Pigment cell mechanism of bluish‐purple nuptial colouration in the Japanese fire‐bellied newt <i>Cynops pyrrhogaster</i>

Abstract Morphological and behavioural characteristics for attracting mates in animal taxa are typically recognized as secondary sexual traits. Among secondary sexual traits, vertebrate body colour has been studied in various taxa. Many researchers have evaluated their functions and genetic basis, but the pigment cell mechanisms underlying such colouration remain to be established. The breeding morphology and behaviour of the Japanese fire‐bellied newt Cynops pyrrhogaster have been well studied; in this species, males exhibit a body colour change to bluish‐purple for attracting females during the breeding season. To obtain a mechanistic understanding of nuptial male colour, we examined the histological characteristics of pigment cells responsible for variation in the light reflectance profiles among males with or without nuptial colours and among females. We found that the nuptial colour of male newts during the breeding season is reflected in the ultraviolet region, as determined using a spectrometer. Observations of skin pigment cells showed that the vivid nuptial colour was a structural colour produced by an increased density of iridophores with a parallel alignment of light‐reflecting platelets and black melanophores. We also discuss potential endocrine mechanisms controlling densities of pigment cells in the nuptial colouration of male newts.