Red clypeal membrane color predicts immune function in a burying beetle (Coleoptera: Silphidae)

Red and orange coloration commonly signals individual quality, especially in non‐mammalian vertebrates. Because the molecules that create red pigments are involved in immune function, acting as antioxidants among other functions, red and orange body coloration can indicate parasite load and the host's ability to mount a strong immune response. Although body color and immunity are associated in fishes, birds and lizards, to our knowledge no studies have examined the role of red or orange color patches as an indicator of immune function in invertebrates. Burying beetles in the genus Nicrophorus often have clypeal membranes directly above their mandibles which scale hyperallometrically with body size and appear red or orange to the human eye. We investigate whether the hue (a chromatic measure) or brightness (an achromatic measure) of this membrane is associated with melanistic encapsulation response to a foreign body inserted into the abdomen. We find that clypeal membrane hue in the red–orange range and brightness are associated with immune response in Nicrophorus pustulatus beetles. Male clypeal membranes show greater hue variation between individuals; however, for both sexes, redder and darker membranes indicate a stronger encapsulation response. Body size was also correlated with the observed variation in immune response, with smaller individuals tending to encapsulate more strongly. Hue and brightness were also associated with body size, with larger individuals having clypeal membranes that were more orange and brighter, especially for males. This represents the first report of an association between hue and brightness of red integumentary color and induced immunity in an invertebrate animal. Investigation into relationships between color and immunity in a broader range of taxa will augment our understanding of relevant ecological factors leading to the evolution of color signals of immune function.