Abstract <p>Exaggerated sexually selected traits are thought to be energetically costly to produce and to maintain. This is especially true of sexually selected weapons, traits that are frequently positively allometric, and therefore likely to be especially energetically costly. However, there have been few studies that directly measure these putative costs. Additionally, metabolic rate, a measure of energy expenditure, is highly variable across individuals, but the causes of this variation are not always well established. Here we tested for the associations between resting metabolic rate, during development and adulthood, and the size of a weapon used during adult male-male competition in the beetle <i>Gnatocerus cornutus</i>. We also tested for genetic variation in metabolic rate and weapon size. We found that metabolic rate was not statistically associated with weapon size, but there were effects of genotype and mass during development (pupal size) on weapons. Additionally, age, mass and genotype all affected metabolic rate in various ways. Despite no evidence of energetic costs to weapon development or maintenance, direct predation costs associated with them have been detected previously, and hence enlarged weapons are costly in this system.</p> Significance <p>The large weapons used in competition between males are thought to be energetically costly. However, there are rather few investigations of these putative costs. We investigate potential costs by testing for associations between metabolic rate and weapon size in a beetle. We found no relationship between weapons and metabolism, indicating weapons are not always energetically costly.</p>