Animals use a spectacular diversity of signals and studying them is central to understanding a range of biological processes including how animals interact socially, and how new species arise. According to animal communication theory, signal design evolves in response to two main selective forces acting on the tactical design and strategic design. Strategic design refers to any element associated with information content (i.e. signal reliability), while the tactical design has to do with the efficacy with which signals are transmitted, perceived, and processed by receivers in the environment (i.e. signal efficacy). Understanding how selection acting on the strategic and tactical components interact to shape signal design is crucial to unravelling the true complexity of animal communication. Colour signals constitute a major and functionally diverse class of signals. It is only recently that the signalling role of structurally produced colours, such as ultraviolet (UV), has been documented. UV colours, invisible to the human eye, have long gone unnoticed by scientists, and yet, their potential to remain hidden from predators lacking UV vision makes them particularly interesting.
During my PhD, I used lizards as a model to study the adaptive co-variation across and between UV colour features in relation to environmental factors and individual traits. More specifically, my objectives were 1.) to improve the general approach used to objectively measure small colour patches, 2.) to study how and to what extent habitat conditions can generate substantial variation in the design of UV signals at various levels of integration (i.e. intra-population and inter-population), 3.) to explore in details the information content of UV signals, that is investigating how design components relate to each other, and to individual quality traits, and 4.) to explore the possible roles UV colours may play in lizards, for example as deimatic (warning) displays.
To answer these questions, the European common wall lizard (Podarcis muralis) is an ideal model because males display sexually selected UV-blue signals on their flanks. First and foremost, after refining the methodology for reflectance spectrophotometry, I provided strong evidence that habitat conditions drive an adaptive response in the design of UV signals, whereby several signal design components respond to different selective pressures both within and between populations. In addition, I demonstrated that UV signals are complex multicomponent signals advertising male quality traits. Finally, I provided an example of UV patterns that potentially function as a deimatic display in the Australian northern blue-tongue skink (Tiliqua scincoides intermedia). This PhD thesis thus provided important insights rooted in animal communication theory and pave the way for future research foci.