Predator–Prey Categorization Beyond Retinal Position

The crab Neohelice is both prey to larger animals (e.g., seagulls) and a predator of smaller conspecifics. Experiments conducted on a horizontal substrate showed that, in response to the same moving dummy, crabs may either flee from or pursue it, depending on whether the dummy moves at ground level or above the substrate. Based on these observations, it was hypothesized that the crab uses a simple rule to categorize visual stimuli: stimuli perceived in the dorsal region of the retina are categorized as dangerous, whereas those perceived in the ventral region are categorized as prey. However, the natural habitats of Neohelice often have considerable slopes. Consequently, when a crab is positioned downslope, it may view a small object moving along the ground with the dorsal part of its retina. Conversely, when positioned upslope, it may view an object moving slightly above the ground with the ventral part of its retina. These conditions challenge the hypothesis that predator–prey categorization relies solely on the retinal position of the stimulus. To test this, we moved a dummy over an inclined substrate. Our results show that crabs can pursue the dummy even when it moves above the level of their eyes, and flee from it even when it moves below them. These finding challenge the simple hypothesis that predator–prey categorization is determined solely by retinal position and indicate that additional visual or contextual cues contribute to the categorization of moving objects.