Interspecific aggression in flat lizards suggests poor species recognition

Mistaken identity and competitive exclusion are two alternative hypotheses proposed to explain interspecific aggression between males. We examined agonistic behaviour in males of two lizard species: Platysaurus minor and P. monotropis. In each of nine outdoor field enclosures, we maintained a male and a female of both species (i.e., four total) and observed the dominance relationship between the males. Interspecific aggression was intense and P. monotropis was dominant in eight of nine enclosures. Furthermore, P. minor males received significantly more bite marks than P. monotropis males during the course of the experiment. To distinguish among the two hypotheses for interspecific aggression (mistaken identity and competitive exclusion), we presented P. minor males from sympatry and allopatry with model lizards of both taxa and measured aggressive responses. All trials with models were conducted in field enclosures where males were kept alone for the duration of the experiment. The model experiment revealed that compared to P. minor from sympatry, allopatric P. minor males were no less aggressive towards the heterospecific model than the conspecific model, a finding that supports the mistaken identity hypothesis. Finally, in the same experiment, we included a supernormal stimulus (pink Platysaurus model) to test if males were simply responding to a brightly coloured male lizard. Males showed some aggression towards the supernormal model, but significantly less than towards the two models (P. minor and P. monotropis) combined.