Responses of galápagos lava lizards (Microlophus bivittatus) to manipulation of female nuptial coloration on lizard robots

Females of some lizard species exhibit conspicuous coloration during the breeding cycle ("nuptial coloration") that elicits male courtship. We conducted two field experiments with San Cristóbal Lava Lizards (Microlophus bivittatus) in the Galápagos Islands to determine how the presence and extent of nuptial coloration on lizard robots affected responses of adult males and females. Robots programmed to perform conspecific bobbing displays had a morphological appearance that mimicked (1) a conspecific female without nuptial coloration (non-red control stimulus); (2) a conspecific female with nuptial coloration (normal red stimulus); or (3) a female with an extent of nuptial coloration beyond the range of conspecific variation (super-normal red stimulus). In Experiment 1, subjects witnessed two stimuli in sequence, being presented first with the side of a robot that exhibited conspecific nuptial coloration or with the opposite side of the same robot that lacked nuptial coloration. Results showed no effect of subject sex or stimulus order, but subjects exhibited more display and shorter display latencies in response to the normal red stimulus than to the non-red stimulus. In Experiment 2, new subjects were shown either non-red, normal red, or super-normal red stimulus. In contrast to Experiment 1, results of this experiment revealed sex differences in the amount of display elicited from subjects. Among the findings, males exhibited less display to the super-normal stimulus than to the non-red and normal red stimuli whereas the quantity of display elicited from females by the super-normal stimulus was similar to that evoked by the other two stimuli. We discuss our results in the context of prior studies and offer suggestions for future research.