Conspecific proximity alters crabs' attention to novel visual stimuli

The ability to prioritize relevant stimuli over distracting ones is essential for adaptive behavior, particularly in species with limited sensory systems. In the mudflat crab Neohelice granulata, a moving object on the ground (dummy) can elicit either avoidance or pursuit, indicating that the same stimulus may be interpreted as predator or prey. Previous studies have shown that the probability of these opposite responses depends on both stimulus features (e.g. size, speed) and intrinsic animal attributes (e.g. sex, hunger level). Here, we investigated how social context - a factor independent of both stimulus properties and individual traits - affects responses to the dummy. Using a controlled experimental design, we tested whether the presence, proximity and behavior of a conspecific of the same or opposite sex modulate individual responses. We found that, irrespective of sex, crabs tested in pairs showed significantly reduced exploratory activity and a lower probability of responding to the dummy compared with solitary crabs. Notably, this reduction occurred only when individuals were within 15 cm of each other. Furthermore, in the presence of a conspecific, crabs initiated avoidance or predatory responses at shorter distances to the dummy, suggesting delayed decision making because of divided attention. In contrast, freezing distance remained unchanged across social conditions, indicating that active responses such as escape or predation are selectively modulated by social proximity. Overall, our results reveal a complex interplay between social context, attention and decision making. Crabs adjust their perceptual and behavioral strategies according to social cues, highlighting the influence of social factors on survival in fluctuating environments.