Vendredi 16 octobre 2026 à 11h00 ; salle 250 ; IUSTI
Animal groups continuously deform as they move, yet the physical principles governing these spontaneous shape fluctuations remain unknown. Using aerial drone recordings of wild milling fish schools, we show that their fluctuating boundary exhibit power-law spectra and propagating shape waves, with scaling laws alike across three species, contrasting ecosystems, and multiple years. Coarse-graining a minimal agent-based model identifies the milling state as a droplet of flocking active matter and yields a symmetry-constrained boundary condition analogous to Laplace’s law in passive fluids. The resulting hydrodynamic theory predicts simulated shape spectra without adjustable parameters. Applied to field observations, it infers effective hydrodynamic parameters from spontaneous boundary fluctuations alone. These results establish the morphodynamics of cohesive animal groups as a non-invasive spectroscopic probe of the laws governing their collective motion.