MEXICAN SPIDER MONKEY

Ateles geoffroyi vellerosus

Geographic Distribution and Habitat
The Mexican spider monkey (Ateles geoffroyi vellerosus) is one of six recognized subspecies of the Geoffroy’s spider monkey (Ateles geoffroyi). Populations of the Mexican spider monkey occur in the Southern Mexican states of Veracruz, Oaxaca, Chiapas, Tabasco, Yucatan, and Quintana Roo. Farther south, beyond its eponymic country, this monkey is found in Guatemala, Belize, El Salvador, and Honduras. Spider monkeys occur in a variety of neotropical forested habitats throughout Central America, including cloud forest, tall evergreen forest, lowland tropical forest, mangrove forest, and dry deciduous forest. Although they can survive in forest remnants that have been disturbed by human activities, population studies and general species ecology suggest that spider monkeys are highly susceptible to the effects of habitat degradation. 

Mexican spider monkey range, IUCN 2020

Size, Weight, and Lifespan
Spider monkeys, or members of the genus Ateles, are large-bodied primates that share consistencies in size and body proportions. Both sexes of the Mexican spider monkey grow similarly throughout life and at maturity possess nearly identical lengths and weights. Adults are 18.5 in (47 cm) long, excluding the dextrous, furred tail, which is longer than the body at 28 in (70 cm). Mature males typically weigh 17 lbs (7.8 kg), and female mass is 16 lbs (7.3 kg). In wild-living populations, individual longevity is strongly affected by habitat quality and susceptibility to local (human) hunting. Assuming infants reach adulthood, members of this species live around 25 years. In captivity, individuals may live significantly longer. One instance of a captive female reaching 47 years old is documented.

Appearance
The Mexican spider monkey has a highly variable color pattern, though variations are not determined by sex. The subspecies is monomorphic, meaning that males and females are nearly identical. Their fur is short and not dense. The majority of their coat, including the portions covering the head, limbs, hands, feet, and tail is brownish-black or black. The hair-coat lightens towards the lumbar region, on the underside, and on the inner aspects of the limbs. Coloring in these areas may range from silvery-white, to yellowish-white, to golden brown. Infants have a uniform, dusky pelage, which transitions into the bicolor pattern at the end of their first year of life.

Due to species monomorphism, the sexes can be difficult to differentiate in the field. However, at a close range, females can be identified by their elongated clitoris or by the act of carrying an infant, a female-only activity. Spider monkeys have a smallish head and a large, pear-shaped, pot-bellied body. Their gracile limbs each possess four long digits and lack an external thumb. These “hook-like” hands are specialized for arboreal locomotion; in the trees, these monkeys adeptly grasp, hang, and swing on branches. The prehensile tail operates as a fifth limb that can support the entire body weight on its own. The end of the tail has a skin-textured strip that is used to grip objects and move in the trees.

Appearance
The Mexican spider monkey has a highly variable color pattern, though variations are not determined by sex. The subspecies is monomorphic, meaning that males and females are nearly identical. Their fur is short and not dense. The majority of their coat, including the portions covering the head, limbs, hands, feet, and tail is brownish-black or black. The hair-coat lightens towards the lumbar region, on the underside, and on the inner aspects of the limbs. Coloring in these areas may range from silvery-white, to yellowish-white, to golden brown. Infants have a uniform, dusky pelage, which transitions into the bicolor pattern at the end of their first year of life.

Due to species monomorphism, the sexes can be difficult to differentiate in the field. However, at a close range, females can be identified by their elongated clitoris or by the act of carrying an infant, a female-only activity. Spider monkeys have a smallish head and a large, pear-shaped, pot-bellied body. Their gracile limbs each possess four long digits and lack an external thumb. These “hook-like” hands are specialized for arboreal locomotion; in the trees, these monkeys adeptly grasp, hang, and swing on branches. The prehensile tail operates as a fifth limb that can support the entire body weight on its own. The end of the tail has a skin-textured strip that is used to grip objects and move in the trees.

What Does It Mean?

Affiliative: 
Social interactions that function to reinforce social bonds with a group or which are of mutual benefit to all animals involved in the interaction.

Agonistic:
Of, relating to, or being aggressive, or defensive social interaction (such as fighting, fleeing, or submitting) between individuals usually of the same species.

Arboreal:
Physically adapted to living primarily or exclusively in trees.

Brachiation:
Also called arm swinging, it’s a form of arboreal locomotion in which primates swing from tree limb to tree limb using only their arms.

Diurnal:
Active during daylight hours.

Extirpation:
When a species ceases to exist in a geographic area they once occupied.

Fission-fusion:
A society in which the members of a social group break off into smaller groups and then rejoin as a larger group. 

Gracile:
Slender, thin.

Interbirth interval:
Refers to the period of time between successive births of an individual female.

Monomorphic:
Showing no differences between the sexes other than their genitalia.

Philopatry:
Returning to one’s birthplace or remaining with one’s birth group.

Prehensile tail:
The tail of an animal that has adapted to be able to grasp or hold objects.

