Dialogues in colour and behaviour: integration of complex signalling traits and physiology

Animal communication is complex, involving multiple static and dynamic traits that vary in redundancy. To understand how multiple signals are integrated and maintained, we staged male–male interactions between size-matched individuals of Psammophilus dorsalis and measured display behaviours, the maximum chromatic contrast of dynamically changing skin colours and size of ultraviolet patches. We also measured testosterone and corticosterone levels induced by the social interaction. All signalling traits condensed in a principal component analysis into two main components—PC1, comprising behaviours (headbobs and shudders) and ultraviolet patch size, and PC2, representing the dorsal and lateral body colours. Testosterone levels were positively associated with PC1, and both testosterone and corticosterone levels were negatively associated with PC2. Our results suggest that the chromatic contrasts of colours are non-redundant with the intensity of behaviours, and that size of UV patches better reflects behavioural intensity than other colour components. We also found that during social interactions, individuals matched the intensity of display behaviours and the contrast of their dorsal (but not lateral) colours with their opponents. Overall, these results highlight how multiple dynamic signals, which change in response to opponents during social interactions, can be maintained both by internal mechanisms (i.e. hormones) and through non-redundancy.