YUCATAN BLACK HOWLER
Geographic Distribution and Habitat
Yucatan black howler monkeys (Alouatta pigra), also known as Central American black howler monkeys, Mexican black howler monkeys, Belizean howler monkeys, Guatemalan black howling monkeys, and Mesoamerican black howler monkeys, are found in southeastern Mexico, northern Guatemala, and throughout Belize. These monkeys can be found in many different ecosystems such as mangroves, swamps, and lowland rainforests. The forest systems they are commonly spotted in are evergreen and semi-evergreen broad-leaved forests and mixed needle forests, as well as deciduous and semi-deciduous broad-leaved forests.
Recent studies are showing more diversity in their habitat beyond lowland rainforests. In the southern point of their geographic range, they can be seen in high montane environments at elevations up to 10,990 feet (3350 m). While primary forests are ideal for supporting monkey populations, the Yucatan black howler monkey can survive in secondary forests that are impacted by people for agricultural purposes. They can be found on plantations and in forests surrounded by urban areas and pasturelands.
Size, Weight, and Lifespan
Adult Yucatan black howler monkeys exhibit some sexual dimorphism. Males tend to weigh more than females. The average weight for males is 16.8 pounds (7.6 kg) and the average weight for females is 12.5 pounds (5.7 kg). The average weight of males can vary depending on their geographic distribution; males in Belize have a heavier weight of 16.5–20 pounds (7.5–9.1 kg), whereas males in Mexico weigh 12–20 pounds (5.5–9.1 kg). This is not the case for females. Their average weight range remains constant at 10–15 pounds (4.6–7 kg) regardless of their geographic range.
Yucatan black howlers’ long prehensile tails are generally 23–27 inches (59–69 cm) long, which can be longer than their body length of about 24 inches (61 cm). As well as providing balance while they navigate the canopy, the tail serves as a fifth limb. A naked patch of skin toward the end of the tail facilitates sensation for grasping, suspension, and arboreal locomotion.
The life span of the Yucatan black howler monkey is 10–15 years.
Howler monkeys are the largest Latin American monkeys, and Yucatan black howler monkeys are the largest of the howlers. Their black coats are sleek and long. Their faces are black and mostly bare, surrounded by a ring of black hair. Both males and females have beards at the chin, with the males’ being noticeably longer. Infants are brown and develop their black coats as they mature.
Large throats, protruding jaws, and high sloping faces accommodate an enlarged hyoid bone, the source of their famous deep booming calls or howls. In most primates, including humans, the hyoid is a small horseshoe-shaped bone that is situated in the midline of the neck. In howlers, it is considerably larger and cup-shaped. The hyoid creates a sort of resonance chamber that makes howler monkey hoots and hollers really ring out. Males’ hyoid bones are up to five times larger than those of females. Their calls can be heard for up to 3 miles (4.8 km) and at 140 decibels. (A jet engine takes off at 150 decibels.) They are considered to be the loudest land animals in the world.
The upper molars of howler monkeys have very sharp shearing crests that are used for grinding the leaves of their heavily leaf-based diet.
Yucatan black howler monkeys are folivores (leaf-eaters) and frugivores (fruit-eaters). An average of 18% of their daily activity is spent foraging. They prefer to eat leaves, fruits, and flowers from many different types of plant species, making diet diversification one of their main priorities. Howler monkeys consume more than 130 different plant species, with more than half of those being from trees. The ratio of leaves to fruits varies by location and by season.
Their diets are flexible and adaptable to accommodates changes in their habitat. Despite their high consumption of leaves, black howler monkeys have a primitive digestive tract more suitable to fruit digestion. It takes them a very long to properly digest leaf matter, so they spend a lot of time lounging in the trees.
In regions that suffer forest fragmentation and loss of land, fruit is less available and they consume more leaves than fruits. Since leaves contain less nutrition than fruit, they must forage more and longer to acquire sufficient energy. In areas in which forests are disturbed or fragmented, the howler monkeys take advantage of foliage that has been introduced by agriculture, like planted orchards. Howlers living in fragmented forests depend more on secondary-growth plants for food. While the number of plant species eaten may be comparable, the quality of the food may be different.
