In resource-limited populations, an increase in averagebody size can occur only with a decline in abundance. This is knownas self-thinning, and the decline in abundance in food-limited populationsis considered proportional to the scaling of metabolism withbody mass. This popular hypothesis may be inaccurate, because selfthinningpopulations can also experience density-dependent competition,which could alter their energy use beyond the predictionsof metabolic scaling. This study tested whether density-dependentcompetition has an energetic role in self-thinning, by manipulatingthe abundance of the fish Macquaria novemaculeata and tank size topartition the effects of competition from metabolic scaling.We foundthat self-thinning can be density dependent and that changes inintraspecific competition may be more influential than metabolicscaling on self-thinning relationships. The energetic mechanism wepropose is that density-dependent competition causes variation inthe allocation of energy to growth, which alters the energetic efficiencyof self-thinning cohorts. The implication is that food-limitedcohorts and populations with competitive strategies that encouragefast-growing individuals will have less body mass at equilibrium andhigher mortality rates. This finding sheds light on the processes structuringpopulations and can be used to explain inconsistencies in themass-abundance scaling of assemblages and communities (theenergetic-equivalence rule).