Mexican Spider Monkey, Ateles geoffroyi vellerosus

Mexican Spider Monkey, Ateles geoffroyi vellerosus. Photographs taken in Cañón del Sumidero, Chiapas, March 2021.  Photographs and identification courtesy of Marina Sutormina, Stockholm, Sweden.

Mexican Spider Monkey, Ateles geoffroyi vellerosus. Photographs taken in coastal Costa Rica, February 2023.  Photograph and identification courtesy of Dr. Tom Bartol, Carlsbad, California.

The Mexican Spider Monkey, Ateles geoffroyi vellerosus, is a member of the Atelidae family of Howler and Prehensile-tailed Monkeys. They are one of six recognized subspecies of Geoffroy’s Spider Monkey, Ateles geoffroyi. The Atelidae family consists of nineteen species placed in four genera. There are seven species in the Ateles genus. The Mexican Spider Monkey is also known as the Northern Mesoamerican or Yucatán Spider Monkey and in Mexico is known as Mono Aranya.

Mexican Spider Monkeys are large-bodied primates with short, thin fur. The majority of their coat, including the portions covering their head, limbs, hands, feet, and tail, is brownish-black or black in color. Their lumbar region, ventral side, and inner aspects of their limbs range from silvery-white, to yellowish-white, to golden brown. Infants possess a uniform, dusky pelage that transitions into the bicolor pattern at the end of their first year. Mexican Spider Monkeys have a small head and a large, pear-shaped body. Their limbs are long and slender, each possessing four long digits. Unlike other species, they lack an external thumb. Their “hook-like” hands are specialized for arboreal locomotion, allowing them to grasp, hang, and swing on branches. Their tail is long and prehensile, operating as a fifth limb that can support their entire body weight on its own. At the end of their tail is a skin-textured strip that is used to grip objects and move in the trees. Mexican Spider Monkeys are monomorphic, meaning males and females are nearly identical. At close range, females can be identified by their elongated clitoris or by the act of carrying an infant, a behavior only performed by females. Both sexes grow similarly throughout life and at maturity, possess nearly identical weights and lengths. Adults measure up to 47 cm (18.5 inches) in snout-to-vent length with a 70 cm (2 foot 4 inches) tail. The mature males are slightly larger having a maximum of 7.8 kg (17 lbs), while mature females weigh 7.3 kg (16 lbs).

Mexican Spider Monkeys live in large groups with several males and females that are organized by a fission-fusion social structure. While their large group, or troop, has a relatively stable membership, members will separate into smaller subgroups on a daily basis. Available research suggests that troops vary in size from 16 to 56 individuals, the majority of whom are male. Troop sizes likely depend upon local factors such as habitat constraints, resource availability, and activity engagement. Subgroups are typically sex-segragated, and all-male subgroups generally travel farther and faster than subgroups of females and their young. While subgroups routinely fluctuate in composition and size, they are determined by a distinct pattern of social relationships. Females are the core of the social network because they form several stable, less selective associations with other group members. Young adult males are initially not tolerated by the older males and form stronger associations with adult females. As males age, they become more tolerable to the resident adult males. New females that immigrate into a troop are initially more tolerated by resident adult males. However, over time, they develop stronger associations with the resident females. Mature males are strongly bonded with a few other members of the troop, who are typically male. The bond between males is maintained through active companionship and is believed to benefit the rest of the group by reducing interspecific competition. In any group, the mother-infant bond is the closest relationship. These relationships help coordinate efforts to patrol boundaries and defend access to food and females in their home range from neighboring males. Mexican Spider Monkeys use vocalizations, facial expressions, body language, physical contact, and contact-promoting behaviors to communicate. Multiple forms of communication are often used simultaneously. Many of their different expressive patterns cannot be interpreted alone, as they convey messages that depend upon the context in which they are used and the sender’s relationship with the recipient, just as in human communication. In general, messages between individuals can be distilled into a few signaling types. These include general alertness or attentiveness, indifference or ambivalence, threatened or threatening arousal, granting attention, sexual interactions, and efforts to maintain group assembly. Physical conflicts are relatively rare. 

