Yucatán Black Howler, Alouatta pigra

Yucatán Black Howler, Alouatta pigra. Photograph taken in Cancún, Quintana Roo, March 2021. Photograph and identifications courtesy of Marina Sutormina, Stockholm, Sweden.

The Yucatán Black Howler, Alouatta pigra, is a member of the Atelidae Family of howler, spider, woolly, and woolly spider monkeys. The Atelidae Family consists of twenty-three species placed into four genera and two subfamilies. There are twelve species in the Alouatta genus. This species is also commonly referred to as the Guatemalan, Central American, and Mexican Black Howler Monkey. In Mexico, they are known as Saraguato Yucateco.

Yucatán Black Howlers have long, sleek, black fur and a long prehensile tail. As well as providing balance while they navigate the tree canopy, their tail serves as a fifth limb. A naked patch of skin toward the end of their tail provides sensation for grasping, suspension, and arboreal locomotion. Their faces are black, mostly bare, and surrounded by a ring of black hair. Both males and females have beards on their chins, with the males’ growing noticeably longer. Their large throats, protruding jaws, and high sloping faces accommodate an enlarged hyoid bone, which is 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 howler monkeys, it is considerably larger and cup-shaped. Hyoid bones in males are up to five times larger than those of females, allowing their calls to be heard for up to 4.8 km (3 mi) at 140 decibels. In comparison, a jet engine takes off at 150 decibels. Yucatán Black Howlers have very sharp upper molars that are used for shearing and grinding leaves. They are the largest species of howler monkey and sexual dimorphism is displayed with males growing larger than females. Males weigh an average of 7.6 kg (16 lbs 13 oz) while females weigh an average of 5.7 kg (12 lbs 8 oz). The average weight of males can vary depending upon their geographic distribution, however, this is not the case for females. Their long prehensile tails are generally 60 cm (13 inches) to 70 cm (2 feet 3 inches) long, which can be longer than their body length of about 61 cm (2 feet 0 inches).

Yucatán Black Howlers are diurnal and arboreal. They travel through the treetops using all of their limbs, grasping a branch with at least two hands (or one hand and a tail) at all times. Their strong prehensile tails are capable of supporting their entire body weight. While fully grown adults do not often rely on their tails for full-body support, juveniles do so more frequently. They spend most of their time in the trees resting, followed by feeding, moving, and socializing. Yucatán Black Howlers are highly social and live in groups called troops. The size  and ratio of males to females in each troop varies depending upon the habitat. They can be as large as sixteen individuals, with an average of six members per group. In general, troops consist of a single male living with multiple females and their young. However, multi-male groups and solitary males living without a troop have been documented. Troops remain stable from year to year and individuals typically feed, sleep, and travel together. Occasionally, a troop will separate only to feed. Male members of the troop awake each morning and give a dawn “chorus” that is answered by other males. Because they do not have an exclusive territory and share parts of their home range with others, the morning call helps to define, defend, and clarify the group’s claim on their home range. When troops move to a new feeding site they also make calls for these reasons. Weaker troops can identify the location of strong troops and avoid those areas, where they will not be able to feed. When two troops do meet, males expend much energy howling, leaping, running, and fighting. This detracts from time that could be spent eating or resting. Instead of spending their energy constantly patrolling territories or fighting with other troops, they howl to let other groups know where they are.

Yucatán Black Howlers are polygynous and only the dominant males or males with high social status will mate with the 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, possibly because their diet of leaves and fruits is not seasonal in availability. To demonstrate interest, both the male and the female flick their pink tongues in and out of their mouths. During the mating period, the pair mutually stay together for a few days and perform multiple copulations. Gestation lasts for approximately 180 days and females give birth to a single offspring. The newborn is licked clean and carried by their mother. Infants are brown in color and develop their black coats as they mature. They are nursed for their first three weeks after birth, at which point they begin to eat leaves. As the young mature, they begin to ride on their mother’s back by grasping the base of her tail with their own. The young are constantly with their mothers for the first four months, never venturing more than 2 m (6.5 feet) away from each other. Occasionally, females other than the mother may care for the young. After about ten to eleven weeks, juveniles start to forage independently and spend much more time on their own. Males reach sexual maturity at about 42 months of age, while females reach sexual maturity at 36 months of age. Females typically remain in their natal groups upon reaching sexual maturity, while males leave. They live between 15 and 20 years in the wild.

Yucatán Black Howlers are herbivores and frugivores. Their diet consists of more than 130 plant species, with more than half of those being from trees. They prefer to consume leaves, fruits, and flowers. Mammals do not have the enzymes capable of digesting cellulose, the carbohydrate that composes the cell wall of plants. Instead, the Yucatán Black Howler receives its energy from rich gasses released during bacteria triggered reactions (fermentation). They have a rather simple acid stomach that contains two enlarged sections in the cecum and colon, in which the fermentative bacteria are found. The gasses produced by this bacteria during digestion serve as an energy source. Even with fermentation they can only extract limited calories from their food, so they must be cautious as to how much energy they spend. As a result, they typically spend half of their waking day resting. Males settle disputes and defend their group from predators, allowing the females to spend more energy on reproduction and care of the young. The greatest predatory threat to young Yucatán Black Howlers is infanticide by adult males. While the exact cause of this event varies and is still being studied, the sexual selection hypothesis offers three potential motivations. First, the attacking male wants to replace the male occupying the position of sole or dominant breeder in the group. Second, the attacking male wants to resume the ovulatory cycling of the mother. Third, the attacking male wants to copulate with the mother, thus shortening the interbirth interval. While most reports provide supportive data for all three predictions, they do not explain all cases of infanticide. Additional predators to the Yucatán Black Howler include jaguars, pumas, large birds of prey, and large arboreal snakes. Humans have been known to illegally catch individuals for pets. They are also occasionally eaten by humans in some areas.

The Yucatán Black Howler may be confused with the Mexican Spider Monkey, Ateles geoffroyi vellerosusdue to their similar appearances and overlapping ranges. Mexican Spider Monkeys have long slender bodies that are covered with coarse black and tan fur. They typically have tan-colored markings around their eyes and chin, and light whiskers around their head. Their social groups are dominated by females, who are also in charge of foraging groups. While Mexican Spider Monkeys also produce vocalizations, they are not nearly as loud as those produced by the Yucatán Black Howler.

The Yucatán Black Howler may be confused with the Mexican Spider Monkey, Ateles geoffroyi vellerosusdue to their similar appearances and overlapping ranges. Mexican Spider Monkeys have long slender bodies that are covered with coarse black and tan fur. They typically have tan-colored markings around their eyes and chin, and light whiskers around their head. Their social groups are dominated by females, who are also in charge of foraging groups. While Mexican Spider Monkeys also produce vocalizations, they are not nearly as loud as those produced by the Yucatán Black Howler.

From a conservation perspective, the Yucatán Black Howler is current considered to be Endangered due to a population reduction of 50% or more over the course of three generations. Data from the Global Forest Watch suggests that should forest loss continue at the same rate that has impacted the previous generation, 31% or more of this species’ suitable habitat is likely to be lost by the year 2048. Yucatán Black Howlers are primarily threatened by deforestation and the direct capture of individuals for the pet trade. In Guatemala, high rates of deforestation have been associated with the rapid expansion of the agricultural and mining fields. While the illegal trade of Yucatán Black Howlers is well-known, there is little available data to determine the extent of its impact on the species.