A sensory ecology of fear: Eye size predicts moonlight avoidance responses in Neotropical electric fishes

Animals continually balance the rewards of activity against the risk of predation. The mere perception of predation risk can trigger cascading effects across “landscapes of fear,” from shifts in individual activity to altered community composition. Despite the established role of sensory systems in predator detection, their role in shaping trade-offs between activity and predator avoidance remains underexplored. For many nocturnal species, moonlight—a key driver of predation risk—varies cyclically, offering a unique opportunity to study how sensory capacity influences activity patterns. We studied four species of Neotropical electric fish inhabiting open areas of a shallow, clearwater Amazon stream, where visually oriented nocturnal predators are abundant. These species were similar in size and diet but differed in body mass-adjusted eye size, a commonly used proxy for visual acuity. Using custom-designed loggers to detect and record the continuously emitted, species-specific electrolocation signals of these fish, we monitored each species' foraging activity across a full lunar cycle while simultaneously measuring moonlight illuminance with a low-light radiometer. The two species with the smallest eyes exhibited strong lunar phobia, hiding during moonlit periods and, as a result, sacrificing ~25% of their nocturnal foraging time. They foraged instead during moonless periods, which follow a complex temporal pattern throughout the lunar cycle. In contrast, the species with the largest eyes foraged continuously, likely using vision to detect and evade predators. A species with intermediate-sized eyes showed a canopy-dependent response: foraging normally under semi-open canopy but exhibiting lunar phobia under closed canopy. Active electroreception, the primary sensory modality of electric fish, enables foraging in complete darkness, but its limited range (<10 cm) makes it ineffective for predator detection. Our findings suggest that large-eyed species can detect approaching predators and adopt a “vigilance” strategy, while the poor low-light vision of small-eyed species promotes a “hiding” strategy. These alternative strategies impose distinct metabolic challenges: the opportunity costs of lost foraging time versus the energetic demands of maintaining larger eyes. Overall, our results highlight how sensory adaptations influence critical trade-offs between foraging and predation risk across both spatial and temporal landscapes of fear.