We found that male L. viridis with experimentally reduced throat UV were more likely to lose the fights than control males, even though the contestants were matching in body and head size. Our results are interesting because, owing to our experimental design, the differently treated males within fighting male pairs could not systematically differ in any other correlated trait; it was the manipulated UV alone that determined the success of the fights with 88 per cent probability. Therefore, throat UV is clearly a male quality signal in L. viridis.
When considering random male pairs, body size alone, or body size combined with relative head size are likely to be the real determinants of fight success [19
]. However, such differences might not be easy and quick to assess for males. Signals advertising fighting ability allow male lizards to assess probability of winning and, therefore, to avoid energetically costly escalated fighting, injuries, and increased predation risk [22
], and to gain time and energy for other tasks like feeding, mate search or thermoregulation [4
]. The role of UV signals in advertising dominance status or aggression has been shown in different taxa [13
]. In our experiment, aggressive displays were abundant, but they rarely escalated to physical fights. Still, in 17 out of 20 cases, one male gave up and tried to escape from the other who pursued the loser, suggesting that males made their decision without taking the risks involved in actual fights. As everything else was equal (or differed randomly), the manipulated UV signal alone must have been the cue used in males' decisions. A possible explanation is that throat colour may be an amplifier of head size, which usually correlates with bite force [24
], and thus males may use throat colour as an indicator of each other's bite force [25
Signals important in one context (intra- versus intersexual) may not be important in the other [1
]. In our case, male throat UV is not only an important cue in female mate choice [11
], but it also determines fight success, making Fisherian runaway [26
] as the process behind the signal's evolution highly unlikely. UV was accepted as conferring comparable costs with pigment-based colours only recently [5
]. For instance, higher UV colour is negatively correlated with health state in the lizard Lacerta schreiberi
]. In L. viridis
, we found that high throat UV is negatively correlated with body condition in the field (O. Molnár, K. Bajer, J. Török & G. Herczeg 2011, unpublished data), and its annual development is dependent on the time available for maintaining optimal body temperature (K. Bajer, O. Molnár, J. Török & G. Herczeg 2011, unpublished data), suggesting that it is a costly signal and thus can honestly reveal individual quality. Our present results are in line with this scenario, suggesting that throat UV is a reliable signal of fight ability, allowing male L. viridis
to judge each other without engaging in costly physical fights. Because UV in this species is important in both settling aggressive encounters between males and female mate choice, we suggest that UV, or other structural colours, can be more important in sexual selection than previously presumed.