throughout the year
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