Can Food Acquisition Priority Alter Emotion, Physiological Responses, and Social Rank in Fish?

Whether non-human animals can perceive and respond to inequitable resource distribution has become a prominent topic in behavioral and cognitive sciences. Fish possess notable cognitive capacities and display physiological and behavioral indicators of "emotion-like states" under stress, pain, or adverse conditions. However, it remains unclear whether fish can perceive inequitable resource allocation and exhibit emotion-related physiological and behavioral responses. Here, we investigated this question in a group-living cichlid, Chindongo demasoni, by manipulating the feeding order to create inequity in food acquisition order (priority-fed vs. delayed-fed). We examined whether this manipulation could elicit emotion-like states and whether these states were associated with variation in growth, hormone levels (dopamine and cortisol), metabolism (standard and routine metabolic rates), personality traits (activity, sociability, and aggression), and anxiety-like behaviors, ultimately influencing social hierarchy. Relative to delayed-fed individuals, priority-fed individuals consumed more food and exhibited a higher condition factor, and showed lower whole-body cortisol levels. However, no significant differences were detected between the two groups in personality traits, anxiety-like behaviors, metabolic rates, dopamine levels, or social rank. These findings suggest that Demasoni cichlids may perceive inequity in food distribution, with differences in food intake and cortisol levels consistent with emotion-like states. However, because cortisol levels and feeding/nutritional state can influence each other, the extent to which these differences reflect emotional processes versus nutritional effects remains unclear. This study provides new evidence for understanding whether and how non-human animals perceive social inequity and highlights the importance of minimizing persistent feeding-order differences in laboratory and aquaculture welfare management.