Experimental Evidence on the Signalling Role of UV ‐Blue Colouration in Male European Wall Lizards ( Podarcis muralis )

ABSTRACT 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. Correlational studies suggest condition‐dependence of that trait, but results are not replicable to all species or populations. On the other hand, 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. A large male acted as the resident lizard and, during the following days, eight different small males were introduced sequentially into the chamber to record the behaviour of both. This simulates 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, above the UV‐blue patches) or treatment (painted covering the UV‐blue patches). Results of 90 trials with 12 large resident males show that aggression by these males was influenced by the small males' experimental manipulation. 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 provide 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.