Predator avoidance in fiddler crabs: 2. The visual cues

The efficiency of predator avoidance strategies depends on the availability and accuracy of sensory information. Although vision can in principle provide instant information on a predator's position, direction of approach and identity, prey animals that face fast predators have to respond so early (close to the limits of detection) that visual cues are unreliable predictors of actual risk. This is a major problem for prey animals that have to balance predation risk with the cost of antipredator action. I investigated the visual cues fiddler crabs Uca vomeris use to decide when to run towards their burrow in response to an approaching (dummy) predator by running towards their burrow. The crabs did not always escape immediately when they first detected the dummy, but continued to monitor and assess its approach. The crabs relied on retinal image speed to trigger a home run. Retinal speed did not correlate well with the actual risk of predation because it confounded a predator's direction of approach with its speed and proximity. In an attempt to reconcile what is known about predator avoidance in semiterrestrial crabs, I propose a two-tier antipredator response system and discuss the crabs' multistage response strategy in the context of the associated costs and the availability of information. Multistage predator avoidance, which is common among prey animals, appears to be designed to improve the quality of information in an attempt to minimize the costs associated with predator avoidance responses.