Behavior and Lifestyle
Yucatan black howler monkeys are both diurnal (active during daylight hours) and arboreal (tree-dwelling). They generally travel quadrupedally (using all limbs) on the tops of branches, grasping a branch with at least two hands—or one hand and a tail—at all times. Their strong prehensile tails are able to support their entire body weight. Fully grown adult howler monkeys do not often rely on their tails for full-body support, but juveniles do so more frequently.
They are slow-moving monkeys and commonly sit on the topmost tree branches, rarely descending to the ground. They get most of the water they need from the plants that they eat. When on the move, howlers progress in orderly groups usually led by an older male.
Yucatan black howler monkeys spend the majority of their day resting. At the Primate Rehabilitation Center in Belize, a caretaker described them as the cats of the monkey world, napping in sunny spots on the forest floor or on a tree limb with their long limbs dangling down. Naptime lengths vary depending on the season and food availability.
Since their hyoid bone amplifies the male howler’s calls, they can locate other males—thereby protecting their territory and families—without expending too much energy. This is important since so much of their diet consists of nutrient-poor leaves that produce low-energy behaviors. Howling occurs primarily at dawn and at dusk.
Yucatan black howler monkeys are sympatric with mantled howler monkeys, sharing geographic range, habitat, and distribution. In fact in their westernmost geographic distribution in Tabasco, Mexico, hybridization between Yucatan black howlers and mantled howlers has been reported and confirmed.
Both male and female howler monkeys participate in howling sessions. Morning calls are used to strengthen group ties and announce group territories. Calling can occur throughout the day and, in captivity, is associated with feedings. Calling is also used for warnings against predators.
Howler monkeys also purr. Monkeys at the Primate Rehabilitation Center in Belize have been observed purring when they are happy or, in the case of juveniles, right before they get into mischief. Other howler sounds include growling during their playfights.
Group sizes of Yucatan black howler monkeys vary in number and in ratio of males to females. Depending on the habitat, groups can be small couples up to larger groups of sixteen individuals. The average size of a group is six members. Troops generally have a single male living with multiple females and their young. However, multi-male groups and solitary males living without a troop have been documented. Howler troops remain stable from year to year and individuals typically feed, sleep, and travel together. Occasionally, the troop will split up to feed but later reform into the original troop. Based on what is known about howler group structure, group cohesion may be the key factor connected to the survival of Yucatan black howlers.
The distribution of the Yucatan black howler monkey varies by region. In Mexico, specifically the states of Campeche, Chiapas, and Quintana Roo, population density is approximately 13–24 individuals per .4 square mile (1 sq km). In northern Guatemala, population density is approximately 5–18 individuals per .4 square mile (1 sq km). In Belize, where the monkeys are found throughout the entire country, population densities have a larger range of 8–257 individuals per .4 square mile (1 sq km).
The size of a group’s range also differs between and within the countries. In the Community Baboon Sanctuary in north central Belize, the home range is only 2.5–10 acres (1–4 hectares). In Lamanai, Belize, the range is larger and averages 35 acres (14 hectares). At Tikal, Guatemala, the average range size is 247 acres (100 hectares). However, at Palenque National Park in Mexico, the average range size is only 15 acres (6.25 hectares).
Walking around the dense rainforests of Central America can be a little ominous, especially in the early morning or at dusk. While it may seem like the T-rex from Jurassic Park is roaring, it is just the songs of the Yucatan black howler monkey. Both males and females howl in what can be described as stereo surround sound, with a chamber quality that is unmatched in the animal world. The haunting calls start early in the day to establish territory and serve as a “roll call” for their groups. The monkeys also howl in the evening, when food is found during foraging, and to warn against predators. Roaring can be contagious, like a yawn. If individuals hear similar sounds, more members tend to join in. Neighboring troops can also join in. Morning and evening howling sessions related to sunrise and sunset. Group howling in these sessions can be an hour in length while sessions throughout the day are shorter. The length of howling is also impacted by environmental factors. It has been noted that reduced sunlight and increased rain shorten the morning and evening group calls and lengthen the frequency and duration of day calling.
