Sensory system properties predict signal modulation in a tropical lizard

Two common challenges faced by signallers, which can affect signal efficacy and are intrinsic to the communication process, are (1) that intended receivers are often inattentive and (2) that the spacing of animals in a habitat is relatively fluid. Signal modulation has been proposed to be an effective strategy to cope with these challenges by providing a mechanism to maintain signal efficacy. We evaluated this hypothesis by testing the use of modulation in the tropical lizard Anolis gundlachi. We characterized the motion properties of the sensory system and measured the physical properties of movement-based headbob displays given during staged social encounters under natural conditions. We found a significant positive association between the maximum amplitude of headbob displays and the distance to intended receivers. Modulation occurred in response to small-scale changes in signaller–receiver distance, and signallers gave displays that fell within a range of amplitudes predicted to optimally stimulate the visual system of A. gundlachi. Our findings strongly suggest that modulation of the physical properties of motion-based signals can be an effective mechanism to tune signals to the combined effects of characteristics of receiver sensory systems and receiver distance, and can serve as a behavioural strategy to cope with relatively frequent changes in the spacing of individuals.