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Sexual dimorphism describes trait differences between males and females of the same species. In primates, body size dimorphism can help with male-male competition for females. Adult male squirrel monkeys (genus Saimiri) undergo a hormonally-induced “fattening” in their 8-week breeding season. Possibly, male fattening is a sexually-selected trait that allows fatter males to have higher access to mates. This field study examined whether the male fattening trait in Saimiri collinsi is linked to advantages acquired during male-male competition events. We hypothesized fatter males would gain more access to females due to their higher competitive abilities. We predicted that more robust males would participate in and win more contests. We collected behavioral data on two squirrel monkey social groups during three mating seasons (2023-2025; N=414 contact hours), in Eastern Amazonia, Brazil. Study groups averaged  46 individuals, 12 were adult males. Males were classified on a visual scale of 1 to 3 (1=smallest, 3=fattest). Focal animal samples were used to construct nearest neighbor budgets for different males. We recorded the initiator, recipient, winner of male-male contests, and copulation occurrences. Grade 3 males were responsible for 88% of the copulations (N=50). During male-male contests, females were within 3m 73% of the time (N=164 observations). Grade 3 males won 67% of competitive interactions, initiating 71%. Fatter males are more competitively dominant and gain more access to females, suggesting that the degree of fattening of male S. collinsi affects their competitive ability and mating success. Funded by NSF (Award no. BCS-2140666).