Natural visual cues eliciting predator avoidance in fiddler crabs

To efficiently provide an animal with relevant information, the design of its visual system should reflect
the distribution of natural signals and the animal’s tasks. In many behavioural contexts, however, we
know comparatively little about the moment-to-moment information-processing challenges animals face
in their daily lives. In predator avoidance, for instance, we lack an accurate description of the natural
signal stream and its value for risk assessment throughout the prey’s defensive behaviour.We characterized
the visual signals generated by real, potentially predatory events by video-recording bird approaches towards
an Uca vomeris colony. Using four synchronized cameras allowed us to simultaneously monitor predator
avoidance responses of crabs. We reconstructed the signals generated by dangerous and non-dangerous
flying animals, identified the cues that triggered escape responses and compared them with those triggering
responses to dummy predators. Fiddler crabs responded to a combination of multiple visual cues
(including retinal speed, elevation and visual flicker) that reflect the visual signatures of distinct bird and
insect behaviours. This allowed crabs to discriminate between dangerous and non-dangerous events. The
results demonstrate the importance of measuring natural sensory signatures of biologically relevant
events in order to understand biological information processing and its effects on behavioural organization.