Nutrient excretion by consumers can alter ecosystems by changing the availability of limiting nutrients. Understanding controls on nutrient excretion is thus necessary to predict ecological dynamics. Ecological stoichiometry predicts nutrient excretion rates based on the principles of homeostasis and mass balance, and this theoretical framework effectively explains variation in excretion among fish species. It fails, however, to explain the substantial variation in excretion within species. This dissertation uses a model species for trait variation, the Trinidadian guppy (Poecilia reticulata), to explore why ecological stoichiometry has fared so poorly in explaining intraspecific variation in excretion. I first used a lab study to assess traditional stoichiometric predictions for nutrient excretion. This study found the expected stoichiometric patterns for phosphorus excretion, but not for nitrogen excretion. A field survey supported these results, and it indicated that the presence of a predatory fish, Crenicichla frenata, explained variation in guppy nitrogen excretion. In ensuing lab studies, I sought to isolate the influence of predation risk on nitrogen excretion by rearing guppies with or without the chemical cues of this predatory fish. This approach highlighted that predator cues reduced guppy nitrogen excretion by nearly 40%, primarily by reducing food intake. Standardizing for food intake, however, revealed that predator cues reduced guppy excretion more than food consumption. Guppies with predator cues thus excreted a smaller fraction of consumed nitrogen, retaining nitrogen more efficiently. We suggest that reduced nitrogen excretion under predation risk reflects a wasteminimizing physiology similar to that caused by extended food deprivation. We show, however, that food deprivation takes up to 14 days to change guppy physiology, while predation risk induces even greater metabolic change within just 24 h. This result raises the possibility that guppies use predation risk as a signal of impending food restriction, immediately inducing metabolic changes to increase retention of nutrients for future needs. In summary, this dissertation highlights that ecological stoichiometry can explain variation in phosphorus excretion but not nitrogen. Nitrogen excretion is better described by models incorporating causes of variation in metabolism. Predation risk, a major influence on consumer metabolism, is likely central to intraspecific variation in nitrogen excretion.