Experimental evidence on the signalling role of UV coloration in male European wall lizards (Podarcis muralis)

Determining the role of a phenotypic trait as a signal often requires a complex and interdisciplinary approach. Most wall lizards (genus Podarcis), as well as many other lacertids, exhibit on their flanks ultraviolet (UV)-blue patches that are thought to inform about fighting ability during male agonistic interactions. Empirical attempts to confirm the communicative function of the UV-blue patches have produced inconclusive results. Here, we set out a behavioural experiment to test the signalling function of the UV-blue patches of the European wall lizard (Podarcis muralis) in staged contests in a large observation chamber. We simulated the situation, often observed in the field, in which a small male intrudes in the territory of a larger male. Small males were randomly selected to act as controls (painted on the flank) or treatment (painted covering the UV-blue patches). Results of 90 trials show that resident males were more aggressive towards males that retained their original UV-blue patches (control) than to those in which this colouration was completely obliterated. These results represent the first experimental evidence of wall lizards using the UV-blue patches to make decisions during agonistic interactions, and confirm that this colouration fulfils a signalling role in this species. Our results also reveal the value of behavioural experiments designed to induce animals to perform tasks or make decisions that closely mimic those in nature and are, therefore, ethologically relevant.