Objectives The physical condition of females depends on access to resources, which vary over space and time. Assessing variation in physical condition can help identify factors affecting reproductive success, but noninvasive measurement is difficult in wild animals. Creatinine concentration relative to the specific gravity (i.e., density) of urine has promise for noninvasively quantifying the relative muscle mass (RMM) of wild primates. We verified the relationship between these urinary parameters for wild white‐faced capuchin monkeys, and assessed temporal changes in the RMM of females across groups and between periods of high and low resource abundance. Materials and Methods We collected urine from 25 adult females in three groups across varying seasons at Sector Santa Rosa, Costa Rica. We measured the specific gravity and creatinine concentration of 692 samples and the effect of specific gravity on creatinine concentration. We used the residuals of this relationship to measure effects of group and season using mixed‐effects models. Results Specific gravity significantly predicted creatinine concentration. Season, group membership and the interaction between these variables were significant predictors of residual creatinine variation. Specifically, RMM was higher during months with high fruit energy density, lower in one social group, and less variable among females in the smallest group. Discussion Our findings suggest that specific gravity and creatinine may be used as urinary parameters to make inferences about the RMM of capuchins. Using this technique, we infer that females experienced changes in muscle mass according to variation in resource energy availability and social group variation.