Fish display a remarkable diversity in social behavior that ranges from highly social to largely solitary. While social behaviors are critical for survival and under stringent selection, surprisingly little is understood about how environmental pressures shape differences in collective behavior. The Mexican tetra, Astyanax mexicanus, is a model for studying the evolution of social behaviors. These fish exist as multiple blind cave populations in the Sierra de El Abra and extant ancestral surface fish populations that inhabit rivers and lakes throughout Mexico. Cavefish populations have converged upon many different morphological, physiological, and behavioral traits including reduced social behavior. Dynamically, groups of surface fish maintain close proximity, persistently shoaling or schooling, while their dark-cave-evolved counterparts swim independently, surveying walls and floors for food. To more carefully study evolved changes in individual and social behavior, we track free-swimming individuals in a large circular tank, varying group size and fish type. We find that group size and isolation impact swimming dynamics of cave and surface fish very differently, underscoring the mechanics of their distinct social dynamics. Highly-social surface fish respond to isolation by becoming more inactive while cavefish swimming becomes less persistent in the presence of others. In stark contrast, we find that cavefish actively evade each other upon encounter and respond to increased social density by slowing down. For abstract consideration of evasive interactions, we consider emergent effects of local evasive interactions by constructing and simulating a minimal active matter model and show how this behavior can generate group dynamics that may uniquely benefit exploration of confined environments in the absence of long-range sensory cues provided by vision. These findings reveal the convergent evolution of avoidance behavior in cavefish and support the use of A. mexicanus as a model for studying the evolution of social behavior.

Analysis of social swimming dynamics in the Mexican cavefish