Mexican Spider Monkeys are polygamous, with one male mating with multiple females. The females are very particular about who they will mate with and may reject several suitors before making their selection. Copulation is rarely observed in the field so little is known about the mating process. It is assumed that most sexual interactions take place at dusk covertly in the trees they sleep in. Births occur year-round and are not disproportionately seasonal. Gestation lasts approximately 225 days (over 7 months) and females give birth to one infant. Mothers form a close bond with their infants and are the primary caregivers in raising their young. For the first four to five months of life, newborns cling to their mothers. The young are then carried on their mothers back for another month or two before they can move around independently. Female ovulation is suppressed by lactation, and a new mother will not give birth again for at least two years, sometimes more than four. In general, female primates are highly attracted to non-related infants in their social groups. While female Mexican Spider Monkeys do not participate in allomothering or frequent infant grooming, they exhibit a marked increase in embraces exchanged between other adult females and a mother with an infant. Both males and females become sexually mature after five years of age. At that time, the females disperse to find neighboring troops, while young adult males remain in the natal group. In wild populations, individual longevity is strongly affected by habitat quality and susceptibility to local hunting. Assuming infants reach adulthood, they have life spans of twenty-five years in the wild and forty-seven years in the captivity.

Mexican Spider Monkeys are diurnal and their daily activities are determined by food availability within their home range, which varies between one to ten square kilometers (0.37 to 3.7 square miles). Troops occupying fragmented forests where food is scarce adjust by traveling more frequently and over greater distances to find resources. In general, Mexican Spider Monkeys spend most of their active time feeding, followed by traveling and resting. They are primarily frugivores and approximately 80% of their diet consists of various fruits. They supplement their diets with leaves, flowers, buds, and occasionally bark, nuts, seeds, insects, spiders, and eggs. Young leaves provide them with some of the protein that is lacking in fruit. Along with providing much of their nutritional needs, fruit and leaves also provide much of their water requirements. They will drink water from tree holes, water gathered on leaves, and even terrestrial sources, a behavior not found in other Spider Monkey species. Besides humans, Mexican Spider Monkeys have few, if any, natural predators. While they have no documented predators, it is possible that they are preyed upon by large Harpy Eagles or predatory cats such as Pumas and Jaguars. Mexican Spider Monkeys are agile and use several forms of locomotion including quadrupedal or upright bipedal walking, leaping, and brachiation. Brachiation, also referred to as arm swinging, is a form of arboreal locomotion in which primates swing from tree limb to tree limb using only their arms. Infants and juveniles may scoot, but not walk, bipedally. Mothers carrying infants move bipedally less often than adults that do not need to support the additional weight. While terrestrial locomotion is common in captivity, wild populations seldom descend to the forest floor. Mexican Spider Monkeys may partake in “sunning”, lying on their backs and basking in the sun, in the early morning, after feeding in the afternoon, or at the first opportunity after a rainstorm. During rest and sleep, they support themselves on a branch or assume a sitting position, lower their head to their chest, and wrap their arms around their legs. At night, groups of two or three individuals usually sleep together while embracing this way. 

The Mexican Spider Monkey may be confused with the Mantled Howler Monkey, Alouatta palliata, due to their similar appearances and overlapping ranges. These specially can be differentiated visually. Mantled Howler Monkeys are large and stocky with black fur and a mantle of lighter-colored guard hairs that reaches down their sides. These long guard hairs give them the common name of “Mantled” Howler Monkey.

From a conservation perspective, the Mexican Spider Monkey is considered Endangered due to a population reduction of greater than 50% over the course of forty-five years, or three generations. Should forest loss continue at the same rate, it is estimated that 40% of their suitable habitat is likely to be lost by the year 2063. Mexican Spider Monkeys are scarce and highly fragmented within their area of occurrence. They are strongly impacted by habitat loss due to large-scale monocultures (such as oil palm and sugar cane), oil extraction, mining, hydroelectric dams, and extensive livestock farming. They are often illegally hunted and captured for pets in homes, businesses, and circuses.