Visit the Glossary for more definitions

Behavior and Lifestyle
Mexican spider monkeys are diurnal, and daily activities are determined by food availability in their home range. Troops occupying fragmented forest where food is scarce adjust by traveling more frequently and over greater distances to find resources. Their home area may range from 0.37 to 3.7 square miles (95–962 hectares). Generally, spider monkeys spend most of their active time feeding, followed by traveling and resting. These primates are agile, versatile movers. Locomotion occurs by brachiation, leaping, and quadrupedal or upright bipedal walking and running. Infants and juveniles may scoot but not walk bipedally, and mothers carrying infants move bipedally less often than adults that do not have to support the additional weight. Wild-living spider monkeys seldom descend to the forest floor, but terrestrial locomotion is common in captivity.

In the early morning, after feeding in the afternoon, or at the first opportunity after a rainstorm, spider monkeys may partake in “sunning,” lying on their backs to bask in the sun. During rest and sleep, they support themselves on a branch or assume a sitting position, lowering their heads to their chests and wrapping their arms around their legs. At night, groups of two or three individuals usually sleep together while embracing in this way. Besides humans, spider monkeys have few, if any, natural predators. While there are no published accounts of predation on these large-bodied primates in Mesoamerica, it is possible that large harpy eagles or predatory cats like pumas and jaguars could occasionally hunt spider monkeys.

Daily Life and Group Dynamics
Mexican spider monkeys live in large, multi-male, multi-female groups organized by a fission-fusion social structure. The large group has a relatively stable membership, but on a daily basis, members will separate into smaller subgroups that routinely fluctuate in composition and size. Field studies of troop dynamics are limited due to the difficult nature of execution, but the available research suggests that groups vary in size from 16 to 56 individuals, the majority of whom are male. More specifically, the female-to-male sex ratio in groups is roughly 0.8 to 4.4. Group sizes likely depend on local factors, like habitat constraints, resource availability, and activity engagement. 

Subgroups are typically sex-segregated, and all-male subgroups travel farther and faster than subgroups with females and their young. While subgroup composition is fluid day-to-day, it is colored by a distinct pattern of social relationships. Females constitute the core of the social network because they form many stable, less selective associations with other group members. Initially, young adult males are not tolerated by older males and have stronger associations with adult females. As males age, they have more affiliative interactions with resident adult males. This species exhibits male philopatry, and female spider monkeys are newcomers to larger groups. New females that immigrate into the troop are initially more tolerated by resident males, but over time, stronger associations with resident females develop. 

The mother-infant bond is the closest relationship in any group, but mature male spider monkeys are strongly bonded with a few other individuals in the troop, who are typically male. The male-male bond is maintained through active companionship and is thought to confer benefits to the larger group by reducing interspecific competition. These relationships help coordinate efforts to patrol boundaries and defend access to food and females in the home range from neighboring males. A similar social and ecological dynamic is observed in other large-bodied primates, like chimpanzees, who face little natural predation but must routinely protect home resources from other encroaching male-conspecifics.

Fun Facts

The naming of this species and subspecies has interesting origins. The genus name Ateles translates from greek as “imperfection or incompleteness of the hand,” since spider monkeys have only four fingers and no thumb. The species geoffroyi is named for the nineteenth-century French vertebrate naturalist, Étienne Geoffroy Saint-Hilaire, who described these monkeys in his writings as having bizarre body proportions. Due to their long limbs, pot bellied form, and smallish head, they became commonly referred to as “spider monkeys.” The subspecies name vellerosus comes from latin and means “hairy.”

Spider monkeys were not observed copulating in the wild until 1970.

Other than the great apes, spider monkeys have the “slowest” life history of all primates due to old age at first reproduction (after 5 years of age) and long interbirth intervals (can exceed 2–4 years).

Communication
Spider monkeys use vocalizations, facial expressions, body language, physical contact, and contact-promoting behaviors to communicate. Often, multiple types of expression are employed simultaneously. As is the case with human communication, many of the different expressive patterns cannot be interpreted in a vacuum, as they convey messages that depend on the context in which they are used and the sender’s relationship with the intended recipient(s).

In general, the messages conveyed between these monkeys can be distilled into a few categories. These signaling types include general alertness or attentiveness, indifference or ambivalence, non-aggressive or aggressive (threatened or threatening) arousal, affiliative soliciting or granting attention, sexual interactions, and efforts to maintain group assembly. Recipients of messages may be affiliative, neutral, or agonistic conspecifics, or other animate and inanimate stimuli in the environment. Agonistic contact behaviors occur relatively rarely, and when spider monkeys perceive that they are being hunted by human predators, they opt to remain inconspicuous and covert instead of directing overt threats at pursuers.

Reproduction and Family
Spider monkeys have a polygamous reproductive structure, and family bands consist of an adult male, several females, and their young. Copulation is rarely observed in the field and remains somewhat mysterious to researchers at present, but it is assumed that most sexual interactions covertly take place at dusk in the sleeping trees. Since the most frequent agonistic interplay observed in spider monkeys is male aggression towards females, and its occurrence is strongly associated with periods when females are reproductively cycling, this interplay might be linked to or representative of a form of sexual behavior. 

