Absence of temporal pattern in courtship signals suggests loss of species recognition in gecko lizards

According to the concept of ritualization, acquisition of signal function (i.e., species recognition) leads to formalization of specific signal structure (i.e., species-specific pattern). By contrast, loss of species recognition may result in specific pattern collapse of the signal, from a temporally regular pattern to an irregular form. Several studies have reported the loss of species recognition driven by release from selective pressure against hybridization under isolation from close relatives, but no study has reported collapse of a regular signal pattern that serves in species recognition. In nocturnal lizards of the genus Gekko, several species pairs hybridize when brought together, whereas other species pairs never hybridize, despite being sympatric. I investigated the relationship between the existence of natural hybridization, courtship call similarity and species recognition ability in eight species of Gekko. I found that males of all eight species court using calls, and four of these species have species-specific regular patterns in the courtship calls, whereas the other four have no temporal patterns. In playback experiments, females of the species-specific pattern call species discriminated species by the call. On the other hand, in the patternless call species, females did not discriminate species by the call, suggesting that natural hybridization has resulted from the loss of species recognition ability in the patternless call species. In insular Gekko isolated from congeners, acoustic species recognition might have been lost, followed by pattern collapse of the courtship calls. This unique example of the signal pattern collapse would provide a key to answer basic questions concerning the causes and consequences of signal evolution.