Seasonal variation in association patterns of wild spider monkeys (Ateles belzebuth belzebuth) at La Macarena, Colombia.

Spider monkeys exhibit a fission-fusion type of social organization. I studied party size and party composition in wild long-haired spider monkeys (Ateles belzebuth belzebuth) in three study periods at La Macarena, Colombia and found that overall party size was larger in the fruit-abundant season. Mean party size in which males were observed was relatively stable across seasons. In contrast, the mean party size of females varied. Females were observed in larger parties in the fruit-abundant season than in the fruit-scarce season. Moreover, whereas males associated with each other at an almost equal frequency across seasons, females associated with each other more frequently in the fruit-abundant season. Females with infants or small juveniles were more often in association with other individuals than were cycling females. The intensity of individual relationships varied according to season, such that even mothers and sons were not always strongly associated. In a large party, females with infants may gain from predation avoidance but they are at a disadvantage in terms of scramble competition. The balance between these factors may change with fruit availability and may influence party size in different periods. For males, party formation may facilitate the defense of resources from neighboring groups more than provide predation avoidance.