As one might expect of these fruit fanatics, fruit availability is one factor that shapes the dynamic relationships within Geoffroy spider monkey society. Essentially, it determines who hangs out with whom, and when. Throughout the year, and even the day, Geoffroy’s spider monkey groups fluctuate in size, a dynamic social relationship that is termed “fission-fusion”. Geoffroy’s Spider Monkeys typically live in large groups of between 16 and 56 members where male members are related to one another and females are from an external group. On occasion, group numbers can reach as high as 100 individuals. On average, there are typically 4 males (4.4) for every 1 female (0.8) in the group and the number of females increases when there is a proliferation of fruit, such as around the wet season. 

At the start of the day, the large group splits into “culinary search committees”, consisting of two to six members. This “fission” of the larger group improves foraging efficiency. Who follows who is based on each individual’s knowledge of food in the area. The high intelligence of Geoffroy’s spider monkeys allows them to memorize the locations of regular food sources. Once a good fruiting source is found, the “search committee” reports back to the group about the feeding sites. By sharing information the whole group obtains a more complete picture of the always changing foraging environment than any one individual could obtain on their own. The fission-fusion relationship is an evolutionary adaptation that allows Geoffroy’s spider monkeys to respond to environments with fluctuating food availability. When fruit availability is high and concentrated in clustered patches, group sizes increase and the amount of area covered in search of food decreases. Groups are larger and change members more often during the rainy season compared to the dry season. Groups of just females are less tightly knit but keep the same members for longer than groups of just males.

Between March and June, the Brosimum–or breadnut tree–an irresistible favorite of Geoffroy’s spider monkeys, grows heavy with fleshy orange fruits, foreshadowing the arrival of the rainy season in May that brings with it an abundance of food. In mature forests, where the density of fruit is ten times greater than in less developed stages of forest, this rich bounty of Brosimum fruits becomes a gathering ground. Large subgroups of Geoffrey’s spider monkeys congregate around these dense patches. Numerous fruits mean that these monkeys don’t have to travel as far to find food. Typically, they forage over large tracts of land. A group of Geoffroy’s spider monkeys can cover up to 6,600 feet (2,000 meters) in a single day, and their home range often exceeds 2,200 acres (900 hectares), but the rainy season marks a time of abundance and less travel. 

However, fruit is not always as readily available throughout the year. The unpredictable and dispersed nature of fruit prevents any one individual or group from monopolizing resources. This has produced a surprisingly egalitarian social structure that lacks any clearly defined dominance hierarchy. Instead of contests for food between individuals and groups, subgroups often scramble to find food, reporting back to the larger group about their findings. Association patterns (time spent together, preferred companions, and group composition) vary between seasons, respond to changes in fruit availability, and are influenced by the sex of individuals. On the other hand, proximity patterns (the physical closeness or nearness of individuals within a social group without social interaction) are minimally affected by changes in fruit availability, suggesting that social factors are more important than food availability in determining cohesion within subgroups.

While there is no one grand poobah of the group and the dominance hierarchy is not as obvious as it is in some other primate species, there are still power dynamics at play. Age-related social status does appear to be important among males, producing several dominant males within a group that appear to mate more frequently than low-ranking males. Despite this, each reproductively mature male mates at least once a year. 

Adult females tend to be more submissive and more nonsocial. When female Geoffroy’s spider monkeys reach reproductive age (around four years old), they break off from their maternal group, sometimes traveling alone or in a coed group. The lack of a cohesive hierarchy in some ways makes strong and permanent bonds less likely to form, particularly among the typically unrelated females. As a result, individuals decide when to leave or join a certain subgroup. Males typically will remain in their kin group. 

Males tend to be both territorial and aggressive, but they direct most of their aggression towards females. Females try to avoid males as much as possible. When they do come together, particularly during the fruiting season, females show a great deal of vigilance. Initially, it was thought that this social vigilance was a way to monitor food sources, but further research has shown that females are wary of aggression from males. While roughly 20 percent of this aggression is physical, the majority (~80%) can be thought of as sexually charged.

This sexually charged aggression occurs most frequently when the female is cycling and is associated with the male smelling the location where the female was previously sitting. The male then ritualistically chases the female through the branches. This is then proceeded by the female courting the male attacker after the chase has ceased. While chases do not culminate immediately in copulation, they do sometimes culminate in the female leaving the subgroup to travel alone with the male aggressor. 

While male-female aggression is more frequent, male-male and female-female aggression does occur on occasion. For females, aggression typically occurs during feeding when in competition for food. Male-on-male aggression does increase in the presence of females, but the amount of diplomacy does. During tense situations, males embrace to clear the air. In the presence of females, the amount of embraces increases as a way to mitigate access to females. “Genital manipulation” is another means for regulating social relationships that can serve to alleviate tension, prompt reconciliation or signal the forming of an alliance. But when it comes to throwing punches, so to speak, Geoffroy’s spider monkeys will grapple with each other. This can include genital manipulation and, in very rare cases, anal penetration. 

However, these cases are rare and males tend to be much more chummy with one another, choosing to galavant around the forest with the boys. Females on the other hand tend to travel alone or with offspring and are more dispersed. Females, in fact, show no interest in being around other females. These intrasex relationships likely hark back to their style of fission-fusion social structure; the females disperse, meaning the males in a subgroup tend to be related to one another.  

Unlike many other primates, social grooming isn’t particularly important to Geoffroy’s spider monkeys. A measly 2.5 percent of their day is dedicated to the task with an average session lasting just two minutes. Instead, their social relationships are shaped by a number of other behaviors, including embraces, kisses, and pectoral sniffs. That said, they still average two grooming sessions a day which occur around noon and late afternoon (4:00-5:00 PM). Nevertheless, grooming still reflects important aspects of their social structure, including intraclass grouping preferences (males like to groom other males), female dispersal and male philopatry, and a long period of juvenile dependence. Females tend to be the ones who groom the most with juveniles being the most frequent recipients of grooming. Males tend to groom other males in this male-bonded fission-fusion society. Both grooming and embraces are valuable social tender between males of the same age. However, as the disparity of age increases so does the reciprocity of grooming. Younger males tend to value relationships with older males more than the reverse, and they give more embraces than they get.