To evaluate whether decreases in muscle metabolic capacities and increases in plasma cortisol explain the effects of neighboring conspecifics on male three-spine sticklebacks Gasterosteus aculeatus, we housed mature males alone, with a mature female, or with a rival mature male. The neighbors were separated from the focal male by a partition that allowed him to smell, see, and hear his neighbor. In the first experiment, focal males were allowed to reproduce, whereas in the second experiment, no reproduction occurred. Coloration and behaviors were monitored while the males tended their nests (or for the same period in the second experiment). The presence of a neighbor markedly affected the reproductive coloration of the focal male, with solitary males being less colorful than males housed with a rival male or a female. Solitary males showed greater aggression toward a model male stickleback than did males with neighbors. The presence of neighbors affected the anatomic and metabolic characteristics of the focal males primarily during nesting, when males housed with rival males had a lower hepatosomatic index and lower activities of mitochondrial and glycolytic enzymes in the axial muscle than did solitary males or males housed with females. Cortisol levels were highly variable in nesting males and did not differ with social condition but were higher in males that had been quick to construct their nests. On the other hand, when focal males were not provided with nesting material, solitary males tended to have lower cortisol levels than did males housed with rival males. While these results do not provide a mechanism for a "femme fatale" effect, they indicate that nesting males decrease metabolic status when housed with rival "macho males."