Both males and females become reproductively mature after five years of age, at which point the females disperse to find neighboring troops, while young adult males remain in the natal group. Births occur year-round and are not disproportionately seasonal. Gestation periods of offspring are long, lasting 225 days (over 7 months). Females give birth to one infant. Mothers bond closely with their infants and are the primary caregivers in raising their young. Newborns cling to their mothers for the first four-to-five months of life and then are carried on the female’s back for another month or two before they finally move about independently. Female ovulation is suppressed by lactation, and a new mother will not give birth again for at least two years and sometimes more than four.

In general, female primates are highly attracted to non-related infants in their social groups. While female spider monkeys do not participate in allomothering or frequent infant grooming, this attraction is exhibited by a marked increase in embraces exchanged between other adult females and the mother with the infant.

Photo credit: Luis Miguel Bugallo Sánchez/Creative Commons

Ecological Role
Spider monkeys symbiotically maintain vegetation structure, diversity, and distribution in their habitats through frugivory and seed dispersal. In tropical rainforests, primates constitute between 25 and 40% of total frugivore biomass. Birds, bats, and other mammals make up the rest of the total and may or may not disperse the same seed types as the primates occupying the same local zones. The role of spider monkeys is especially crucial in the reproduction of large-seeded tree species that rely upon large-bodied animals for dispersal. These large new-world primates feed on unripe fruits by consuming a small amount of fruit pulp or aril (the pod surrounding a seed) and spitting out or dropping the seeds directly under the parent tree. Whole, ripe fruits are fully consumed, and the seeds travel through the digestive tract undamaged and are defecated in clumps or scatters of feces some distance away from the parent tree. Distances from the origin can exceed 328 feet (100 meters). Seed germination via this dispersal method is successful when there is a greater number of seeds present in stools, a wider variety of seed species present, and a higher number of scattered stool deposits. 

A study examining Mexican spider monkey feeding ecology in the forests in southern Mexico found that stools contained mostly intact (86% undamaged versus 14% damaged) seeds of 71 different species of plants, and 95% of all stools examined contained seeds. In continuous forests in this region, ripe fruits are more available and frugivory occurs more frequently. In fragmented forests, spider monkeys more frequently resort to eating unripe fruits and other foods. This shift was reflected by behavioral observations and fecal analysis: the monkeys dropped or spit seeds more regularly, and fewer stools contained seeds (82%). While their ecological role is increasingly important to regenerate vegetative structure in degraded forest zones, unfortunately in these areas they face greater threat of extirpation due to hunting pressures, larger home ranges, and low reproductive rates.  

Conservation Status and Threats
The International Union for Conservation of Nature (IUCN) categorizes the Mexican spider monkey as Endangered (IUCN, 2020). An estimated population reduction of over 50% is projected over the next three generations, or 45 years. Habitat degradation and hunting threaten species survival. Intensity of development has deforested Mesoamerica by 70%, and Global Forest Watch projects that 34% of the species’ habitat will be lost by 2063. Construction of additional highways in spider monkey habitats has accelerated in recent years, which incentivizes hunting and capture for sale through increased access to markets. Spider monkeys are the most commonly captured native species for the primate pet trade, and they are usually sold along highways in southern Mexico. 

Spider monkeys are locally sold to relatives, friends, or people searching for an infant and can fetch at least $18 to $260. Adult females and other adults in the group are usually killed during infant capture. Without further data on population size, distribution, and number of monkeys killed, assessing the magnitude of hunting is not possible. However, even if hunting is only occasional, the removal of multiple individuals from a low-reproductive-index species will inherently affect local groups and have consequences for broader spider monkey populations.

One study conducted in the early 2000s interviewed four hunters, who routinely hunted Mexican spider monkeys in the forests of Oaxaca, Mexico, as complementary to their principal occupation. The interviews suggested that the hunters were not knowledgeable of current legislation, national conservation status of the monkeys, or ecological consequences of hunting. The hunters were very knowledgeable about the land and its animals and knew generally that hunting could be punished but were not deterred. They were aware of the monkeys’ commercial value and would attempt to capture the animals when possible.

Conservation Efforts
​The Convention on International Trade in Endangered Species (CITES) lists the Goeffroy’s spider monkey (Ateles geoffroyiz), the parent species of the Mexican spider monkey (Ateles geoffroyi vellerosus) in Appendix II (CITES, 2018), and Mexico considers the species endangered by the NOM-059-SEMARNAT-2010 (SEMARNAT 2010). Despite intense deforestation, Mesoamerica officially recognizes 400 protected areas within the spider monkey distribution range. Yet these areas are small, relatively isolated, and have no fixed boundaries. Though the sites are “recognized,” they lack actual protection due to limited staff members and virtually no instances of official recourse. Since the absence of spider monkeys in formerly occupied zones is almost exclusively due to human activities, in order to preserve current populations, it is necessary to inform local inhabitants of conservation status and facilitate local involvement in national and international efforts for species conservation.

References:

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Written by Cookie Koch, April 2021