Group size shapes interactions in confined minimal active biological collectives.

Abstract

We demonstrate that group size reshapes the effective interaction network in confined biological collectives. Using minimal groups of the shrimp Neocaridina davidi (N=2,3,4) as a representative nonequilibrium system, we reconstruct the asymmetric coupling matrix via an inverse asymmetric Ising model. This mapping is physically enabled by thigmotactic behavior, which restricts trajectories to a discrete decision space, allowing for a maximum-entropy description appropriate for steady states characterized by broken detailed balance. Our analysis reveals a structured progression with increasing group size: Mean coupling strength undergoes social screening, while heterogeneity, structural frustration, and nonreciprocal asymmetry emerge as defining architectural features. Group-size identity is encoded in a subspace orthogonal to the global coupling scale and statistical volatility. Furthermore, the eigenvalue spectrum signals a stationary log-probability distribution in which collective rotational order is amplified while statistical barriers between configurations are lowered, thereby expanding the repertoire of accessible states. Confinement thus drives a well-defined architectural transition from simple reciprocal coordination toward a complex, heterogeneous, and nonreciprocal nonequilibrium steady state.