Claw waving display changes with receiver distance in fiddler crabs, Uca perplexa

Effective communication is critically dependent on the successful transfer of information and, because environmental and social conditions can affect signal transmission, animals should be able to adjust their signals to optimize reliability. We show, apparently for the first time in a movement-based signal, that visual displays are adjusted with respect to the distance of signal receivers. Not only does this show the ability of the fiddler crab to judge distance, but this also shows that signalling is context dependent on surprisingly fine spatial and temporal scales. We elicited courtship behaviour in the crabs with tethered females and simultaneously recorded the displays of males from above and from crab-eye level. As females approached, males increased signal intensity by shortening display duration and altered signal form by reducing the lateral movement component of the waving signal. We suggest that males tune their waving display depending on receiver distance (a) to balance energetic costs with reproductive benefits, (b) to alter the information content of the signal and (c) to avoid signal misinterpretation. Such fine-scale context sensitivity is likely to be far more widespread in animal communication than hitherto recognized from similar signal modifications in auditory communication.