We present a model and an experimental test for a nonshareable parental-investment behavior (fanning of eggs) using parental male bluegill sunfish. The model predicts that as brood size increases, nonshareable investment into the brood should increase if a parent is to maximize its remaining lifetime reproductive success. The experimental test manipulated the number of eggs in nests and quantified the frequency and tempo of fanning in the field using underwater video equipment. As predicted, male bluegill fanned larger broods more frequently than smaller broods. The fanning tempo (beats per min) did not change with changes in brood size. Parental males lost substantial amounts of body weight during the period of parental care, and lipid extractions revealed a loss in nonpolar lipids. The difference in fanning frequency between large and small broods did not result in differential weight loss for the parents, but parents that fanned more tended to lose more nonpolar lipids from the body tissue than males that fanned less.