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

Animal communication can be complex and costly, involving multiple traits that vary in redundancy. To understand how dynamic signals are integrated, 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 of behaviours (number of headbobs and shudders) and ultraviolet (UV) 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 reflect behavioural intensity than other colour components. We also found that 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 by both internal mechanisms (i.e. hormones) as well as through non-redundancy. 

Methods
We staged male-male interactions in the lab and measured the entire repertoire of display behaviour using a video camera. We also measured dorsal and lateral body colours in both visible and ultraviolet ranges from photographs using a multispectral camera. An X-Rite colour checker was used to linearise and normalise all the colours measured. ImageJ was used to measure the area of UV patches. Additionally, we measured plasma testosterone and corticosterone levels before and after the social trials. Snout-vent length and weight for each lizard were measured using a linear scale and a weighing balance respectively before the trials.