Violin plots of Δdarkness of the iris and of the line for males and females treated with melanocyte-stimulating hormone (MSH) or noradrenaline (NA); higher values indicate greater difference, i.e. a darker line. Filled circles and error bars show the mean ± SE (SE adjusted for within-subject variance using the method of Morey, 2008). Open circles show the observed Δdarkness scores. The Δdarkness was calculated as the relative difference in darkness between the surrounding skin and the line or iris, respectively; in two cases the skin was darker than the iris, and in one case the skin was darker than the line, resulting in negative values of Δdarkness.