This study describes several of the proximate and ultimate factors influencing the flexible "fission-fusion" grouping patterns of wild spider monkeys ( Ateles belzebuth belzebuth) living in a lowland tropical rainforest at the Tiputini Biodiversity Station (TBS), in eastern Ecuador. During 22 months we conducted behavioral follows using focal animal sampling on all adult members of one habituated group of spider monkeys. We collected systematic data on their activity patterns, diet and ranging behaviors. Data on the size and composition of the subgroup containing the focal animal were also collected in detail, recording all events related with subgroup change in size and composition (fissions and fusions). Furthermore, in order to monitor the mineral licks used by spider monkeys at the TBS and study their lick visitation patterns we used camera and video traps that recorded the activity at the licks continuously. The main goal of this study was to describe the flexible grouping associations of spider monkeys and evaluate the influence of resource availability, predation pressure and intergroup competition on the ranging and grouping decisions of a group of spider monkeys.
In order to describe the flexible grouping patterns of spider monkeys we conducted simultaneous follows on two spider monkeys, attempting to sample all possible dyads, and recorded their inter-individual distances. We were able to describe the spatial cohesion between the members of a group of spider monkeys and found that while on average approximately 40% of the time any two members of the group might range together cohesively in a subgroup, in approximately 60% of the time they would travel and forage in complete independence from each other. Nonetheless, the study group had a stable composition and group members would interact almost exclusively with each other rather than with any other conspecifics. This results support the idea that fission-fusion grouping patterns are indeed extremely flexible as subgroup size and composition, as well as the spatial cohesion of groups' members vary within short spatio-temporal intervals.
Fruit availability at the Tiputini Biodiversity Station was less seasonal when compared to the phenological patterns of most Neotropical lowland forests where spider monkeys have been studied. Resource availability at the TBS and intragroup feeding competition seem to only explain a minor proportion of the variance in spider monkey subgroup size and grouping strategies. The strong correlation between resource availability and spider monkeys subgroup size found elsewhere was not found at TBS.
These results might be supported by the fact that periods of fruit scarcity are shorter at TBS and social groups use extremely large areas to forage and exploit resources. Other factors, such as predation risk and intergroup competition, amongst others, might be playing a stronger role on the grouping patterns of spider monkeys.
This study evidenced that mineral licks are key areas for spider monkey and howler monkeys (Alouatta seniculus) at the TBS. They are the most frequently and intensively used area within the spider monkeys' home range, and both primate species visit the lick frequently, on approximately 14% of days when the licks were monitored. The results suggest mineral licks are perceived as areas of high predation risk as spider monkeys only visit mineral licks on days with ideal weather conditions (sunny and clear days), spend several hours vigilant at the lick area prior to coming down to the ground only for one or two minutes, and leave the lick immediately after eating soil form the forest ground. Visits to mineral licks strongly influenced the grouping patterns of spider monkeys at TBS, suggesting that perceived predation risk influences the grouping strategies of spider monkeys. Spider monkeys might be gathering at the lick area in order to increase vigilance and as a strategy to reduce the chance of not detecting a predator while attempting to gather the only resources found available in the forest floor.
Intergroup competition amongst groups of spider monkeys is intense at the TBS. The behavioral patterns displayed by spider monkeys in the context of intergroup competition closely resemble those displayed by common chimpanzees (Pan troglodytes). Male spider monkeys were observed engaging in almost all behaviors that characterize lethal coalitionary aggression in wild chimpanzees: territorial defense, boundary patrols, deep incursions, coalitionary attacks and border avoidance, and indirect evidence largely suggests that coalitionary kills might also be part of the behavioral repertoire of spider monkeys at TBS. This study further documents that warfare occurs outside the Human-Chmpanzee lineage and that the unique sociality of spider monkeys, chimpanzees and small scale human societies (fission-fusion grouping patterns, male philopatry and strong male bonds in territorial defense) might be escential factors in the evolution of warfare and lethal intergroup aggression in two distant primate lineages. (Abstract shortened by UMI.)
|Advisor:||Fiore, Anthony F. Di|
|Commitee:||Cords, Marina, Disotell, Todd R., Jolly, Clifford J., Rothman, Jessica|
|School:||New York University|
|School Location:||United States -- New York|
|Source:||DAI-A 72/11, Dissertation Abstracts International|
|Subjects:||Physical anthropology, Zoology|
|Keywords:||Ateles belzebuth, Ecuador, Fission fusion, Intergroup competition, Predation risk, Primates, Resource availability, Spider monkeys, White-bellied spider monkey|
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