It takes two peaks to tango: the importance of UVB and UVA in sexual signalling in jumping spiders

Colourful male ornaments such as the bright plumage of birds or flashy wing patterns of butterflies serve to highlight the fitness of the bearer and are subject to sexual selection via female mate choice and/or direct male-male competition. Males of many species have markings that reflect ultraviolet (UV: wavelengths 280-400 nm). Recently, studies have focused on the role of either UVA (315-400 nm) or UVB reflectance (280-315 nm) in intraspecific communication, but the specific importance of UVA and UVB colours in sexual signalling remains unexplored. Here, we address this using Cosmophasis umbratica, a sexually dimorphic jumping spider species (Salticidae) in which males possess both UVA and UVB reflective patches. By conducting a series of female mate choice and male-male interaction trials with manipulated UVA and UVB coloration of males, we provide the first evidence that both UV components are prerequisites for intraspecific communication. Furthermore, UVA but not UVB is specifically used by both sexes for sex recognition, as shown by distinct, unexpected changes in behaviour by spiders encountering conspecifics with UVA removed. Our study revealed the adaptive significance of the presence of dual UV peaks in male ornamentation.