Effect of hypoxia on opercular displays: evidence for an honest signal?

One of the most common displays made by fish is the opercular display. This display appears in virtually all taxa of fish, and is typically associated with aggressive contests between individuals. In addition, the frequency and duration of this display is often correlated with success in aggressive contests. Despite the ubiquitous nature of this display, no explanation has been provided to explain why this display is used. Here, we test the hypothesis that the opercular display functions to honestly convey information about physiological condition. While making this display, fish are unable to pass water efficiently over their gills and hence, display duration may be dependent upon underlying body condition. We tested this hypothesis by measuring the duration and frequency of opercular displays of Siamese fighting fish, Betta splendens, displaying to their mirror images in normoxic or hypoxic water. We observed that fish significantly reduced their displays under hypoxic conditions. This result is consistent with the hypothesis that opercular displays provide an honest indication of current physiological condition.