Conspecific chemical cues drive density-dependent metabolic suppression independently of resource intake.

Within species, individuals of the same size can vary substantially in their metabolic rate. One source of variation in metabolism is conspecific density - individuals in denser populations may have lower metabolism than those in sparser populations. However, the mechanisms through which conspecifics drive metabolic suppression remain unclear. Although food competition is a potential driver, other density-mediated factors could act independently or in combination to drive metabolic suppression, but these drivers have rarely been investigated. We used sessile marine invertebrates to test how food availability interacts with oxygen availability, water flow and chemical cues to affect metabolism. We show that conspecific chemical cues induce metabolic suppression independently of food and this metabolic reduction is associated with the downregulation of physiological processes rather than feeding activity. Conspecific cues should be considered when predicting metabolic variation and competitive outcomes as they are an important, but underexplored, source of variation in metabolic traits.