Yucatan black howlers are polygynous, which means that each male may mate with multiple females. Females reach sexual maturity at four years of age. Males reach sexual maturity between six and eight years old. Only the dominant males or males with high social status will copulate with females. Alpha males have the opportunity to copulate more frequently and with more females than those with lower status. Due to the necessity of having to wait to gain high social status, males often mate later in life than females. They breed non-seasonally and typically produce one offspring annually.
After a six-month gestation, the mother gives birth to one offspring. The newborn is licked clean and carried by her or his mother. During the first few weeks after birth, the mother nurses the infant. After 3 weeks, the infant begins to eat leaves. As the infant matures, he or she begins to ride on the mother’s back. When traveling on the mother’s back, the infant grasps the base of the mother’s tail with his or her own. The child is constantly with the mother for the first four months. During this time, mother and infant do not venture more than 6.5 feet (2 m) away from each other. Occasionally, females other than the mother may care for the child. At about 10–11 weeks, the baby starts to forage independently and spends much more time on his or her own. Males reach maturity at about 42 months and females at about 36 months.
Males leave their natal group upon reaching sexual maturity, but females generally remain with their natal group.
With their diet rich in both young and mature leaves, howler monkeys play an important role as gardeners of the forest, pruning the variety of trees they feed from. Additionally, the inclusion of fruits in their diet means that they also participate in seed dispersal—spreading the seeds of fruits across the forest. Seeds may also stick to their fur while they feed. The seeds drop as the monkeys travel, further playing a role in seed dispersal. Their feces are used by dung beetles, which helps to recycle nutrients into the soil—an added benefit.
The International Union for Conservation of Nature lists the Yucatan black howler monkey as Endangered (IUCN, 2020), appearing on the IUCN Red List of Threatened Species. The IUCN reports that populations are expected to decrease by 50% over the next three generations due to the current rate of deforestation. It is expected that 31% of suitable habitat will be lost by 2048. Loss of forests at Rio Polochic Valley and Lake Izabel, both in Guatemala, are noted as having a significant impact on the availability of suitable habitats for the monkeys.
Loss of forest is caused by several factors. Development for housing and urbanization and for agricultural purposes is the most noted. Logging for timber is also common. The land is needed to build farms, focusing on monoculture farms for non-timber crops like sugar cane or eucalyptus. The number of cattle ranches has also grown. Forest is also lost to natural disasters like hurricanes. Storms and flooding due to climate change are significant conditions that negatively impact their ecosystems.
Hunting has also been noted as a factor in species decline. It is more common in Mexico than in Belize or Guatemala. Another factor that impacts population size and viability is the illegal pet trade. In order for an infant to be taken for the pet trade, its mother has to be shot and killed. This removes not only an infant from its troop, but it also removes a fertile female from the population. Infants rarely receive the proper diet once sold into the pet trade and many do not survive as pets. In Mexico, it is legal to own exotic pets, even endangered species, if it is registered under certain conditions. For example, it cannot be taken from the wild. However, Mexican officials have a hard time regulating the permits. In Belize, it is illegal to own a monkey, while in Guatemala it is illegal to own exotic animals like monkeys under the Protected Areas Law unless permitted for research or conservation.
The Yucatan black howler monkey is listed in Appendix I of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival.
Wildtracks, which is home to the Primate Rehabilitation Center in Belize, rescues and rehabilitates Yucatan howler monkeys and Central American spider monkeys (Ateles geoffroyi) impacted by deforestation and the illegal pet trade. The Primate Rehabilitation Center (PRC) at Wildtracks helps design and support conservation initiatives for primate conservation as well as assist the Forest Department with confiscations and rehabilitations of primates taken in the illegal pet trade. Most new admissions to the center are infants too young to be separated from their mothers. They require years of rehabilitation before they can be release back to the forest. Animals capable of release from the PRC are freed in the North East Biological Corridor (NEBC), home to some of the last remaining forests in Belize. The NEBC allows for forest connectivity to Yucatan and forest resilience in the southern areas.
Written by Stefanie Buxel and Debra Curtin, February 2023