Author's School

Graduate School of Arts & Sciences

Author's Department/Program



English (en)

Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Robert Sussma


I conducted a 15 month study of the activity patterns, diet, ranging behavior, and group cohesiveness of northern bearded sakis: Chiropotes sagulatus) in a continuous lowland rainforest in southern Guyana. My study combined observational data with a novel GIS-based method for assessing food patch quality to assess how bearded sakis adjusted their ranging and grouping patterns in response to changes in resource quality. My study group consisted of at least 65 animals, making it the largest group of Chiropotes yet reported. Bearded sakis were highly active, spending most of their time traveling: 40% of activity) and feeding: 35% of activity). Compared to other platyrrhines, they spent relatively little time resting. Sakis changed their activity patterns monthly, increasing foraging effort when fruit was most abundant, and increasing resting when fruit was most scarce. Consistent with their dental adaptations for seed predation, seeds made up 75% of feeding time. Sakis consumed over 215 plant species during the study period, including many types of dry and fleshy fruits. Due to their ability to exploit food items of varies states of maturity, sakis often used the same tree species for three months or more. Monthly seed consumption was correlated with fruit availability. When seeds became scarce seasonally, they incorporated a high percentage of insects, flowers, and mature fruit in their diet. The study group had home and day ranges that were larger than previously reported for Chiropotes anywhere in its geographic range. Sakis exhibited a highly fluid social structure that likely mitigates the effects of intragroup feeding competition despite their large group size. Throughout much of the year, the group moved like an amoeba through their home range, with expansions and contractions dictated by intragroup social dynamics and not changes in food patch quality. When resources became scarcer, and food patch quality was lower, the group divided into smaller, independently functioning foraging parties. The results of this study suggest that their unique sclerocarpic seed predator niche may necessitate bearded sakis range over large areas to balance the effects of seed secondary compounds. High predation pressure may explain the large group sizes of the genus.


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